include/linux/sched.h | 2 + kernel/sched/core.c | 52 ++---- kernel/sched/debug.c | 18 ++- kernel/sched/fair.c | 340 +++++++++++++++++++++++++++++++++++++++- kernel/sched/features.h | 1 + kernel/sched/sched.h | 56 ++++++- kernel/sched/topology.c | 4 +- 7 files changed, 420 insertions(+), 53 deletions(-)
Changes
-------
This is v3 of the shared runqueue patchset. This patch set is based off
of commit 88c56cfeaec4 ("sched/fair: Block nohz tick_stop when cfs
bandwidth in use") on the sched/core branch of tip.git.
v1 (RFC): https://lore.kernel.org/lkml/20230613052004.2836135-1-void@manifault.com/
v2: https://lore.kernel.org/lkml/20230710200342.358255-1-void@manifault.com/
v2 -> v3 changes:
- Don't leave stale tasks in the lists when the SHARED_RUNQ feature is
disabled (Abel Wu)
- Use raw spin lock instead of spinlock_t (Peter)
- Fix return value from shared_runq_pick_next_task() to match the
semantics expected by newidle_balance() (Gautham, Abel)
- Fold patch __enqueue_entity() / __dequeue_entity() into previous patch
(Peter)
- Skip <= LLC domains in newidle_balance() if SHARED_RUNQ is enabled
(Peter)
- Properly support hotplug and recreating sched domains (Peter)
- Avoid unnecessary task_rq_unlock() + raw_spin_rq_lock() when src_rq ==
target_rq in shared_runq_pick_next_task() (Abel)
- Only issue list_del_init() in shared_runq_dequeue_task() if the task
is still in the list after acquiring the lock (Aaron Lu)
- Slightly change shared_runq_shard_idx() to make it more likely to keep
SMT siblings on the same bucket (Peter)
v1 -> v2 changes:
- Change name from swqueue to shared_runq (Peter)
- Shard per-LLC shared runqueues to avoid contention on scheduler-heavy
workloads (Peter)
- Pull tasks from the shared_runq in newidle_balance() rather than in
pick_next_task_fair() (Peter and Vincent)
- Rename a few functions to reflect their actual purpose. For example,
shared_runq_dequeue_task() instead of swqueue_remove_task() (Peter)
- Expose move_queued_task() from core.c rather than migrate_task_to()
(Peter)
- Properly check is_cpu_allowed() when pulling a task from a shared_runq
to ensure it can actually be migrated (Peter and Gautham)
- Dropped RFC tag
Overview
========
The scheduler must constantly strike a balance between work
conservation, and avoiding costly migrations which harm performance due
to e.g. decreased cache locality. The matter is further complicated by
the topology of the system. Migrating a task between cores on the same
LLC may be more optimal than keeping a task local to the CPU, whereas
migrating a task between LLCs or NUMA nodes may tip the balance in the
other direction.
With that in mind, while CFS is by and large mostly a work conserving
scheduler, there are certain instances where the scheduler will choose
to keep a task local to a CPU, when it would have been more optimal to
migrate it to an idle core.
An example of such a workload is the HHVM / web workload at Meta. HHVM
is a VM that JITs Hack and PHP code in service of web requests. Like
other JIT / compilation workloads, it tends to be heavily CPU bound, and
exhibit generally poor cache locality. To try and address this, we set
several debugfs (/sys/kernel/debug/sched) knobs on our HHVM workloads:
- migration_cost_ns -> 0
- latency_ns -> 20000000
- min_granularity_ns -> 10000000
- wakeup_granularity_ns -> 12000000
These knobs are intended both to encourage the scheduler to be as work
conserving as possible (migration_cost_ns -> 0), and also to keep tasks
running for relatively long time slices so as to avoid the overhead of
context switching (the other knobs). Collectively, these knobs provide a
substantial performance win; resulting in roughly a 20% improvement in
throughput. Worth noting, however, is that this improvement is _not_ at
full machine saturation.
That said, even with these knobs, we noticed that CPUs were still going
idle even when the host was overcommitted. In response, we wrote the
"shared runqueue" (SHARED_RUNQ) feature proposed in this patch set. The
idea behind SHARED_RUNQ is simple: it enables the scheduler to be more
aggressively work conserving by placing a waking task into a sharded
per-LLC FIFO queue that can be pulled from by another core in the LLC
FIFO queue which can then be pulled from before it goes idle.
With this simple change, we were able to achieve a 1 - 1.6% improvement
in throughput, as well as a small, consistent improvement in p95 and p99
latencies, in HHVM. These performance improvements were in addition to
the wins from the debugfs knobs mentioned above, and to other benchmarks
outlined below in the Results section.
Design
======
Note that the design described here reflects sharding, which is the
implementation added in the final patch of the series (following the
initial unsharded implementation added in patch 6/7). The design is
described that way in this commit summary as the benchmarks described in
the results section below all reflect a sharded SHARED_RUNQ.
The design of SHARED_RUNQ is quite simple. A shared_runq is simply a
list of struct shared_runq_shard objects, which itself is simply a
struct list_head of tasks, and a spinlock:
struct shared_runq_shard {
struct list_head list;
raw_spinlock_t lock;
} ____cacheline_aligned;
struct shared_runq {
u32 num_shards;
struct shared_runq_shard shards[];
} ____cacheline_aligned;
We create a struct shared_runq per LLC, ensuring they're in their own
cachelines to avoid false sharing between CPUs on different LLCs, and we
create a number of struct shared_runq_shard objects that are housed
there.
When a task first wakes up, it enqueues itself in the shared_runq_shard
of its current LLC at the end of enqueue_task_fair(). Enqueues only
happen if the task was not manually migrated to the current core by
select_task_rq(), and is not pinned to a specific CPU.
A core will pull a task from the shards in its LLC's shared_runq at the
beginning of newidle_balance().
Difference between SHARED_RUNQ and SIS_NODE
===========================================
In [0] Peter proposed a patch that addresses Tejun's observations that
when workqueues are targeted towards a specific LLC on his Zen2 machine
with small CCXs, that there would be significant idle time due to
select_idle_sibling() not considering anything outside of the current
LLC.
This patch (SIS_NODE) is essentially the complement to the proposal
here. SID_NODE causes waking tasks to look for idle cores in neighboring
LLCs on the same die, whereas SHARED_RUNQ causes cores about to go idle
to look for enqueued tasks. That said, in its current form, the two
features at are a different scope as SIS_NODE searches for idle cores
between LLCs, while SHARED_RUNQ enqueues tasks within a single LLC.
The patch was since removed in [1], and we compared the results to
SHARED_RUNQ (previously called "swqueue") in [2]. SIS_NODE did not
outperform SHARED_RUNQ on any of the benchmarks, so we elect to not
compare against it again for this v2 patch set.
[0]: https://lore.kernel.org/all/20230530113249.GA156198@hirez.programming.kicks-ass.net/
[1]: https://lore.kernel.org/all/20230605175636.GA4253@hirez.programming.kicks-ass.net/
[2]: https://lore.kernel.org/lkml/20230613052004.2836135-1-void@manifault.com/
Worth noting as well is that pointed out in [3] that the logic behind
including SIS_NODE in the first place should apply to SHARED_RUNQ
(meaning that e.g. very small Zen2 CPUs with only 3/4 cores per LLC
should benefit from having a single shared_runq stretch across multiple
LLCs). I drafted a patch that implements this by having a minimum LLC
size for creating a shard, and stretches a shared_runq across multiple
LLCs if they're smaller than that size, and sent it to Tejun to test on
his Zen2. Tejun reported back that SIS_NODE did not seem to make a
difference:
[3]: https://lore.kernel.org/lkml/20230711114207.GK3062772@hirez.programming.kicks-ass.net/
o____________o__________o
| mean | Variance |
o------------o----------o
Vanilla: | 108.84s | 0.0057 |
NO_SHARED_RUNQ: | 108.82s | 0.119s |
SHARED_RUNQ: | 108.17s | 0.038s |
SHARED_RUNQ w/ SIS_NODE: | 108.87s | 0.111s |
o------------o----------o
I similarly tried running kcompile on SHARED_RUNQ with SIS_NODE on my
7950X Zen3, but didn't see any gain relative to plain SHARED_RUNQ (though
a gain was observed relative to NO_SHARED_RUNQ, as described below).
Results
=======
Note that the motivation for the shared runqueue feature was originally
arrived at using experiments in the sched_ext framework that's currently
being proposed upstream. The ~1 - 1.6% improvement in HHVM throughput
is similarly visible using work-conserving sched_ext schedulers (even
very simple ones like global FIFO).
In both single and multi socket / CCX hosts, this can measurably improve
performance. In addition to the performance gains observed on our
internal web workloads, we also observed an improvement in common
workloads such as kernel compile and hackbench, when running shared
runqueue.
On the other hand, some workloads suffer from SHARED_RUNQ. Workloads
that hammer the runqueue hard, such as netperf UDP_RR, or schbench -L
-m 52 -p 512 -r 10 -t 1. This can be mitigated somewhat by sharding the
shared datastructures within a CCX, but it doesn't seem to eliminate all
contention in every scenario. On the positive side, it seems that
sharding does not materially harm the benchmarks run for this patch
series; and in fact seems to improve some workloads such as kernel
compile.
Note that for the kernel compile workloads below, the compilation was
done by running make -j$(nproc) built-in.a on several different types of
hosts configured with make allyesconfig on commit a27648c74210 ("afs:
Fix setting of mtime when creating a file/dir/symlink") on Linus' tree
(boost and turbo were disabled on all of these hosts when the
experiments were performed).
Finally, note that these results were from the patch set built off of
commit ebb83d84e49b ("sched/core: Avoid multiple calling
update_rq_clock() in __cfsb_csd_unthrottle()") on the sched/core branch
of tip.git for easy comparison with the v2 patch set results. The
patches in their final form from this set were rebased onto commit
88c56cfeaec4 ("sched/fair: Block nohz tick_stop when cfs bandwidth in
use") on the sched/core branch of tip.git.
=== Single-socket | 16 core / 32 thread | 2-CCX | AMD 7950X Zen4 ===
CPU max MHz: 5879.8818
CPU min MHz: 3000.0000
Command: make -j$(nproc) built-in.a
o____________o__________o
| mean | Variance |
o------------o----------o
NO_SHARED_RUNQ: | 581.95s | 2.639s |
SHARED_RUNQ: | 577.02s | 0.084s |
o------------o----------o
Takeaway: SHARED_RUNQ results in a statistically significant ~.85%
improvement over NO_SHARED_RUNQ. This suggests that enqueuing tasks in
the shared runqueue on every enqueue improves work conservation, and
thanks to sharding, does not result in contention.
Command: hackbench --loops 10000
o____________o__________o
| mean | Variance |
o------------o----------o
NO_SHARED_RUNQ: | 2.2492s | .00001s |
SHARED_RUNQ: | 2.0217s | .00065s |
o------------o----------o
Takeaway: SHARED_RUNQ in both forms performs exceptionally well compared
to NO_SHARED_RUNQ here, beating it by over 10%. This was a surprising
result given that it seems advantageous to err on the side of avoiding
migration in hackbench given that tasks are short lived in sending only
10k bytes worth of messages, but the results of the benchmark would seem
to suggest that minimizing runqueue delays is preferable.
Command:
for i in `seq 128`; do
netperf -6 -t UDP_RR -c -C -l $runtime &
done
o_______________________o
| Throughput | Variance |
o-----------------------o
NO_SHARED_RUNQ: | 25037.45 | 2243.44 |
SHARED_RUNQ: | 24952.50 | 1268.06 |
o-----------------------o
Takeaway: No statistical significance, though it is worth noting that
there is no regression for shared runqueue on the 7950X, while there is
a small regression on the Skylake and Milan hosts for SHARED_RUNQ as
described below.
=== Single-socket | 18 core / 36 thread | 1-CCX | Intel Skylake ===
CPU max MHz: 1601.0000
CPU min MHz: 800.0000
Command: make -j$(nproc) built-in.a
o____________o__________o
| mean | Variance |
o------------o----------o
NO_SHARED_RUNQ: | 1517.44s | 2.8322s |
SHARED_RUNQ: | 1516.51s | 2.9450s |
o------------o----------o
Takeaway: There's on statistically significant gain here. I observed
what I claimed was a .23% win in v2, but it appears that this is not
actually statistically significant.
Command: hackbench --loops 10000
o____________o__________o
| mean | Variance |
o------------o----------o
NO_SHARED_RUNQ: | 5.3370s | .0012s |
SHARED_RUNQ: | 5.2668s | .0033s |
o------------o----------o
Takeaway: SHARED_RUNQ results in a ~1.3% improvement over
NO_SHARED_RUNQ. Also statistically significant, but smaller than the
10+% improvement observed on the 7950X.
Command: netperf -n $(nproc) -l 60 -t TCP_RR
for i in `seq 128`; do
netperf -6 -t UDP_RR -c -C -l $runtime &
done
o_______________________o
| Throughput | Variance |
o-----------------------o
NO_SHARED_RUNQ: | 15699.32 | 377.01 |
SHARED_RUNQ: | 14966.42 | 714.13 |
o-----------------------o
Takeaway: NO_SHARED_RUNQ beats SHARED_RUNQ by ~4.6%. This result makes
sense -- the workload is very heavy on the runqueue, so enqueuing tasks
in the shared runqueue in __enqueue_entity() would intuitively result in
increased contention on the shard lock.
=== Single-socket | 72-core | 6-CCX | AMD Milan Zen3 ===
CPU max MHz: 700.0000
CPU min MHz: 700.0000
Command: make -j$(nproc) built-in.a
o____________o__________o
| mean | Variance |
o------------o----------o
NO_SHARED_RUNQ: | 1568.55s | 0.1568s |
SHARED_RUNQ: | 1568.26s | 1.2168s |
o------------o----------o
Takeaway: No statistically significant difference here. It might be
worth experimenting with work stealing in a follow-on patch set.
Command: hackbench --loops 10000
o____________o__________o
| mean | Variance |
o------------o----------o
NO_SHARED_RUNQ: | 5.2716s | .00143s |
SHARED_RUNQ: | 5.1716s | .00289s |
o------------o----------o
Takeaway: SHARED_RUNQ again wins, by about 2%.
Command: netperf -n $(nproc) -l 60 -t TCP_RR
for i in `seq 128`; do
netperf -6 -t UDP_RR -c -C -l $runtime &
done
o_______________________o
| Throughput | Variance |
o-----------------------o
NO_SHARED_RUNQ: | 17482.03 | 4675.99 |
SHARED_RUNQ: | 16697.25 | 9812.23 |
o-----------------------o
Takeaway: Similar to the Skylake runs, NO_SHARED_RUNQ still beats
SHARED_RUNQ, this time by ~4.5%. It's worth noting that in v2, the
NO_SHARED_RUNQ was only ~1.8% faster. The variance is very high here, so
the results of this benchmark should be taken with a large grain of
salt (noting that we do consistently see NO_SHARED_RUNQ on top due to
not contending on the shard lock).
Finally, let's look at how sharding affects the following schbench
incantation suggested by Chris in [4]:
schbench -L -m 52 -p 512 -r 10 -t 1
[4]: https://lore.kernel.org/lkml/c8419d9b-2b31-2190-3058-3625bdbcb13d@meta.com/
The TL;DR is that sharding improves things a lot, but doesn't completely
fix the problem. Here are the results from running the schbench command
on the 18 core / 36 thread single CCX, single-socket Skylake:
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
&shard->lock: 31510503 31510711 0.08 19.98 168932319.64 5.36 31700383 31843851 0.03 17.50 10273968.33 0.32
------------
&shard->lock 15731657 [<0000000068c0fd75>] pick_next_task_fair+0x4dd/0x510
&shard->lock 15756516 [<000000001faf84f9>] enqueue_task_fair+0x459/0x530
&shard->lock 21766 [<00000000126ec6ab>] newidle_balance+0x45a/0x650
&shard->lock 772 [<000000002886c365>] dequeue_task_fair+0x4c9/0x540
------------
&shard->lock 23458 [<00000000126ec6ab>] newidle_balance+0x45a/0x650
&shard->lock 16505108 [<000000001faf84f9>] enqueue_task_fair+0x459/0x530
&shard->lock 14981310 [<0000000068c0fd75>] pick_next_task_fair+0x4dd/0x510
&shard->lock 835 [<000000002886c365>] dequeue_task_fair+0x4c9/0x540
These results are when we create only 3 shards (16 logical cores per
shard), so the contention may be a result of overly-coarse sharding. If
we run the schbench incantation with no sharding whatsoever, we see the
following significantly worse lock stats contention:
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
&shard->lock: 117868635 118361486 0.09 393.01 1250954097.25 10.57 119345882 119780601 0.05 343.35 38313419.51 0.32
------------
&shard->lock 59169196 [<0000000060507011>] __enqueue_entity+0xdc/0x110
&shard->lock 59084239 [<00000000f1c67316>] __dequeue_entity+0x78/0xa0
&shard->lock 108051 [<00000000084a6193>] newidle_balance+0x45a/0x650
------------
&shard->lock 60028355 [<0000000060507011>] __enqueue_entity+0xdc/0x110
&shard->lock 119882 [<00000000084a6193>] newidle_balance+0x45a/0x650
&shard->lock 58213249 [<00000000f1c67316>] __dequeue_entity+0x78/0xa0
The contention is ~3-4x worse if we don't shard at all. This roughly
matches the fact that we had 3 shards on the first workload run above.
If we make the shards even smaller, the contention is comparably much
lower:
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
&shard->lock: 13839849 13877596 0.08 13.23 5389564.95 0.39 46910241 48069307 0.06 16.40 16534469.35 0.34
------------
&shard->lock 3559 [<00000000ea455dcc>] newidle_balance+0x45a/0x650
&shard->lock 6992418 [<000000002266f400>] __dequeue_entity+0x78/0xa0
&shard->lock 6881619 [<000000002a62f2e0>] __enqueue_entity+0xdc/0x110
------------
&shard->lock 6640140 [<000000002266f400>] __dequeue_entity+0x78/0xa0
&shard->lock 3523 [<00000000ea455dcc>] newidle_balance+0x45a/0x650
&shard->lock 7233933 [<000000002a62f2e0>] __enqueue_entity+0xdc/0x110
Interestingly, SHARED_RUNQ performs worse than NO_SHARED_RUNQ on the schbench
benchmark on Milan as well, but we contend more on the rq lock than the
shard lock:
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
&rq->__lock: 9617614 9656091 0.10 79.64 69665812.00 7.21 18092700 67652829 0.11 82.38 344524858.87 5.09
-----------
&rq->__lock 6301611 [<000000003e63bf26>] task_rq_lock+0x43/0xe0
&rq->__lock 2530807 [<00000000516703f0>] __schedule+0x72/0xaa0
&rq->__lock 109360 [<0000000011be1562>] raw_spin_rq_lock_nested+0xa/0x10
&rq->__lock 178218 [<00000000c38a30f9>] sched_ttwu_pending+0x3d/0x170
-----------
&rq->__lock 3245506 [<00000000516703f0>] __schedule+0x72/0xaa0
&rq->__lock 1294355 [<00000000c38a30f9>] sched_ttwu_pending+0x3d/0x170
&rq->__lock 2837804 [<000000003e63bf26>] task_rq_lock+0x43/0xe0
&rq->__lock 1627866 [<0000000011be1562>] raw_spin_rq_lock_nested+0xa/0x10
..................................................................................................................................................................................................
&shard->lock: 7338558 7343244 0.10 35.97 7173949.14 0.98 30200858 32679623 0.08 35.59 16270584.52 0.50
------------
&shard->lock 2004142 [<00000000f8aa2c91>] __dequeue_entity+0x78/0xa0
&shard->lock 2611264 [<00000000473978cc>] newidle_balance+0x45a/0x650
&shard->lock 2727838 [<0000000028f55bb5>] __enqueue_entity+0xdc/0x110
------------
&shard->lock 2737232 [<00000000473978cc>] newidle_balance+0x45a/0x650
&shard->lock 1693341 [<00000000f8aa2c91>] __dequeue_entity+0x78/0xa0
&shard->lock 2912671 [<0000000028f55bb5>] __enqueue_entity+0xdc/0x110
...................................................................................................................................................................................................
If we look at the lock stats with SHARED_RUNQ disabled, the rq lock still
contends the most, but it's significantly less than with it enabled:
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
&rq->__lock: 791277 791690 0.12 110.54 4889787.63 6.18 1575996 62390275 0.13 112.66 316262440.56 5.07
-----------
&rq->__lock 263343 [<00000000516703f0>] __schedule+0x72/0xaa0
&rq->__lock 19394 [<0000000011be1562>] raw_spin_rq_lock_nested+0xa/0x10
&rq->__lock 4143 [<000000003b542e83>] __task_rq_lock+0x51/0xf0
&rq->__lock 51094 [<00000000c38a30f9>] sched_ttwu_pending+0x3d/0x170
-----------
&rq->__lock 23756 [<0000000011be1562>] raw_spin_rq_lock_nested+0xa/0x10
&rq->__lock 379048 [<00000000516703f0>] __schedule+0x72/0xaa0
&rq->__lock 677 [<000000003b542e83>] __task_rq_lock+0x51/0xf0
Worth noting is that increasing the granularity of the shards in general
improves very runqueue-heavy workloads such as netperf UDP_RR and this
schbench command, but it doesn't necessarily make a big difference for
every workload, or for sufficiently small CCXs such as the 7950X. It may
make sense to eventually allow users to control this with a debugfs
knob, but for now we'll elect to choose a default that resulted in good
performance for the benchmarks run for this patch series.
Conclusion
==========
SHARED_RUNQ in this form provides statistically significant wins for
several types of workloads, and various CPU topologies. The reason for
this is roughly the same for all workloads: SHARED_RUNQ encourages work
conservation inside of a CCX by having a CPU do an O(# per-LLC shards)
iteration over the shared_runq shards in an LLC. We could similarly do
an O(n) iteration over all of the runqueues in the current LLC when a
core is going idle, but that's quite costly (especially for larger
LLCs), and sharded SHARED_RUNQ seems to provide a performant middle
ground between doing an O(n) walk, and doing an O(1) pull from a single
per-LLC shared runq.
For the workloads above, kernel compile and hackbench were clear winners
for SHARED_RUNQ (especially in __enqueue_entity()). The reason for the
improvement in kernel compile is of course that we have a heavily
CPU-bound workload where cache locality doesn't mean much; getting a CPU
is the #1 goal. As mentioned above, while I didn't expect to see an
improvement in hackbench, the results of the benchmark suggest that
minimizing runqueue delays is preferable to optimizing for L1/L2
locality.
Not all workloads benefit from SHARED_RUNQ, however. Workloads that
hammer the runqueue hard, such as netperf UDP_RR, or schbench -L -m 52
-p 512 -r 10 -t 1, tend to run into contention on the shard locks;
especially when enqueuing tasks in __enqueue_entity(). This can be
mitigated significantly by sharding the shared datastructures within a
CCX, but it doesn't eliminate all contention, as described above.
Worth noting as well is that Gautham Shenoy ran some interesting
experiments on a few more ideas in [5], such as walking the shared_runq
on the pop path until a task is found that can be migrated to the
calling CPU. I didn't run those experiments in this patch set, but it
might be worth doing so.
[5]: https://lore.kernel.org/lkml/ZJkqeXkPJMTl49GB@BLR-5CG11610CF.amd.com/
Gautham also ran some other benchmarks in [6], which we may want to
again try on this v3, but with boost disabled.
[6]: https://lore.kernel.org/lkml/ZLpMGVPDXqWEu+gm@BLR-5CG11610CF.amd.com/
Finally, while SHARED_RUNQ in this form encourages work conservation, it
of course does not guarantee it given that we don't implement any kind
of work stealing between shared_runq's. In the future, we could
potentially push CPU utilization even higher by enabling work stealing
between shared_runq's, likely between CCXs on the same NUMA node.
Originally-by: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: David Vernet <void@manifault.com>
David Vernet (7):
sched: Expose move_queued_task() from core.c
sched: Move is_cpu_allowed() into sched.h
sched: Check cpu_active() earlier in newidle_balance()
sched: Enable sched_feat callbacks on enable/disable
sched/fair: Add SHARED_RUNQ sched feature and skeleton calls
sched: Implement shared runqueue in CFS
sched: Shard per-LLC shared runqueues
include/linux/sched.h | 2 +
kernel/sched/core.c | 52 ++----
kernel/sched/debug.c | 18 ++-
kernel/sched/fair.c | 340 +++++++++++++++++++++++++++++++++++++++-
kernel/sched/features.h | 1 +
kernel/sched/sched.h | 56 ++++++-
kernel/sched/topology.c | 4 +-
7 files changed, 420 insertions(+), 53 deletions(-)
--
2.41.0
On Wed, Aug 09, 2023 at 05:12:11PM -0500, David Vernet wrote:
> Changes
> -------
>
> This is v3 of the shared runqueue patchset. This patch set is based off
> of commit 88c56cfeaec4 ("sched/fair: Block nohz tick_stop when cfs
> bandwidth in use") on the sched/core branch of tip.git.
>
> v1 (RFC): https://lore.kernel.org/lkml/20230613052004.2836135-1-void@manifault.com/
> v2: https://lore.kernel.org/lkml/20230710200342.358255-1-void@manifault.com/
Hello everyone,
I wanted to give an update on this, as I've been promising a v4 of the
patch set for quite some time. I ran more experiments over the last few
weeks, and EEVDF has changed the performance profile of the SHARED_RUNQ
feature quite a bit compared to what we were observing with CFS. We may
pick this back up again at a later point, but for now we're going to
take a step back and re-evaluate.
Thanks,
David
On Wed, 2023-08-09 at 17:12 -0500, David Vernet wrote:
Hi David,
I have been benchmarking the patch-set on POWER9 machine to understand
its impact. However, I've run into a recurring hard-lockups in
newidle_balance, specifically when SHARED_RUNQ feature is enabled. It
doesn't happen all the time, but it's something worth noting. I wanted
to inform you about this, and I can provide more details if needed.
-----------------------------------------
Some inital information regarding the hard-lockup:
Base Kernel:
-----------
Base kernel is upto commit 88c56cfeaec4 ("sched/fair: Block nohz
tick_stop when cfs bandwidth in use").
Patched Kernel:
-------------
Base Kernel + v3 (shared runqueue patch-set)(
https://lore.kernel.org/all/20230809221218.163894-1-void@manifault.com/
)
The hard-lockup moslty occurs when running the Apache2 benchmarks with
ab (Apache HTTP benchmarking tool) on the patched kernel. However, this
problem is not exclusive to the mentioned benchmark and only occurs
while the SHARED_RUNQ feature is enabled. Disabling SHARED_RUNQ feature
prevents the occurrence of the lockup.
ab (Apache HTTP benchmarking tool):
https://httpd.apache.org/docs/2.4/programs/ab.html
Hardlockup with Patched Kernel:
------------------------------
[ 3289.727912][ C123] rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
[ 3289.727943][ C123] rcu: 124-...0: (1 GPs behind) idle=f174/1/0x4000000000000000 softirq=12283/12289 fqs=732
[ 3289.727976][ C123] rcu: (detected by 123, t=2103 jiffies, g=127061, q=5517 ncpus=128)
[ 3289.728008][ C123] Sending NMI from CPU 123 to CPUs 124:
[ 3295.182378][ C123] CPU 124 didn't respond to backtrace IPI, inspecting paca.
[ 3295.182403][ C123] irq_soft_mask: 0x01 in_mce: 0 in_nmi: 0 current: 15 (ksoftirqd/124)
[ 3295.182421][ C123] Back trace of paca->saved_r1 (0xc000000de13e79b0) (possibly stale):
[ 3295.182437][ C123] Call Trace:
[ 3295.182456][ C123] [c000000de13e79b0] [c000000de13e7a70] 0xc000000de13e7a70 (unreliable)
[ 3295.182477][ C123] [c000000de13e7ac0] [0000000000000008] 0x8
[ 3295.182500][ C123] [c000000de13e7b70] [c000000de13e7c98] 0xc000000de13e7c98
[ 3295.182519][ C123] [c000000de13e7ba0] [c0000000001da8bc] move_queued_task+0x14c/0x280
[ 3295.182557][ C123] [c000000de13e7c30] [c0000000001f22d8] newidle_balance+0x648/0x940
[ 3295.182602][ C123] [c000000de13e7d30] [c0000000001f26ac] pick_next_task_fair+0x7c/0x680
[ 3295.182647][ C123] [c000000de13e7dd0] [c0000000010f175c] __schedule+0x15c/0x1040
[ 3295.182675][ C123] [c000000de13e7ec0] [c0000000010f26b4] schedule+0x74/0x140
[ 3295.182694][ C123] [c000000de13e7f30] [c0000000001c4994] smpboot_thread_fn+0x244/0x250
[ 3295.182731][ C123] [c000000de13e7f90] [c0000000001bc6e8] kthread+0x138/0x140
[ 3295.182769][ C123] [c000000de13e7fe0] [c00000000000ded8] start_kernel_thread+0x14/0x18
[ 3295.182806][ C123] rcu: rcu_sched kthread starved for 544 jiffies! g127061 f0x0 RCU_GP_DOING_FQS(6) ->state=0x0 ->cpu=66
[ 3295.182845][ C123] rcu: Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior.
[ 3295.182878][ C123] rcu: RCU grace-period kthread stack dump:
-----------------------------------------
[ 3943.438625][ C112] watchdog: CPU 112 self-detected hard LOCKUP @ _raw_spin_lock_irqsave+0x4c/0xc0
[ 3943.438631][ C112] watchdog: CPU 112 TB:115060212303626, last heartbeat TB:115054309631589 (11528ms ago)
[ 3943.438673][ C112] CPU: 112 PID: 2090 Comm: kworker/112:2 Tainted: G W L 6.5.0-rc2-00028-g7475adccd76b #51
[ 3943.438676][ C112] Hardware name: 8335-GTW POWER9 (raw) 0x4e1203 opal:skiboot-v6.5.3-35-g1851b2a06 PowerNV
[ 3943.438678][ C112] Workqueue: 0x0 (events)
[ 3943.438682][ C112] NIP: c0000000010ff01c LR: c0000000001d1064 CTR: c0000000001e8580
[ 3943.438684][ C112] REGS: c000007fffb6bd60 TRAP: 0900 Tainted: G W L (6.5.0-rc2-00028-g7475adccd76b)
[ 3943.438686][ C112] MSR: 9000000000009033 <SF,HV,EE,ME,IR,DR,RI,LE> CR: 24082222 XER: 00000000
[ 3943.438693][ C112] CFAR: 0000000000000000 IRQMASK: 1
[ 3943.438693][ C112] GPR00: c0000000001d1064 c000000e16d1fb20 c0000000014e8200 c000000e092fed3c
[ 3943.438693][ C112] GPR04: c000000e16d1fc58 c000000e092fe3c8 00000000000000e1 fffffffffffe0000
[ 3943.438693][ C112] GPR08: 0000000000000000 00000000000000e1 0000000000000000 c00000000299ccd8
[ 3943.438693][ C112] GPR12: 0000000024088222 c000007ffffb8300 c0000000001bc5b8 c000000deb46f740
[ 3943.438693][ C112] GPR16: 0000000000000008 c000000e092fe280 0000000000000001 c000007ffedd7b00
[ 3943.438693][ C112] GPR20: 0000000000000001 c0000000029a1280 0000000000000000 0000000000000001
[ 3943.438693][ C112] GPR24: 0000000000000000 c000000e092fed3c c000000e16d1fdf0 c00000000299ccd8
[ 3943.438693][ C112] GPR28: c000000e16d1fc58 c0000000021fbf00 c000007ffee6bf00 0000000000000001
[ 3943.438722][ C112] NIP [c0000000010ff01c] _raw_spin_lock_irqsave+0x4c/0xc0
[ 3943.438725][ C112] LR [c0000000001d1064] task_rq_lock+0x64/0x1b0
[ 3943.438727][ C112] Call Trace:
[ 3943.438728][ C112] [c000000e16d1fb20] [c000000e16d1fb60] 0xc000000e16d1fb60 (unreliable)
[ 3943.438731][ C112] [c000000e16d1fb50] [c000000e16d1fbf0] 0xc000000e16d1fbf0
[ 3943.438733][ C112] [c000000e16d1fbf0] [c0000000001f214c] newidle_balance+0x4bc/0x940
[ 3943.438737][ C112] [c000000e16d1fcf0] [c0000000001f26ac] pick_next_task_fair+0x7c/0x680
[ 3943.438739][ C112] [c000000e16d1fd90] [c0000000010f175c] __schedule+0x15c/0x1040
[ 3943.438743][ C112] [c000000e16d1fe80] [c0000000010f26b4] schedule+0x74/0x140
[ 3943.438747][ C112] [c000000e16d1fef0] [c0000000001afd44] worker_thread+0x134/0x580
[ 3943.438749][ C112] [c000000e16d1ff90] [c0000000001bc6e8] kthread+0x138/0x140
[ 3943.438753][ C112] [c000000e16d1ffe0] [c00000000000ded8] start_kernel_thread+0x14/0x18
[ 3943.438756][ C112] Code: 63e90001 992d0932 a12d0008 3ce0fffe 5529083c 61290001 7d001
-----------------------------------------
System configuration:
--------------------
# lscpu
Architecture: ppc64le
Byte Order: Little Endian
CPU(s): 128
On-line CPU(s) list: 0-127
Thread(s) per core: 4
Core(s) per socket: 16
Socket(s): 2
NUMA node(s): 8
Model: 2.3 (pvr 004e 1203)
Model name: POWER9 (raw), altivec supported
Frequency boost: enabled
CPU max MHz: 3800.0000
CPU min MHz: 2300.0000
L1d cache: 1 MiB
L1i cache: 1 MiB
NUMA node0 CPU(s): 64-127
NUMA node8 CPU(s): 0-63
NUMA node250 CPU(s):
NUMA node251 CPU(s):
NUMA node252 CPU(s):
NUMA node253 CPU(s):
NUMA node254 CPU(s):
NUMA node255 CPU(s):
# uname -r
6.5.0-rc2-00028-g7475adccd76b
# cat /sys/kernel/debug/sched/features
GENTLE_FAIR_SLEEPERS START_DEBIT NO_NEXT_BUDDY LAST_BUDDY
CACHE_HOT_BUDDY WAKEUP_PREEMPTION NO_HRTICK NO_HRTICK_DL NO_DOUBLE_TICK
NONTASK_CAPACITY TTWU_QUEUE NO_SIS_PROP SIS_UTIL NO_WARN_DOUBLE_CLOCK
RT_PUSH_IPI NO_RT_RUNTIME_SHARE NO_LB_MIN ATTACH_AGE_LOAD WA_IDLE
WA_WEIGHT WA_BIAS UTIL_EST UTIL_EST_FASTUP NO_LATENCY_WARN ALT_PERIOD
BASE_SLICE HZ_BW SHARED_RUNQ
-----------------------------------------
Please let me know if I've missed anything here. I'll continue
investigating and share any additional information I find.
Thanks and Regards,
Aboorva
> Changes
> -------
>
> This is v3 of the shared runqueue patchset. This patch set is based
> off
> of commit 88c56cfeaec4 ("sched/fair: Block nohz tick_stop when cfs
> bandwidth in use") on the sched/core branch of tip.git.
>
> v1 (RFC):
> https://lore.kernel.org/lkml/20230613052004.2836135-1-void@manifault.com/
> v2:
> https://lore.kernel.org/lkml/20230710200342.358255-1-void@manifault.com/
>
> v2 -> v3 changes:
> - Don't leave stale tasks in the lists when the SHARED_RUNQ feature
> is
> disabled (Abel Wu)
>
> - Use raw spin lock instead of spinlock_t (Peter)
>
> - Fix return value from shared_runq_pick_next_task() to match the
> semantics expected by newidle_balance() (Gautham, Abel)
>
> - Fold patch __enqueue_entity() / __dequeue_entity() into previous
> patch
> (Peter)
>
> - Skip <= LLC domains in newidle_balance() if SHARED_RUNQ is enabled
> (Peter)
>
> - Properly support hotplug and recreating sched domains (Peter)
>
> - Avoid unnecessary task_rq_unlock() + raw_spin_rq_lock() when src_rq
> ==
> target_rq in shared_runq_pick_next_task() (Abel)
>
> - Only issue list_del_init() in shared_runq_dequeue_task() if the
> task
> is still in the list after acquiring the lock (Aaron Lu)
>
> - Slightly change shared_runq_shard_idx() to make it more likely to
> keep
> SMT siblings on the same bucket (Peter)
>
> v1 -> v2 changes:
> - Change name from swqueue to shared_runq (Peter)
>
> - Shard per-LLC shared runqueues to avoid contention on scheduler-
> heavy
> workloads (Peter)
>
> - Pull tasks from the shared_runq in newidle_balance() rather than in
> pick_next_task_fair() (Peter and Vincent)
>
> - Rename a few functions to reflect their actual purpose. For
> example,
> shared_runq_dequeue_task() instead of swqueue_remove_task() (Peter)
>
> - Expose move_queued_task() from core.c rather than migrate_task_to()
> (Peter)
>
> - Properly check is_cpu_allowed() when pulling a task from a
> shared_runq
> to ensure it can actually be migrated (Peter and Gautham)
>
> - Dropped RFC tag
>
> Overview
> ========
>
> The scheduler must constantly strike a balance between work
> conservation, and avoiding costly migrations which harm performance
> due
> to e.g. decreased cache locality. The matter is further complicated
> by
> the topology of the system. Migrating a task between cores on the
> same
> LLC may be more optimal than keeping a task local to the CPU, whereas
> migrating a task between LLCs or NUMA nodes may tip the balance in
> the
> other direction.
>
> With that in mind, while CFS is by and large mostly a work conserving
> scheduler, there are certain instances where the scheduler will
> choose
> to keep a task local to a CPU, when it would have been more optimal
> to
> migrate it to an idle core.
>
> An example of such a workload is the HHVM / web workload at Meta.
> HHVM
> is a VM that JITs Hack and PHP code in service of web requests. Like
> other JIT / compilation workloads, it tends to be heavily CPU bound,
> and
> exhibit generally poor cache locality. To try and address this, we
> set
> several debugfs (/sys/kernel/debug/sched) knobs on our HHVM
> workloads:
>
> - migration_cost_ns -> 0
> - latency_ns -> 20000000
> - min_granularity_ns -> 10000000
> - wakeup_granularity_ns -> 12000000
>
> These knobs are intended both to encourage the scheduler to be as
> work
> conserving as possible (migration_cost_ns -> 0), and also to keep
> tasks
> running for relatively long time slices so as to avoid the overhead
> of
> context switching (the other knobs). Collectively, these knobs
> provide a
> substantial performance win; resulting in roughly a 20% improvement
> in
> throughput. Worth noting, however, is that this improvement is _not_
> at
> full machine saturation.
>
> That said, even with these knobs, we noticed that CPUs were still
> going
> idle even when the host was overcommitted. In response, we wrote the
> "shared runqueue" (SHARED_RUNQ) feature proposed in this patch set.
> The
> idea behind SHARED_RUNQ is simple: it enables the scheduler to be
> more
> aggressively work conserving by placing a waking task into a sharded
> per-LLC FIFO queue that can be pulled from by another core in the LLC
> FIFO queue which can then be pulled from before it goes idle.
>
> With this simple change, we were able to achieve a 1 - 1.6%
> improvement
> in throughput, as well as a small, consistent improvement in p95 and
> p99
> latencies, in HHVM. These performance improvements were in addition
> to
> the wins from the debugfs knobs mentioned above, and to other
> benchmarks
> outlined below in the Results section.
>
> Design
> ======
>
> Note that the design described here reflects sharding, which is the
> implementation added in the final patch of the series (following the
> initial unsharded implementation added in patch 6/7). The design is
> described that way in this commit summary as the benchmarks described
> in
> the results section below all reflect a sharded SHARED_RUNQ.
>
> The design of SHARED_RUNQ is quite simple. A shared_runq is simply a
> list of struct shared_runq_shard objects, which itself is simply a
> struct list_head of tasks, and a spinlock:
>
> struct shared_runq_shard {
> struct list_head list;
> raw_spinlock_t lock;
> } ____cacheline_aligned;
>
> struct shared_runq {
> u32 num_shards;
> struct shared_runq_shard shards[];
> } ____cacheline_aligned;
>
> We create a struct shared_runq per LLC, ensuring they're in their own
> cachelines to avoid false sharing between CPUs on different LLCs, and
> we
> create a number of struct shared_runq_shard objects that are housed
> there.
>
> When a task first wakes up, it enqueues itself in the
> shared_runq_shard
> of its current LLC at the end of enqueue_task_fair(). Enqueues only
> happen if the task was not manually migrated to the current core by
> select_task_rq(), and is not pinned to a specific CPU.
>
> A core will pull a task from the shards in its LLC's shared_runq at
> the
> beginning of newidle_balance().
>
> Difference between SHARED_RUNQ and SIS_NODE
> ===========================================
>
> In [0] Peter proposed a patch that addresses Tejun's observations
> that
> when workqueues are targeted towards a specific LLC on his Zen2
> machine
> with small CCXs, that there would be significant idle time due to
> select_idle_sibling() not considering anything outside of the current
> LLC.
>
> This patch (SIS_NODE) is essentially the complement to the proposal
> here. SID_NODE causes waking tasks to look for idle cores in
> neighboring
> LLCs on the same die, whereas SHARED_RUNQ causes cores about to go
> idle
> to look for enqueued tasks. That said, in its current form, the two
> features at are a different scope as SIS_NODE searches for idle cores
> between LLCs, while SHARED_RUNQ enqueues tasks within a single LLC.
>
> The patch was since removed in [1], and we compared the results to
> SHARED_RUNQ (previously called "swqueue") in [2]. SIS_NODE did not
> outperform SHARED_RUNQ on any of the benchmarks, so we elect to not
> compare against it again for this v2 patch set.
>
> [0]:
> https://lore.kernel.org/all/20230530113249.GA156198@hirez.programming.kicks-ass.net/
> [1]:
> https://lore.kernel.org/all/20230605175636.GA4253@hirez.programming.kicks-ass.net/
> [2]:
> https://lore.kernel.org/lkml/20230613052004.2836135-1-void@manifault.com/
>
> Worth noting as well is that pointed out in [3] that the logic behind
> including SIS_NODE in the first place should apply to SHARED_RUNQ
> (meaning that e.g. very small Zen2 CPUs with only 3/4 cores per LLC
> should benefit from having a single shared_runq stretch across
> multiple
> LLCs). I drafted a patch that implements this by having a minimum LLC
> size for creating a shard, and stretches a shared_runq across
> multiple
> LLCs if they're smaller than that size, and sent it to Tejun to test
> on
> his Zen2. Tejun reported back that SIS_NODE did not seem to make a
> difference:
>
> [3]:
> https://lore.kernel.org/lkml/20230711114207.GK3062772@hirez.programming.kicks-ass.net/
>
> o____________o__________o
> | mean | Variance |
> o------------o----------o
> Vanilla: | 108.84s | 0.0057 |
> NO_SHARED_RUNQ: | 108.82s | 0.119s |
> SHARED_RUNQ: | 108.17s | 0.038s |
> SHARED_RUNQ w/ SIS_NODE: | 108.87s | 0.111s |
> o------------o----------o
>
> I similarly tried running kcompile on SHARED_RUNQ with SIS_NODE on my
> 7950X Zen3, but didn't see any gain relative to plain SHARED_RUNQ
> (though
> a gain was observed relative to NO_SHARED_RUNQ, as described below).
>
> Results
> =======
>
> Note that the motivation for the shared runqueue feature was
> originally
> arrived at using experiments in the sched_ext framework that's
> currently
> being proposed upstream. The ~1 - 1.6% improvement in HHVM throughput
> is similarly visible using work-conserving sched_ext schedulers (even
> very simple ones like global FIFO).
>
> In both single and multi socket / CCX hosts, this can measurably
> improve
> performance. In addition to the performance gains observed on our
> internal web workloads, we also observed an improvement in common
> workloads such as kernel compile and hackbench, when running shared
> runqueue.
>
> On the other hand, some workloads suffer from SHARED_RUNQ. Workloads
> that hammer the runqueue hard, such as netperf UDP_RR, or schbench -L
> -m 52 -p 512 -r 10 -t 1. This can be mitigated somewhat by sharding
> the
> shared datastructures within a CCX, but it doesn't seem to eliminate
> all
> contention in every scenario. On the positive side, it seems that
> sharding does not materially harm the benchmarks run for this patch
> series; and in fact seems to improve some workloads such as kernel
> compile.
>
> Note that for the kernel compile workloads below, the compilation was
> done by running make -j$(nproc) built-in.a on several different types
> of
> hosts configured with make allyesconfig on commit a27648c74210 ("afs:
> Fix setting of mtime when creating a file/dir/symlink") on Linus'
> tree
> (boost and turbo were disabled on all of these hosts when the
> experiments were performed).
>
> Finally, note that these results were from the patch set built off of
> commit ebb83d84e49b ("sched/core: Avoid multiple calling
> update_rq_clock() in __cfsb_csd_unthrottle()") on the sched/core
> branch
> of tip.git for easy comparison with the v2 patch set results. The
> patches in their final form from this set were rebased onto commit
> 88c56cfeaec4 ("sched/fair: Block nohz tick_stop when cfs bandwidth in
> use") on the sched/core branch of tip.git.
>
> === Single-socket | 16 core / 32 thread | 2-CCX | AMD 7950X Zen4 ===
>
> CPU max MHz: 5879.8818
> CPU min MHz: 3000.0000
>
> Command: make -j$(nproc) built-in.a
> o____________o__________o
> | mean | Variance |
> o------------o----------o
> NO_SHARED_RUNQ: | 581.95s | 2.639s |
> SHARED_RUNQ: | 577.02s | 0.084s |
> o------------o----------o
>
> Takeaway: SHARED_RUNQ results in a statistically significant ~.85%
> improvement over NO_SHARED_RUNQ. This suggests that enqueuing tasks
> in
> the shared runqueue on every enqueue improves work conservation, and
> thanks to sharding, does not result in contention.
>
> Command: hackbench --loops 10000
> o____________o__________o
> | mean | Variance |
> o------------o----------o
> NO_SHARED_RUNQ: | 2.2492s | .00001s |
> SHARED_RUNQ: | 2.0217s | .00065s |
> o------------o----------o
>
> Takeaway: SHARED_RUNQ in both forms performs exceptionally well
> compared
> to NO_SHARED_RUNQ here, beating it by over 10%. This was a surprising
> result given that it seems advantageous to err on the side of
> avoiding
> migration in hackbench given that tasks are short lived in sending
> only
> 10k bytes worth of messages, but the results of the benchmark would
> seem
> to suggest that minimizing runqueue delays is preferable.
>
> Command:
> for i in `seq 128`; do
> netperf -6 -t UDP_RR -c -C -l $runtime &
> done
> o_______________________o
> | Throughput | Variance |
> o-----------------------o
> NO_SHARED_RUNQ: | 25037.45 | 2243.44 |
> SHARED_RUNQ: | 24952.50 | 1268.06 |
> o-----------------------o
>
> Takeaway: No statistical significance, though it is worth noting that
> there is no regression for shared runqueue on the 7950X, while there
> is
> a small regression on the Skylake and Milan hosts for SHARED_RUNQ as
> described below.
>
> === Single-socket | 18 core / 36 thread | 1-CCX | Intel Skylake ===
>
> CPU max MHz: 1601.0000
> CPU min MHz: 800.0000
>
> Command: make -j$(nproc) built-in.a
> o____________o__________o
> | mean | Variance |
> o------------o----------o
> NO_SHARED_RUNQ: | 1517.44s | 2.8322s |
> SHARED_RUNQ: | 1516.51s | 2.9450s |
> o------------o----------o
>
> Takeaway: There's on statistically significant gain here. I observed
> what I claimed was a .23% win in v2, but it appears that this is not
> actually statistically significant.
>
> Command: hackbench --loops 10000
> o____________o__________o
> | mean | Variance |
> o------------o----------o
> NO_SHARED_RUNQ: | 5.3370s | .0012s |
> SHARED_RUNQ: | 5.2668s | .0033s |
> o------------o----------o
>
> Takeaway: SHARED_RUNQ results in a ~1.3% improvement over
> NO_SHARED_RUNQ. Also statistically significant, but smaller than the
> 10+% improvement observed on the 7950X.
>
> Command: netperf -n $(nproc) -l 60 -t TCP_RR
> for i in `seq 128`; do
> netperf -6 -t UDP_RR -c -C -l $runtime &
> done
> o_______________________o
> | Throughput | Variance |
> o-----------------------o
> NO_SHARED_RUNQ: | 15699.32 | 377.01 |
> SHARED_RUNQ: | 14966.42 | 714.13 |
> o-----------------------o
>
> Takeaway: NO_SHARED_RUNQ beats SHARED_RUNQ by ~4.6%. This result
> makes
> sense -- the workload is very heavy on the runqueue, so enqueuing
> tasks
> in the shared runqueue in __enqueue_entity() would intuitively result
> in
> increased contention on the shard lock.
>
> === Single-socket | 72-core | 6-CCX | AMD Milan Zen3 ===
>
> CPU max MHz: 700.0000
> CPU min MHz: 700.0000
>
> Command: make -j$(nproc) built-in.a
> o____________o__________o
> | mean | Variance |
> o------------o----------o
> NO_SHARED_RUNQ: | 1568.55s | 0.1568s |
> SHARED_RUNQ: | 1568.26s | 1.2168s |
> o------------o----------o
>
> Takeaway: No statistically significant difference here. It might be
> worth experimenting with work stealing in a follow-on patch set.
>
> Command: hackbench --loops 10000
> o____________o__________o
> | mean | Variance |
> o------------o----------o
> NO_SHARED_RUNQ: | 5.2716s | .00143s |
> SHARED_RUNQ: | 5.1716s | .00289s |
> o------------o----------o
>
> Takeaway: SHARED_RUNQ again wins, by about 2%.
>
> Command: netperf -n $(nproc) -l 60 -t TCP_RR
> for i in `seq 128`; do
> netperf -6 -t UDP_RR -c -C -l $runtime &
> done
> o_______________________o
> | Throughput | Variance |
> o-----------------------o
> NO_SHARED_RUNQ: | 17482.03 | 4675.99 |
> SHARED_RUNQ: | 16697.25 | 9812.23 |
> o-----------------------o
>
> Takeaway: Similar to the Skylake runs, NO_SHARED_RUNQ still beats
> SHARED_RUNQ, this time by ~4.5%. It's worth noting that in v2, the
> NO_SHARED_RUNQ was only ~1.8% faster. The variance is very high here,
> so
> the results of this benchmark should be taken with a large grain of
> salt (noting that we do consistently see NO_SHARED_RUNQ on top due to
> not contending on the shard lock).
>
> Finally, let's look at how sharding affects the following schbench
> incantation suggested by Chris in [4]:
>
> schbench -L -m 52 -p 512 -r 10 -t 1
>
> [4]:
> https://lore.kernel.org/lkml/c8419d9b-2b31-2190-3058-3625bdbcb13d@meta.com/
>
> The TL;DR is that sharding improves things a lot, but doesn't
> completely
> fix the problem. Here are the results from running the schbench
> command
> on the 18 core / 36 thread single CCX, single-socket Skylake:
>
> -------------------------------------------------------------------
> -------------------------------------------------------------------
> -----------------------------------------------------------------
> class name con-bounces contentions waittime-
> min waittime-max waittime-total waittime-avg acq-
> bounces acquisitions holdtime-min holdtime-max holdtime-
> total holdtime-avg
> -------------------------------------------------------------------
> -------------------------------------------------------------------
> -----------------------------------------------------------------
>
> &shard-
> >lock: 31510503 31510711 0.08 19.98
> 168932319.64 5.36 31700383 31843851 0.0
> 3 17.50 10273968.33 0.32
> ------------
> &shard->lock 15731657 [<0000000068c0fd75>]
> pick_next_task_fair+0x4dd/0x510
> &shard->lock 15756516 [<000000001faf84f9>]
> enqueue_task_fair+0x459/0x530
> &shard->lock 21766 [<00000000126ec6ab>]
> newidle_balance+0x45a/0x650
> &shard->lock 772 [<000000002886c365>]
> dequeue_task_fair+0x4c9/0x540
> ------------
> &shard->lock 23458 [<00000000126ec6ab>]
> newidle_balance+0x45a/0x650
> &shard->lock 16505108 [<000000001faf84f9>]
> enqueue_task_fair+0x459/0x530
> &shard->lock 14981310 [<0000000068c0fd75>]
> pick_next_task_fair+0x4dd/0x510
> &shard->lock 835 [<000000002886c365>]
> dequeue_task_fair+0x4c9/0x540
>
> These results are when we create only 3 shards (16 logical cores per
> shard), so the contention may be a result of overly-coarse sharding.
> If
> we run the schbench incantation with no sharding whatsoever, we see
> the
> following significantly worse lock stats contention:
>
> -------------------------------------------------------------------
> -------------------------------------------------------------------
> -------------------------------------------------------------------
> ----
> class name con-bounces contentions waittime-
> min waittime-max waittime-total waittime-avg acq-
> bounces acquisitions holdtime-min holdtime-max holdtime-
> total holdtime-avg
> -------------------------------------------------------------------
> -------------------------------------------------------------------
> -------------------------------------------------------------------
> ----
>
> &shard-
> >lock: 117868635 118361486 0.09 393.01
> 1250954097.25 10.57 119345882 119780601
> 0.05 343.35 38313419.51 0.32
> ------------
> &shard->lock 59169196 [<0000000060507011>]
> __enqueue_entity+0xdc/0x110
> &shard->lock 59084239 [<00000000f1c67316>]
> __dequeue_entity+0x78/0xa0
> &shard->lock 108051 [<00000000084a6193>]
> newidle_balance+0x45a/0x650
> ------------
> &shard->lock 60028355 [<0000000060507011>]
> __enqueue_entity+0xdc/0x110
> &shard->lock 119882 [<00000000084a6193>]
> newidle_balance+0x45a/0x650
> &shard->lock 58213249 [<00000000f1c67316>]
> __dequeue_entity+0x78/0xa0
>
> The contention is ~3-4x worse if we don't shard at all. This roughly
> matches the fact that we had 3 shards on the first workload run
> above.
> If we make the shards even smaller, the contention is comparably much
> lower:
>
> -------------------------------------------------------------------
> -------------------------------------------------------------------
> ----------------------------------------------------------
> class name con-bounces contentions waittime-
> min waittime-max waittime-total waittime-avg acq-
> bounces acquisitions holdtime-min holdtime-max holdtime-
> total holdtime-avg
> -------------------------------------------------------------------
> -------------------------------------------------------------------
> ----------------------------------------------------------
>
> &shard-
> >lock: 13839849 13877596 0.08 13.23 5
> 389564.95 0.39 46910241 48069307 0.06
> 16.40 16534469.35 0.34
> ------------
> &shard->lock 3559 [<00000000ea455dcc>]
> newidle_balance+0x45a/0x650
> &shard->lock 6992418 [<000000002266f400>]
> __dequeue_entity+0x78/0xa0
> &shard->lock 6881619 [<000000002a62f2e0>]
> __enqueue_entity+0xdc/0x110
> ------------
> &shard->lock 6640140 [<000000002266f400>]
> __dequeue_entity+0x78/0xa0
> &shard->lock 3523 [<00000000ea455dcc>]
> newidle_balance+0x45a/0x650
> &shard->lock 7233933 [<000000002a62f2e0>]
> __enqueue_entity+0xdc/0x110
>
> Interestingly, SHARED_RUNQ performs worse than NO_SHARED_RUNQ on the
> schbench
> benchmark on Milan as well, but we contend more on the rq lock than
> the
> shard lock:
>
> -------------------------------------------------------------------
> -------------------------------------------------------------------
> -----------------------------------------------------------
> class name con-bounces contentions waittime-
> min waittime-max waittime-total waittime-avg acq-
> bounces acquisitions holdtime-min holdtime-max holdtime-
> total holdtime-avg
> -------------------------------------------------------------------
> -------------------------------------------------------------------
> -----------------------------------------------------------
>
> &rq-
> >__lock: 9617614 9656091 0.10 79.64
> 69665812.00 7.21 18092700 67652829 0.11
> 82.38 344524858.87 5.09
> -----------
> &rq->__lock 6301611 [<000000003e63bf26>]
> task_rq_lock+0x43/0xe0
> &rq->__lock 2530807 [<00000000516703f0>]
> __schedule+0x72/0xaa0
> &rq->__lock 109360 [<0000000011be1562>]
> raw_spin_rq_lock_nested+0xa/0x10
> &rq->__lock 178218 [<00000000c38a30f9>]
> sched_ttwu_pending+0x3d/0x170
> -----------
> &rq->__lock 3245506 [<00000000516703f0>]
> __schedule+0x72/0xaa0
> &rq->__lock 1294355 [<00000000c38a30f9>]
> sched_ttwu_pending+0x3d/0x170
> &rq->__lock 2837804 [<000000003e63bf26>]
> task_rq_lock+0x43/0xe0
> &rq->__lock 1627866 [<0000000011be1562>]
> raw_spin_rq_lock_nested+0xa/0x10
>
> .....................................................................
> .....................................................................
> ........................................................
>
> &shard-
> >lock: 7338558 7343244 0.10 35.97 7
> 173949.14 0.98 30200858 32679623 0.08
> 35.59 16270584.52 0.50
> ------------
> &shard->lock 2004142 [<00000000f8aa2c91>]
> __dequeue_entity+0x78/0xa0
> &shard->lock 2611264 [<00000000473978cc>]
> newidle_balance+0x45a/0x650
> &shard->lock 2727838 [<0000000028f55bb5>]
> __enqueue_entity+0xdc/0x110
> ------------
> &shard->lock 2737232 [<00000000473978cc>]
> newidle_balance+0x45a/0x650
> &shard->lock 1693341 [<00000000f8aa2c91>]
> __dequeue_entity+0x78/0xa0
> &shard->lock 2912671 [<0000000028f55bb5>]
> __enqueue_entity+0xdc/0x110
>
> .....................................................................
> .....................................................................
> .........................................................
>
> If we look at the lock stats with SHARED_RUNQ disabled, the rq lock
> still
> contends the most, but it's significantly less than with it enabled:
>
> -------------------------------------------------------------------
> -------------------------------------------------------------------
> --------------------------------------------------------------
> class name con-bounces contentions waittime-
> min waittime-max waittime-total waittime-avg acq-
> bounces acquisitions holdtime-min holdtime-max holdtime-
> total holdtime-avg
> -------------------------------------------------------------------
> -------------------------------------------------------------------
> --------------------------------------------------------------
>
> &rq-
> >__lock: 791277 791690 0.12 110.54
> 4889787.63 6.18 1575996 62390275 0.1
> 3 112.66 316262440.56 5.07
> -----------
> &rq->__lock 263343 [<00000000516703f0>]
> __schedule+0x72/0xaa0
> &rq->__lock 19394 [<0000000011be1562>]
> raw_spin_rq_lock_nested+0xa/0x10
> &rq->__lock 4143 [<000000003b542e83>]
> __task_rq_lock+0x51/0xf0
> &rq->__lock 51094 [<00000000c38a30f9>]
> sched_ttwu_pending+0x3d/0x170
> -----------
> &rq->__lock 23756 [<0000000011be1562>]
> raw_spin_rq_lock_nested+0xa/0x10
> &rq->__lock 379048 [<00000000516703f0>]
> __schedule+0x72/0xaa0
> &rq->__lock 677 [<000000003b542e83>]
> __task_rq_lock+0x51/0xf0
>
> Worth noting is that increasing the granularity of the shards in
> general
> improves very runqueue-heavy workloads such as netperf UDP_RR and
> this
> schbench command, but it doesn't necessarily make a big difference
> for
> every workload, or for sufficiently small CCXs such as the 7950X. It
> may
> make sense to eventually allow users to control this with a debugfs
> knob, but for now we'll elect to choose a default that resulted in
> good
> performance for the benchmarks run for this patch series.
>
> Conclusion
> ==========
>
> SHARED_RUNQ in this form provides statistically significant wins for
> several types of workloads, and various CPU topologies. The reason
> for
> this is roughly the same for all workloads: SHARED_RUNQ encourages
> work
> conservation inside of a CCX by having a CPU do an O(# per-LLC
> shards)
> iteration over the shared_runq shards in an LLC. We could similarly
> do
> an O(n) iteration over all of the runqueues in the current LLC when a
> core is going idle, but that's quite costly (especially for larger
> LLCs), and sharded SHARED_RUNQ seems to provide a performant middle
> ground between doing an O(n) walk, and doing an O(1) pull from a
> single
> per-LLC shared runq.
>
> For the workloads above, kernel compile and hackbench were clear
> winners
> for SHARED_RUNQ (especially in __enqueue_entity()). The reason for
> the
> improvement in kernel compile is of course that we have a heavily
> CPU-bound workload where cache locality doesn't mean much; getting a
> CPU
> is the #1 goal. As mentioned above, while I didn't expect to see an
> improvement in hackbench, the results of the benchmark suggest that
> minimizing runqueue delays is preferable to optimizing for L1/L2
> locality.
>
> Not all workloads benefit from SHARED_RUNQ, however. Workloads that
> hammer the runqueue hard, such as netperf UDP_RR, or schbench -L -m
> 52
> -p 512 -r 10 -t 1, tend to run into contention on the shard locks;
> especially when enqueuing tasks in __enqueue_entity(). This can be
> mitigated significantly by sharding the shared datastructures within
> a
> CCX, but it doesn't eliminate all contention, as described above.
>
> Worth noting as well is that Gautham Shenoy ran some interesting
> experiments on a few more ideas in [5], such as walking the
> shared_runq
> on the pop path until a task is found that can be migrated to the
> calling CPU. I didn't run those experiments in this patch set, but it
> might be worth doing so.
>
> [5]:
> https://lore.kernel.org/lkml/ZJkqeXkPJMTl49GB@BLR-5CG11610CF.amd.com/
>
> Gautham also ran some other benchmarks in [6], which we may want to
> again try on this v3, but with boost disabled.
>
> [6]:
> https://lore.kernel.org/lkml/ZLpMGVPDXqWEu+gm@BLR-5CG11610CF.amd.com/
>
> Finally, while SHARED_RUNQ in this form encourages work conservation,
> it
> of course does not guarantee it given that we don't implement any
> kind
> of work stealing between shared_runq's. In the future, we could
> potentially push CPU utilization even higher by enabling work
> stealing
> between shared_runq's, likely between CCXs on the same NUMA node.
>
> Originally-by: Roman Gushchin <roman.gushchin@linux.dev>
> Signed-off-by: David Vernet <void@manifault.com>
>
> David Vernet (7):
> sched: Expose move_queued_task() from core.c
> sched: Move is_cpu_allowed() into sched.h
> sched: Check cpu_active() earlier in newidle_balance()
> sched: Enable sched_feat callbacks on enable/disable
> sched/fair: Add SHARED_RUNQ sched feature and skeleton calls
> sched: Implement shared runqueue in CFS
> sched: Shard per-LLC shared runqueues
>
> include/linux/sched.h | 2 +
> kernel/sched/core.c | 52 ++----
> kernel/sched/debug.c | 18 ++-
> kernel/sched/fair.c | 340
> +++++++++++++++++++++++++++++++++++++++-
> kernel/sched/features.h | 1 +
> kernel/sched/sched.h | 56 ++++++-
> kernel/sched/topology.c | 4 +-
> 7 files changed, 420 insertions(+), 53 deletions(-)
>
On Mon, Nov 27, 2023 at 01:58:34PM +0530, Aboorva Devarajan wrote:
> On Wed, 2023-08-09 at 17:12 -0500, David Vernet wrote:
>
> Hi David,
>
> I have been benchmarking the patch-set on POWER9 machine to understand
> its impact. However, I've run into a recurring hard-lockups in
> newidle_balance, specifically when SHARED_RUNQ feature is enabled. It
> doesn't happen all the time, but it's something worth noting. I wanted
> to inform you about this, and I can provide more details if needed.
Hello Aboorva,
Thank you for testing out this patch set and for the report. One issue
that v4 will correct is that the shared_runq list could become corrupted
if you enable and disable the feature, as a stale task could remain in
the list after the feature has been disabled. I'll be including a fix
for that in v4, which I'm currently benchmarking, but other stuff keeps
seeming to preempt it.
By any chance, did you run into this when you were enabling / disabling
the feature? Or did you just enable it once and then hit this issue
after some time, which would indicate a different issue? I'm trying to
repro using ab, but haven't been successful thus far. If you're able to
repro consistently, it might be useful to run with CONFIG_LIST_DEBUG=y.
Thanks,
David
> -----------------------------------------
>
> Some inital information regarding the hard-lockup:
>
> Base Kernel:
> -----------
>
> Base kernel is upto commit 88c56cfeaec4 ("sched/fair: Block nohz
> tick_stop when cfs bandwidth in use").
>
> Patched Kernel:
> -------------
>
> Base Kernel + v3 (shared runqueue patch-set)(
> https://lore.kernel.org/all/20230809221218.163894-1-void@manifault.com/
> )
>
> The hard-lockup moslty occurs when running the Apache2 benchmarks with
> ab (Apache HTTP benchmarking tool) on the patched kernel. However, this
> problem is not exclusive to the mentioned benchmark and only occurs
> while the SHARED_RUNQ feature is enabled. Disabling SHARED_RUNQ feature
> prevents the occurrence of the lockup.
>
> ab (Apache HTTP benchmarking tool):
> https://httpd.apache.org/docs/2.4/programs/ab.html
>
> Hardlockup with Patched Kernel:
> ------------------------------
>
> [ 3289.727912][ C123] rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
> [ 3289.727943][ C123] rcu: 124-...0: (1 GPs behind) idle=f174/1/0x4000000000000000 softirq=12283/12289 fqs=732
> [ 3289.727976][ C123] rcu: (detected by 123, t=2103 jiffies, g=127061, q=5517 ncpus=128)
> [ 3289.728008][ C123] Sending NMI from CPU 123 to CPUs 124:
> [ 3295.182378][ C123] CPU 124 didn't respond to backtrace IPI, inspecting paca.
> [ 3295.182403][ C123] irq_soft_mask: 0x01 in_mce: 0 in_nmi: 0 current: 15 (ksoftirqd/124)
> [ 3295.182421][ C123] Back trace of paca->saved_r1 (0xc000000de13e79b0) (possibly stale):
> [ 3295.182437][ C123] Call Trace:
> [ 3295.182456][ C123] [c000000de13e79b0] [c000000de13e7a70] 0xc000000de13e7a70 (unreliable)
> [ 3295.182477][ C123] [c000000de13e7ac0] [0000000000000008] 0x8
> [ 3295.182500][ C123] [c000000de13e7b70] [c000000de13e7c98] 0xc000000de13e7c98
> [ 3295.182519][ C123] [c000000de13e7ba0] [c0000000001da8bc] move_queued_task+0x14c/0x280
> [ 3295.182557][ C123] [c000000de13e7c30] [c0000000001f22d8] newidle_balance+0x648/0x940
> [ 3295.182602][ C123] [c000000de13e7d30] [c0000000001f26ac] pick_next_task_fair+0x7c/0x680
> [ 3295.182647][ C123] [c000000de13e7dd0] [c0000000010f175c] __schedule+0x15c/0x1040
> [ 3295.182675][ C123] [c000000de13e7ec0] [c0000000010f26b4] schedule+0x74/0x140
> [ 3295.182694][ C123] [c000000de13e7f30] [c0000000001c4994] smpboot_thread_fn+0x244/0x250
> [ 3295.182731][ C123] [c000000de13e7f90] [c0000000001bc6e8] kthread+0x138/0x140
> [ 3295.182769][ C123] [c000000de13e7fe0] [c00000000000ded8] start_kernel_thread+0x14/0x18
> [ 3295.182806][ C123] rcu: rcu_sched kthread starved for 544 jiffies! g127061 f0x0 RCU_GP_DOING_FQS(6) ->state=0x0 ->cpu=66
> [ 3295.182845][ C123] rcu: Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior.
> [ 3295.182878][ C123] rcu: RCU grace-period kthread stack dump:
>
> -----------------------------------------
>
> [ 3943.438625][ C112] watchdog: CPU 112 self-detected hard LOCKUP @ _raw_spin_lock_irqsave+0x4c/0xc0
> [ 3943.438631][ C112] watchdog: CPU 112 TB:115060212303626, last heartbeat TB:115054309631589 (11528ms ago)
> [ 3943.438673][ C112] CPU: 112 PID: 2090 Comm: kworker/112:2 Tainted: G W L 6.5.0-rc2-00028-g7475adccd76b #51
> [ 3943.438676][ C112] Hardware name: 8335-GTW POWER9 (raw) 0x4e1203 opal:skiboot-v6.5.3-35-g1851b2a06 PowerNV
> [ 3943.438678][ C112] Workqueue: 0x0 (events)
> [ 3943.438682][ C112] NIP: c0000000010ff01c LR: c0000000001d1064 CTR: c0000000001e8580
> [ 3943.438684][ C112] REGS: c000007fffb6bd60 TRAP: 0900 Tainted: G W L (6.5.0-rc2-00028-g7475adccd76b)
> [ 3943.438686][ C112] MSR: 9000000000009033 <SF,HV,EE,ME,IR,DR,RI,LE> CR: 24082222 XER: 00000000
> [ 3943.438693][ C112] CFAR: 0000000000000000 IRQMASK: 1
> [ 3943.438693][ C112] GPR00: c0000000001d1064 c000000e16d1fb20 c0000000014e8200 c000000e092fed3c
> [ 3943.438693][ C112] GPR04: c000000e16d1fc58 c000000e092fe3c8 00000000000000e1 fffffffffffe0000
> [ 3943.438693][ C112] GPR08: 0000000000000000 00000000000000e1 0000000000000000 c00000000299ccd8
> [ 3943.438693][ C112] GPR12: 0000000024088222 c000007ffffb8300 c0000000001bc5b8 c000000deb46f740
> [ 3943.438693][ C112] GPR16: 0000000000000008 c000000e092fe280 0000000000000001 c000007ffedd7b00
> [ 3943.438693][ C112] GPR20: 0000000000000001 c0000000029a1280 0000000000000000 0000000000000001
> [ 3943.438693][ C112] GPR24: 0000000000000000 c000000e092fed3c c000000e16d1fdf0 c00000000299ccd8
> [ 3943.438693][ C112] GPR28: c000000e16d1fc58 c0000000021fbf00 c000007ffee6bf00 0000000000000001
> [ 3943.438722][ C112] NIP [c0000000010ff01c] _raw_spin_lock_irqsave+0x4c/0xc0
> [ 3943.438725][ C112] LR [c0000000001d1064] task_rq_lock+0x64/0x1b0
> [ 3943.438727][ C112] Call Trace:
> [ 3943.438728][ C112] [c000000e16d1fb20] [c000000e16d1fb60] 0xc000000e16d1fb60 (unreliable)
> [ 3943.438731][ C112] [c000000e16d1fb50] [c000000e16d1fbf0] 0xc000000e16d1fbf0
> [ 3943.438733][ C112] [c000000e16d1fbf0] [c0000000001f214c] newidle_balance+0x4bc/0x940
> [ 3943.438737][ C112] [c000000e16d1fcf0] [c0000000001f26ac] pick_next_task_fair+0x7c/0x680
> [ 3943.438739][ C112] [c000000e16d1fd90] [c0000000010f175c] __schedule+0x15c/0x1040
> [ 3943.438743][ C112] [c000000e16d1fe80] [c0000000010f26b4] schedule+0x74/0x140
> [ 3943.438747][ C112] [c000000e16d1fef0] [c0000000001afd44] worker_thread+0x134/0x580
> [ 3943.438749][ C112] [c000000e16d1ff90] [c0000000001bc6e8] kthread+0x138/0x140
> [ 3943.438753][ C112] [c000000e16d1ffe0] [c00000000000ded8] start_kernel_thread+0x14/0x18
> [ 3943.438756][ C112] Code: 63e90001 992d0932 a12d0008 3ce0fffe 5529083c 61290001 7d001
>
> -----------------------------------------
>
> System configuration:
> --------------------
>
> # lscpu
> Architecture: ppc64le
> Byte Order: Little Endian
> CPU(s): 128
> On-line CPU(s) list: 0-127
> Thread(s) per core: 4
> Core(s) per socket: 16
> Socket(s): 2
> NUMA node(s): 8
> Model: 2.3 (pvr 004e 1203)
> Model name: POWER9 (raw), altivec supported
> Frequency boost: enabled
> CPU max MHz: 3800.0000
> CPU min MHz: 2300.0000
> L1d cache: 1 MiB
> L1i cache: 1 MiB
> NUMA node0 CPU(s): 64-127
> NUMA node8 CPU(s): 0-63
> NUMA node250 CPU(s):
> NUMA node251 CPU(s):
> NUMA node252 CPU(s):
> NUMA node253 CPU(s):
> NUMA node254 CPU(s):
> NUMA node255 CPU(s):
>
> # uname -r
> 6.5.0-rc2-00028-g7475adccd76b
>
> # cat /sys/kernel/debug/sched/features
> GENTLE_FAIR_SLEEPERS START_DEBIT NO_NEXT_BUDDY LAST_BUDDY
> CACHE_HOT_BUDDY WAKEUP_PREEMPTION NO_HRTICK NO_HRTICK_DL NO_DOUBLE_TICK
> NONTASK_CAPACITY TTWU_QUEUE NO_SIS_PROP SIS_UTIL NO_WARN_DOUBLE_CLOCK
> RT_PUSH_IPI NO_RT_RUNTIME_SHARE NO_LB_MIN ATTACH_AGE_LOAD WA_IDLE
> WA_WEIGHT WA_BIAS UTIL_EST UTIL_EST_FASTUP NO_LATENCY_WARN ALT_PERIOD
> BASE_SLICE HZ_BW SHARED_RUNQ
>
> -----------------------------------------
>
> Please let me know if I've missed anything here. I'll continue
> investigating and share any additional information I find.
>
> Thanks and Regards,
> Aboorva
>
>
> > Changes
> > -------
> >
> > This is v3 of the shared runqueue patchset. This patch set is based
> > off
> > of commit 88c56cfeaec4 ("sched/fair: Block nohz tick_stop when cfs
> > bandwidth in use") on the sched/core branch of tip.git.
> >
> > v1 (RFC):
> > https://lore.kernel.org/lkml/20230613052004.2836135-1-void@manifault.com/
> > v2:
> > https://lore.kernel.org/lkml/20230710200342.358255-1-void@manifault.com/
> >
> > v2 -> v3 changes:
> > - Don't leave stale tasks in the lists when the SHARED_RUNQ feature
> > is
> > disabled (Abel Wu)
> >
> > - Use raw spin lock instead of spinlock_t (Peter)
> >
> > - Fix return value from shared_runq_pick_next_task() to match the
> > semantics expected by newidle_balance() (Gautham, Abel)
> >
> > - Fold patch __enqueue_entity() / __dequeue_entity() into previous
> > patch
> > (Peter)
> >
> > - Skip <= LLC domains in newidle_balance() if SHARED_RUNQ is enabled
> > (Peter)
> >
> > - Properly support hotplug and recreating sched domains (Peter)
> >
> > - Avoid unnecessary task_rq_unlock() + raw_spin_rq_lock() when src_rq
> > ==
> > target_rq in shared_runq_pick_next_task() (Abel)
> >
> > - Only issue list_del_init() in shared_runq_dequeue_task() if the
> > task
> > is still in the list after acquiring the lock (Aaron Lu)
> >
> > - Slightly change shared_runq_shard_idx() to make it more likely to
> > keep
> > SMT siblings on the same bucket (Peter)
> >
> > v1 -> v2 changes:
> > - Change name from swqueue to shared_runq (Peter)
> >
> > - Shard per-LLC shared runqueues to avoid contention on scheduler-
> > heavy
> > workloads (Peter)
> >
> > - Pull tasks from the shared_runq in newidle_balance() rather than in
> > pick_next_task_fair() (Peter and Vincent)
> >
> > - Rename a few functions to reflect their actual purpose. For
> > example,
> > shared_runq_dequeue_task() instead of swqueue_remove_task() (Peter)
> >
> > - Expose move_queued_task() from core.c rather than migrate_task_to()
> > (Peter)
> >
> > - Properly check is_cpu_allowed() when pulling a task from a
> > shared_runq
> > to ensure it can actually be migrated (Peter and Gautham)
> >
> > - Dropped RFC tag
> >
> > Overview
> > ========
> >
> > The scheduler must constantly strike a balance between work
> > conservation, and avoiding costly migrations which harm performance
> > due
> > to e.g. decreased cache locality. The matter is further complicated
> > by
> > the topology of the system. Migrating a task between cores on the
> > same
> > LLC may be more optimal than keeping a task local to the CPU, whereas
> > migrating a task between LLCs or NUMA nodes may tip the balance in
> > the
> > other direction.
> >
> > With that in mind, while CFS is by and large mostly a work conserving
> > scheduler, there are certain instances where the scheduler will
> > choose
> > to keep a task local to a CPU, when it would have been more optimal
> > to
> > migrate it to an idle core.
> >
> > An example of such a workload is the HHVM / web workload at Meta.
> > HHVM
> > is a VM that JITs Hack and PHP code in service of web requests. Like
> > other JIT / compilation workloads, it tends to be heavily CPU bound,
> > and
> > exhibit generally poor cache locality. To try and address this, we
> > set
> > several debugfs (/sys/kernel/debug/sched) knobs on our HHVM
> > workloads:
> >
> > - migration_cost_ns -> 0
> > - latency_ns -> 20000000
> > - min_granularity_ns -> 10000000
> > - wakeup_granularity_ns -> 12000000
> >
> > These knobs are intended both to encourage the scheduler to be as
> > work
> > conserving as possible (migration_cost_ns -> 0), and also to keep
> > tasks
> > running for relatively long time slices so as to avoid the overhead
> > of
> > context switching (the other knobs). Collectively, these knobs
> > provide a
> > substantial performance win; resulting in roughly a 20% improvement
> > in
> > throughput. Worth noting, however, is that this improvement is _not_
> > at
> > full machine saturation.
> >
> > That said, even with these knobs, we noticed that CPUs were still
> > going
> > idle even when the host was overcommitted. In response, we wrote the
> > "shared runqueue" (SHARED_RUNQ) feature proposed in this patch set.
> > The
> > idea behind SHARED_RUNQ is simple: it enables the scheduler to be
> > more
> > aggressively work conserving by placing a waking task into a sharded
> > per-LLC FIFO queue that can be pulled from by another core in the LLC
> > FIFO queue which can then be pulled from before it goes idle.
> >
> > With this simple change, we were able to achieve a 1 - 1.6%
> > improvement
> > in throughput, as well as a small, consistent improvement in p95 and
> > p99
> > latencies, in HHVM. These performance improvements were in addition
> > to
> > the wins from the debugfs knobs mentioned above, and to other
> > benchmarks
> > outlined below in the Results section.
> >
> > Design
> > ======
> >
> > Note that the design described here reflects sharding, which is the
> > implementation added in the final patch of the series (following the
> > initial unsharded implementation added in patch 6/7). The design is
> > described that way in this commit summary as the benchmarks described
> > in
> > the results section below all reflect a sharded SHARED_RUNQ.
> >
> > The design of SHARED_RUNQ is quite simple. A shared_runq is simply a
> > list of struct shared_runq_shard objects, which itself is simply a
> > struct list_head of tasks, and a spinlock:
> >
> > struct shared_runq_shard {
> > struct list_head list;
> > raw_spinlock_t lock;
> > } ____cacheline_aligned;
> >
> > struct shared_runq {
> > u32 num_shards;
> > struct shared_runq_shard shards[];
> > } ____cacheline_aligned;
> >
> > We create a struct shared_runq per LLC, ensuring they're in their own
> > cachelines to avoid false sharing between CPUs on different LLCs, and
> > we
> > create a number of struct shared_runq_shard objects that are housed
> > there.
> >
> > When a task first wakes up, it enqueues itself in the
> > shared_runq_shard
> > of its current LLC at the end of enqueue_task_fair(). Enqueues only
> > happen if the task was not manually migrated to the current core by
> > select_task_rq(), and is not pinned to a specific CPU.
> >
> > A core will pull a task from the shards in its LLC's shared_runq at
> > the
> > beginning of newidle_balance().
> >
> > Difference between SHARED_RUNQ and SIS_NODE
> > ===========================================
> >
> > In [0] Peter proposed a patch that addresses Tejun's observations
> > that
> > when workqueues are targeted towards a specific LLC on his Zen2
> > machine
> > with small CCXs, that there would be significant idle time due to
> > select_idle_sibling() not considering anything outside of the current
> > LLC.
> >
> > This patch (SIS_NODE) is essentially the complement to the proposal
> > here. SID_NODE causes waking tasks to look for idle cores in
> > neighboring
> > LLCs on the same die, whereas SHARED_RUNQ causes cores about to go
> > idle
> > to look for enqueued tasks. That said, in its current form, the two
> > features at are a different scope as SIS_NODE searches for idle cores
> > between LLCs, while SHARED_RUNQ enqueues tasks within a single LLC.
> >
> > The patch was since removed in [1], and we compared the results to
> > SHARED_RUNQ (previously called "swqueue") in [2]. SIS_NODE did not
> > outperform SHARED_RUNQ on any of the benchmarks, so we elect to not
> > compare against it again for this v2 patch set.
> >
> > [0]:
> > https://lore.kernel.org/all/20230530113249.GA156198@hirez.programming.kicks-ass.net/
> > [1]:
> > https://lore.kernel.org/all/20230605175636.GA4253@hirez.programming.kicks-ass.net/
> > [2]:
> > https://lore.kernel.org/lkml/20230613052004.2836135-1-void@manifault.com/
> >
> > Worth noting as well is that pointed out in [3] that the logic behind
> > including SIS_NODE in the first place should apply to SHARED_RUNQ
> > (meaning that e.g. very small Zen2 CPUs with only 3/4 cores per LLC
> > should benefit from having a single shared_runq stretch across
> > multiple
> > LLCs). I drafted a patch that implements this by having a minimum LLC
> > size for creating a shard, and stretches a shared_runq across
> > multiple
> > LLCs if they're smaller than that size, and sent it to Tejun to test
> > on
> > his Zen2. Tejun reported back that SIS_NODE did not seem to make a
> > difference:
> >
> > [3]:
> > https://lore.kernel.org/lkml/20230711114207.GK3062772@hirez.programming.kicks-ass.net/
> >
> > o____________o__________o
> > | mean | Variance |
> > o------------o----------o
> > Vanilla: | 108.84s | 0.0057 |
> > NO_SHARED_RUNQ: | 108.82s | 0.119s |
> > SHARED_RUNQ: | 108.17s | 0.038s |
> > SHARED_RUNQ w/ SIS_NODE: | 108.87s | 0.111s |
> > o------------o----------o
> >
> > I similarly tried running kcompile on SHARED_RUNQ with SIS_NODE on my
> > 7950X Zen3, but didn't see any gain relative to plain SHARED_RUNQ
> > (though
> > a gain was observed relative to NO_SHARED_RUNQ, as described below).
> >
> > Results
> > =======
> >
> > Note that the motivation for the shared runqueue feature was
> > originally
> > arrived at using experiments in the sched_ext framework that's
> > currently
> > being proposed upstream. The ~1 - 1.6% improvement in HHVM throughput
> > is similarly visible using work-conserving sched_ext schedulers (even
> > very simple ones like global FIFO).
> >
> > In both single and multi socket / CCX hosts, this can measurably
> > improve
> > performance. In addition to the performance gains observed on our
> > internal web workloads, we also observed an improvement in common
> > workloads such as kernel compile and hackbench, when running shared
> > runqueue.
> >
> > On the other hand, some workloads suffer from SHARED_RUNQ. Workloads
> > that hammer the runqueue hard, such as netperf UDP_RR, or schbench -L
> > -m 52 -p 512 -r 10 -t 1. This can be mitigated somewhat by sharding
> > the
> > shared datastructures within a CCX, but it doesn't seem to eliminate
> > all
> > contention in every scenario. On the positive side, it seems that
> > sharding does not materially harm the benchmarks run for this patch
> > series; and in fact seems to improve some workloads such as kernel
> > compile.
> >
> > Note that for the kernel compile workloads below, the compilation was
> > done by running make -j$(nproc) built-in.a on several different types
> > of
> > hosts configured with make allyesconfig on commit a27648c74210 ("afs:
> > Fix setting of mtime when creating a file/dir/symlink") on Linus'
> > tree
> > (boost and turbo were disabled on all of these hosts when the
> > experiments were performed).
> >
> > Finally, note that these results were from the patch set built off of
> > commit ebb83d84e49b ("sched/core: Avoid multiple calling
> > update_rq_clock() in __cfsb_csd_unthrottle()") on the sched/core
> > branch
> > of tip.git for easy comparison with the v2 patch set results. The
> > patches in their final form from this set were rebased onto commit
> > 88c56cfeaec4 ("sched/fair: Block nohz tick_stop when cfs bandwidth in
> > use") on the sched/core branch of tip.git.
> >
> > === Single-socket | 16 core / 32 thread | 2-CCX | AMD 7950X Zen4 ===
> >
> > CPU max MHz: 5879.8818
> > CPU min MHz: 3000.0000
> >
> > Command: make -j$(nproc) built-in.a
> > o____________o__________o
> > | mean | Variance |
> > o------------o----------o
> > NO_SHARED_RUNQ: | 581.95s | 2.639s |
> > SHARED_RUNQ: | 577.02s | 0.084s |
> > o------------o----------o
> >
> > Takeaway: SHARED_RUNQ results in a statistically significant ~.85%
> > improvement over NO_SHARED_RUNQ. This suggests that enqueuing tasks
> > in
> > the shared runqueue on every enqueue improves work conservation, and
> > thanks to sharding, does not result in contention.
> >
> > Command: hackbench --loops 10000
> > o____________o__________o
> > | mean | Variance |
> > o------------o----------o
> > NO_SHARED_RUNQ: | 2.2492s | .00001s |
> > SHARED_RUNQ: | 2.0217s | .00065s |
> > o------------o----------o
> >
> > Takeaway: SHARED_RUNQ in both forms performs exceptionally well
> > compared
> > to NO_SHARED_RUNQ here, beating it by over 10%. This was a surprising
> > result given that it seems advantageous to err on the side of
> > avoiding
> > migration in hackbench given that tasks are short lived in sending
> > only
> > 10k bytes worth of messages, but the results of the benchmark would
> > seem
> > to suggest that minimizing runqueue delays is preferable.
> >
> > Command:
> > for i in `seq 128`; do
> > netperf -6 -t UDP_RR -c -C -l $runtime &
> > done
> > o_______________________o
> > | Throughput | Variance |
> > o-----------------------o
> > NO_SHARED_RUNQ: | 25037.45 | 2243.44 |
> > SHARED_RUNQ: | 24952.50 | 1268.06 |
> > o-----------------------o
> >
> > Takeaway: No statistical significance, though it is worth noting that
> > there is no regression for shared runqueue on the 7950X, while there
> > is
> > a small regression on the Skylake and Milan hosts for SHARED_RUNQ as
> > described below.
> >
> > === Single-socket | 18 core / 36 thread | 1-CCX | Intel Skylake ===
> >
> > CPU max MHz: 1601.0000
> > CPU min MHz: 800.0000
> >
> > Command: make -j$(nproc) built-in.a
> > o____________o__________o
> > | mean | Variance |
> > o------------o----------o
> > NO_SHARED_RUNQ: | 1517.44s | 2.8322s |
> > SHARED_RUNQ: | 1516.51s | 2.9450s |
> > o------------o----------o
> >
> > Takeaway: There's on statistically significant gain here. I observed
> > what I claimed was a .23% win in v2, but it appears that this is not
> > actually statistically significant.
> >
> > Command: hackbench --loops 10000
> > o____________o__________o
> > | mean | Variance |
> > o------------o----------o
> > NO_SHARED_RUNQ: | 5.3370s | .0012s |
> > SHARED_RUNQ: | 5.2668s | .0033s |
> > o------------o----------o
> >
> > Takeaway: SHARED_RUNQ results in a ~1.3% improvement over
> > NO_SHARED_RUNQ. Also statistically significant, but smaller than the
> > 10+% improvement observed on the 7950X.
> >
> > Command: netperf -n $(nproc) -l 60 -t TCP_RR
> > for i in `seq 128`; do
> > netperf -6 -t UDP_RR -c -C -l $runtime &
> > done
> > o_______________________o
> > | Throughput | Variance |
> > o-----------------------o
> > NO_SHARED_RUNQ: | 15699.32 | 377.01 |
> > SHARED_RUNQ: | 14966.42 | 714.13 |
> > o-----------------------o
> >
> > Takeaway: NO_SHARED_RUNQ beats SHARED_RUNQ by ~4.6%. This result
> > makes
> > sense -- the workload is very heavy on the runqueue, so enqueuing
> > tasks
> > in the shared runqueue in __enqueue_entity() would intuitively result
> > in
> > increased contention on the shard lock.
> >
> > === Single-socket | 72-core | 6-CCX | AMD Milan Zen3 ===
> >
> > CPU max MHz: 700.0000
> > CPU min MHz: 700.0000
> >
> > Command: make -j$(nproc) built-in.a
> > o____________o__________o
> > | mean | Variance |
> > o------------o----------o
> > NO_SHARED_RUNQ: | 1568.55s | 0.1568s |
> > SHARED_RUNQ: | 1568.26s | 1.2168s |
> > o------------o----------o
> >
> > Takeaway: No statistically significant difference here. It might be
> > worth experimenting with work stealing in a follow-on patch set.
> >
> > Command: hackbench --loops 10000
> > o____________o__________o
> > | mean | Variance |
> > o------------o----------o
> > NO_SHARED_RUNQ: | 5.2716s | .00143s |
> > SHARED_RUNQ: | 5.1716s | .00289s |
> > o------------o----------o
> >
> > Takeaway: SHARED_RUNQ again wins, by about 2%.
> >
> > Command: netperf -n $(nproc) -l 60 -t TCP_RR
> > for i in `seq 128`; do
> > netperf -6 -t UDP_RR -c -C -l $runtime &
> > done
> > o_______________________o
> > | Throughput | Variance |
> > o-----------------------o
> > NO_SHARED_RUNQ: | 17482.03 | 4675.99 |
> > SHARED_RUNQ: | 16697.25 | 9812.23 |
> > o-----------------------o
> >
> > Takeaway: Similar to the Skylake runs, NO_SHARED_RUNQ still beats
> > SHARED_RUNQ, this time by ~4.5%. It's worth noting that in v2, the
> > NO_SHARED_RUNQ was only ~1.8% faster. The variance is very high here,
> > so
> > the results of this benchmark should be taken with a large grain of
> > salt (noting that we do consistently see NO_SHARED_RUNQ on top due to
> > not contending on the shard lock).
> >
> > Finally, let's look at how sharding affects the following schbench
> > incantation suggested by Chris in [4]:
> >
> > schbench -L -m 52 -p 512 -r 10 -t 1
> >
> > [4]:
> > https://lore.kernel.org/lkml/c8419d9b-2b31-2190-3058-3625bdbcb13d@meta.com/
> >
> > The TL;DR is that sharding improves things a lot, but doesn't
> > completely
> > fix the problem. Here are the results from running the schbench
> > command
> > on the 18 core / 36 thread single CCX, single-socket Skylake:
> >
> > -------------------------------------------------------------------
> > -------------------------------------------------------------------
> > -----------------------------------------------------------------
> > class name con-bounces contentions waittime-
> > min waittime-max waittime-total waittime-avg acq-
> > bounces acquisitions holdtime-min holdtime-max holdtime-
> > total holdtime-avg
> > -------------------------------------------------------------------
> > -------------------------------------------------------------------
> > -----------------------------------------------------------------
> >
> > &shard-
> > >lock: 31510503 31510711 0.08 19.98
> > 168932319.64 5.36 31700383 31843851 0.0
> > 3 17.50 10273968.33 0.32
> > ------------
> > &shard->lock 15731657 [<0000000068c0fd75>]
> > pick_next_task_fair+0x4dd/0x510
> > &shard->lock 15756516 [<000000001faf84f9>]
> > enqueue_task_fair+0x459/0x530
> > &shard->lock 21766 [<00000000126ec6ab>]
> > newidle_balance+0x45a/0x650
> > &shard->lock 772 [<000000002886c365>]
> > dequeue_task_fair+0x4c9/0x540
> > ------------
> > &shard->lock 23458 [<00000000126ec6ab>]
> > newidle_balance+0x45a/0x650
> > &shard->lock 16505108 [<000000001faf84f9>]
> > enqueue_task_fair+0x459/0x530
> > &shard->lock 14981310 [<0000000068c0fd75>]
> > pick_next_task_fair+0x4dd/0x510
> > &shard->lock 835 [<000000002886c365>]
> > dequeue_task_fair+0x4c9/0x540
> >
> > These results are when we create only 3 shards (16 logical cores per
> > shard), so the contention may be a result of overly-coarse sharding.
> > If
> > we run the schbench incantation with no sharding whatsoever, we see
> > the
> > following significantly worse lock stats contention:
> >
> > -------------------------------------------------------------------
> > -------------------------------------------------------------------
> > -------------------------------------------------------------------
> > ----
> > class name con-bounces contentions waittime-
> > min waittime-max waittime-total waittime-avg acq-
> > bounces acquisitions holdtime-min holdtime-max holdtime-
> > total holdtime-avg
> > -------------------------------------------------------------------
> > -------------------------------------------------------------------
> > -------------------------------------------------------------------
> > ----
> >
> > &shard-
> > >lock: 117868635 118361486 0.09 393.01
> > 1250954097.25 10.57 119345882 119780601
> > 0.05 343.35 38313419.51 0.32
> > ------------
> > &shard->lock 59169196 [<0000000060507011>]
> > __enqueue_entity+0xdc/0x110
> > &shard->lock 59084239 [<00000000f1c67316>]
> > __dequeue_entity+0x78/0xa0
> > &shard->lock 108051 [<00000000084a6193>]
> > newidle_balance+0x45a/0x650
> > ------------
> > &shard->lock 60028355 [<0000000060507011>]
> > __enqueue_entity+0xdc/0x110
> > &shard->lock 119882 [<00000000084a6193>]
> > newidle_balance+0x45a/0x650
> > &shard->lock 58213249 [<00000000f1c67316>]
> > __dequeue_entity+0x78/0xa0
> >
> > The contention is ~3-4x worse if we don't shard at all. This roughly
> > matches the fact that we had 3 shards on the first workload run
> > above.
> > If we make the shards even smaller, the contention is comparably much
> > lower:
> >
> > -------------------------------------------------------------------
> > -------------------------------------------------------------------
> > ----------------------------------------------------------
> > class name con-bounces contentions waittime-
> > min waittime-max waittime-total waittime-avg acq-
> > bounces acquisitions holdtime-min holdtime-max holdtime-
> > total holdtime-avg
> > -------------------------------------------------------------------
> > -------------------------------------------------------------------
> > ----------------------------------------------------------
> >
> > &shard-
> > >lock: 13839849 13877596 0.08 13.23 5
> > 389564.95 0.39 46910241 48069307 0.06
> > 16.40 16534469.35 0.34
> > ------------
> > &shard->lock 3559 [<00000000ea455dcc>]
> > newidle_balance+0x45a/0x650
> > &shard->lock 6992418 [<000000002266f400>]
> > __dequeue_entity+0x78/0xa0
> > &shard->lock 6881619 [<000000002a62f2e0>]
> > __enqueue_entity+0xdc/0x110
> > ------------
> > &shard->lock 6640140 [<000000002266f400>]
> > __dequeue_entity+0x78/0xa0
> > &shard->lock 3523 [<00000000ea455dcc>]
> > newidle_balance+0x45a/0x650
> > &shard->lock 7233933 [<000000002a62f2e0>]
> > __enqueue_entity+0xdc/0x110
> >
> > Interestingly, SHARED_RUNQ performs worse than NO_SHARED_RUNQ on the
> > schbench
> > benchmark on Milan as well, but we contend more on the rq lock than
> > the
> > shard lock:
> >
> > -------------------------------------------------------------------
> > -------------------------------------------------------------------
> > -----------------------------------------------------------
> > class name con-bounces contentions waittime-
> > min waittime-max waittime-total waittime-avg acq-
> > bounces acquisitions holdtime-min holdtime-max holdtime-
> > total holdtime-avg
> > -------------------------------------------------------------------
> > -------------------------------------------------------------------
> > -----------------------------------------------------------
> >
> > &rq-
> > >__lock: 9617614 9656091 0.10 79.64
> > 69665812.00 7.21 18092700 67652829 0.11
> > 82.38 344524858.87 5.09
> > -----------
> > &rq->__lock 6301611 [<000000003e63bf26>]
> > task_rq_lock+0x43/0xe0
> > &rq->__lock 2530807 [<00000000516703f0>]
> > __schedule+0x72/0xaa0
> > &rq->__lock 109360 [<0000000011be1562>]
> > raw_spin_rq_lock_nested+0xa/0x10
> > &rq->__lock 178218 [<00000000c38a30f9>]
> > sched_ttwu_pending+0x3d/0x170
> > -----------
> > &rq->__lock 3245506 [<00000000516703f0>]
> > __schedule+0x72/0xaa0
> > &rq->__lock 1294355 [<00000000c38a30f9>]
> > sched_ttwu_pending+0x3d/0x170
> > &rq->__lock 2837804 [<000000003e63bf26>]
> > task_rq_lock+0x43/0xe0
> > &rq->__lock 1627866 [<0000000011be1562>]
> > raw_spin_rq_lock_nested+0xa/0x10
> >
> > .....................................................................
> > .....................................................................
> > ........................................................
> >
> > &shard-
> > >lock: 7338558 7343244 0.10 35.97 7
> > 173949.14 0.98 30200858 32679623 0.08
> > 35.59 16270584.52 0.50
> > ------------
> > &shard->lock 2004142 [<00000000f8aa2c91>]
> > __dequeue_entity+0x78/0xa0
> > &shard->lock 2611264 [<00000000473978cc>]
> > newidle_balance+0x45a/0x650
> > &shard->lock 2727838 [<0000000028f55bb5>]
> > __enqueue_entity+0xdc/0x110
> > ------------
> > &shard->lock 2737232 [<00000000473978cc>]
> > newidle_balance+0x45a/0x650
> > &shard->lock 1693341 [<00000000f8aa2c91>]
> > __dequeue_entity+0x78/0xa0
> > &shard->lock 2912671 [<0000000028f55bb5>]
> > __enqueue_entity+0xdc/0x110
> >
> > .....................................................................
> > .....................................................................
> > .........................................................
> >
> > If we look at the lock stats with SHARED_RUNQ disabled, the rq lock
> > still
> > contends the most, but it's significantly less than with it enabled:
> >
> > -------------------------------------------------------------------
> > -------------------------------------------------------------------
> > --------------------------------------------------------------
> > class name con-bounces contentions waittime-
> > min waittime-max waittime-total waittime-avg acq-
> > bounces acquisitions holdtime-min holdtime-max holdtime-
> > total holdtime-avg
> > -------------------------------------------------------------------
> > -------------------------------------------------------------------
> > --------------------------------------------------------------
> >
> > &rq-
> > >__lock: 791277 791690 0.12 110.54
> > 4889787.63 6.18 1575996 62390275 0.1
> > 3 112.66 316262440.56 5.07
> > -----------
> > &rq->__lock 263343 [<00000000516703f0>]
> > __schedule+0x72/0xaa0
> > &rq->__lock 19394 [<0000000011be1562>]
> > raw_spin_rq_lock_nested+0xa/0x10
> > &rq->__lock 4143 [<000000003b542e83>]
> > __task_rq_lock+0x51/0xf0
> > &rq->__lock 51094 [<00000000c38a30f9>]
> > sched_ttwu_pending+0x3d/0x170
> > -----------
> > &rq->__lock 23756 [<0000000011be1562>]
> > raw_spin_rq_lock_nested+0xa/0x10
> > &rq->__lock 379048 [<00000000516703f0>]
> > __schedule+0x72/0xaa0
> > &rq->__lock 677 [<000000003b542e83>]
> > __task_rq_lock+0x51/0xf0
> >
> > Worth noting is that increasing the granularity of the shards in
> > general
> > improves very runqueue-heavy workloads such as netperf UDP_RR and
> > this
> > schbench command, but it doesn't necessarily make a big difference
> > for
> > every workload, or for sufficiently small CCXs such as the 7950X. It
> > may
> > make sense to eventually allow users to control this with a debugfs
> > knob, but for now we'll elect to choose a default that resulted in
> > good
> > performance for the benchmarks run for this patch series.
> >
> > Conclusion
> > ==========
> >
> > SHARED_RUNQ in this form provides statistically significant wins for
> > several types of workloads, and various CPU topologies. The reason
> > for
> > this is roughly the same for all workloads: SHARED_RUNQ encourages
> > work
> > conservation inside of a CCX by having a CPU do an O(# per-LLC
> > shards)
> > iteration over the shared_runq shards in an LLC. We could similarly
> > do
> > an O(n) iteration over all of the runqueues in the current LLC when a
> > core is going idle, but that's quite costly (especially for larger
> > LLCs), and sharded SHARED_RUNQ seems to provide a performant middle
> > ground between doing an O(n) walk, and doing an O(1) pull from a
> > single
> > per-LLC shared runq.
> >
> > For the workloads above, kernel compile and hackbench were clear
> > winners
> > for SHARED_RUNQ (especially in __enqueue_entity()). The reason for
> > the
> > improvement in kernel compile is of course that we have a heavily
> > CPU-bound workload where cache locality doesn't mean much; getting a
> > CPU
> > is the #1 goal. As mentioned above, while I didn't expect to see an
> > improvement in hackbench, the results of the benchmark suggest that
> > minimizing runqueue delays is preferable to optimizing for L1/L2
> > locality.
> >
> > Not all workloads benefit from SHARED_RUNQ, however. Workloads that
> > hammer the runqueue hard, such as netperf UDP_RR, or schbench -L -m
> > 52
> > -p 512 -r 10 -t 1, tend to run into contention on the shard locks;
> > especially when enqueuing tasks in __enqueue_entity(). This can be
> > mitigated significantly by sharding the shared datastructures within
> > a
> > CCX, but it doesn't eliminate all contention, as described above.
> >
> > Worth noting as well is that Gautham Shenoy ran some interesting
> > experiments on a few more ideas in [5], such as walking the
> > shared_runq
> > on the pop path until a task is found that can be migrated to the
> > calling CPU. I didn't run those experiments in this patch set, but it
> > might be worth doing so.
> >
> > [5]:
> > https://lore.kernel.org/lkml/ZJkqeXkPJMTl49GB@BLR-5CG11610CF.amd.com/
> >
> > Gautham also ran some other benchmarks in [6], which we may want to
> > again try on this v3, but with boost disabled.
> >
> > [6]:
> > https://lore.kernel.org/lkml/ZLpMGVPDXqWEu+gm@BLR-5CG11610CF.amd.com/
> >
> > Finally, while SHARED_RUNQ in this form encourages work conservation,
> > it
> > of course does not guarantee it given that we don't implement any
> > kind
> > of work stealing between shared_runq's. In the future, we could
> > potentially push CPU utilization even higher by enabling work
> > stealing
> > between shared_runq's, likely between CCXs on the same NUMA node.
> >
> > Originally-by: Roman Gushchin <roman.gushchin@linux.dev>
> > Signed-off-by: David Vernet <void@manifault.com>
> >
> > David Vernet (7):
> > sched: Expose move_queued_task() from core.c
> > sched: Move is_cpu_allowed() into sched.h
> > sched: Check cpu_active() earlier in newidle_balance()
> > sched: Enable sched_feat callbacks on enable/disable
> > sched/fair: Add SHARED_RUNQ sched feature and skeleton calls
> > sched: Implement shared runqueue in CFS
> > sched: Shard per-LLC shared runqueues
> >
> > include/linux/sched.h | 2 +
> > kernel/sched/core.c | 52 ++----
> > kernel/sched/debug.c | 18 ++-
> > kernel/sched/fair.c | 340
> > +++++++++++++++++++++++++++++++++++++++-
> > kernel/sched/features.h | 1 +
> > kernel/sched/sched.h | 56 ++++++-
> > kernel/sched/topology.c | 4 +-
> > 7 files changed, 420 insertions(+), 53 deletions(-)
> >
>
On Mon, 2023-11-27 at 13:49 -0600, David Vernet wrote:
> On Mon, Nov 27, 2023 at 01:58:34PM +0530, Aboorva Devarajan wrote:
> > On Wed, 2023-08-09 at 17:12 -0500, David Vernet wrote:
> >
> > Hi David,
> >
> > I have been benchmarking the patch-set on POWER9 machine to understand
> > its impact. However, I've run into a recurring hard-lockups in
> > newidle_balance, specifically when SHARED_RUNQ feature is enabled. It
> > doesn't happen all the time, but it's something worth noting. I wanted
> > to inform you about this, and I can provide more details if needed.
>
> Hello Aboorva,
>
> Thank you for testing out this patch set and for the report. One issue
> that v4 will correct is that the shared_runq list could become corrupted
> if you enable and disable the feature, as a stale task could remain in
> the list after the feature has been disabled. I'll be including a fix
> for that in v4, which I'm currently benchmarking, but other stuff keeps
> seeming to preempt it.
Hi David,
Thank you for your response. While testing, I did observe the
shared_runq list becoming corrupted when enabling and disabling the
feature.
Please find the logs below with CONFIG_DEBUG_LIST enabled:
------------------------------------------
[ 4952.270819] list_add corruption. prev->next should be next (c0000003fae87a80), but was c0000000ba027ec8. (prev=c0000000ba027ec8).
[ 4952.270926] ------------[ cut here ]------------
[ 4952.270935] kernel BUG at lib/list_debug.c:30!
[ 4952.270947] Oops: Exception in kernel mode, sig: 5 [#1]
[ 4952.270956] LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA pSeries
[ 4952.271029] CPU: 10 PID: 31426 Comm: cc1 Kdump: loaded Not tainted 6.5.0-rc2+ #1
[ 4952.271042] Hardware name: IBM,9080-HEX POWER10 (raw) 0x800200 0xf000006 of:IBM,FW1060.00 (NH1060_012) hv:phyp pSeries
[ 4952.271054] NIP: c000000000872f88 LR: c000000000872f84 CTR: 00000000006d1a1c
[ 4952.271070] REGS: c00000006e1b34e0 TRAP: 0700 Not tainted (6.5.0-rc2+)
[ 4952.271079] MSR: 8000000002029033 <SF,VEC,EE,ME,IR,DR,RI,LE> CR: 28048222 XER: 00000006
[ 4952.271102] CFAR: c0000000001ffa24 IRQMASK: 1
[ 4952.271102] GPR00: c000000000872f84 c00000006e1b3780 c0000000019a3b00 0000000000000075
[ 4952.271102] GPR04: c0000003faff2c08 c0000003fb077e80 c00000006e1b35c8 00000003f8e70000
[ 4952.271102] GPR08: 0000000000000027 c000000002185f30 00000003f8e70000 0000000000000001
[ 4952.271102] GPR12: 0000000000000000 c0000003fffe2c80 c000000068ecb100 0000000000000000
[ 4952.271102] GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
[ 4952.271102] GPR20: 0000000000000000 0000000000000000 0000000000000041 c00000006e1b3bb0
[ 4952.271102] GPR24: c000000002c72058 00000003f8e70000 0000000000000001 c00000000e919948
[ 4952.271102] GPR28: c0000000ba027ec8 c0000003fae87a80 c000000080ce6c00 c00000000e919980
[ 4952.271212] NIP [c000000000872f88] __list_add_valid+0xb8/0x100
[ 4952.271236] LR [c000000000872f84] __list_add_valid+0xb4/0x100
[ 4952.271248] Call Trace:
[ 4952.271254] [c00000006e1b3780] [c000000000872f84] __list_add_valid+0xb4/0x100 (unreliable)
[ 4952.271270] [c00000006e1b37e0] [c0000000001b8f50] __enqueue_entity+0x110/0x1c0
[ 4952.271288] [c00000006e1b3830] [c0000000001bec9c] enqueue_entity+0x16c/0x690
[ 4952.271301] [c00000006e1b38e0] [c0000000001bf280] enqueue_task_fair+0xc0/0x490
[ 4952.271315] [c00000006e1b3980] [c0000000001ada0c] ttwu_do_activate+0xac/0x410
[ 4952.271328] [c00000006e1b3a10] [c0000000001ae59c] try_to_wake_up+0x5fc/0x8b0
[ 4952.271341] [c00000006e1b3ae0] [c0000000001df6dc] autoremove_wake_function+0x2c/0xc0
[ 4952.271359] [c00000006e1b3b20] [c0000000001e1018] __wake_up_common+0xc8/0x240
[ 4952.271372] [c00000006e1b3b90] [c0000000001e123c] __wake_up_common_lock+0xac/0x120
[ 4952.271385] [c00000006e1b3c20] [c0000000005bd4a4] pipe_write+0xd4/0x980
[ 4952.271401] [c00000006e1b3d00] [c0000000005ad720] vfs_write+0x350/0x4b0
[ 4952.271420] [c00000006e1b3dc0] [c0000000005adc24] ksys_write+0xf4/0x140
[ 4952.271433] [c00000006e1b3e10] [c000000000031108] system_call_exception+0x128/0x340
[ 4952.271449] [c00000006e1b3e50] [c00000000000cedc] system_call_vectored_common+0x15c/0x2ec
[ 4952.271470] --- interrupt: 3000 at 0x7fff8df3aa34
[ 4952.271482] NIP: 00007fff8df3aa34 LR: 0000000000000000 CTR: 0000000000000000
[ 4952.271492] REGS: c00000006e1b3e80 TRAP: 3000 Not tainted (6.5.0-rc2+)
[ 4952.271502] MSR: 800000000000f033 <SF,EE,PR,FP,ME,IR,DR,RI,LE> CR: 44002822 XER: 00000000
[ 4952.271526] IRQMASK: 0
[ 4952.271526] GPR00: 0000000000000004 00007fffea094d00 0000000112467a00 0000000000000001
[ 4952.271526] GPR04: 0000000132c6a810 0000000000002000 00000000000004e4 0000000000000036
[ 4952.271526] GPR08: 0000000132c6c810 0000000000000000 0000000000000000 0000000000000000
[ 4952.271526] GPR12: 0000000000000000 00007fff8e71cac0 0000000000000000 0000000000000000
[ 4952.271526] GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
[ 4952.271526] GPR20: 00007fffea09c76f 00000001123b6898 0000000000000003 0000000132c6c820
[ 4952.271526] GPR24: 0000000112469d88 00000001124686b8 0000000132c6a810 0000000000002000
[ 4952.271526] GPR28: 0000000000002000 00007fff8e0418e0 0000000132c6a810 0000000000002000
[ 4952.271627] NIP [00007fff8df3aa34] 0x7fff8df3aa34
[ 4952.271635] LR [0000000000000000] 0x0
[ 4952.271642] --- interrupt: 3000
[ 4952.271648] Code: f8010070 4b98ca81 60000000 0fe00000 7c0802a6 3c62ffa6 7d064378 7d244b78 38637f68 f8010070 4b98ca5d 60000000 <0fe00000> 7c0802a6 3c62ffa6 7ca62b78
[ 4952.271685] ---[ end trace 0000000000000000 ]---
[ 4952.282562] pstore: backend (nvram) writing error (-1)
------------------------------------------
>
> By any chance, did you run into this when you were enabling / disabling
> the feature? Or did you just enable it once and then hit this issue
> after some time, which would indicate a different issue? I'm trying to
> repro using ab, but haven't been successful thus far. If you're able to
> repro consistently, it might be useful to run with CONFIG_LIST_DEBUG=y.
>
Additionally, I noticed a sporadic issue persisting even after enabling
the feature once, and the issue surfaced over time. However, it
occurred specifically on a particular system, and my attempts to
recreate it were unsuccessful. I will provide more details if I can
successfully reproduce the issue with debug enabled. But looks like the
primary issue revolves around the shared_runq list getting corrupted
when toggling the feature on and off repeatedly as you pointed out.
I will keep an eye out for v4 and test if it's available later.
Thanks,
Aboorva
> Thanks,
> David
>
> > -----------------------------------------
> >
> > Some inital information regarding the hard-lockup:
> >
> > Base Kernel:
> > -----------
> >
> > Base kernel is upto commit 88c56cfeaec4 ("sched/fair: Block nohz
> > tick_stop when cfs bandwidth in use").
> >
> > Patched Kernel:
> > -------------
> >
> > Base Kernel + v3 (shared runqueue patch-set)(
> > https://lore.kernel.org/all/20230809221218.163894-1-void@manifault.com/
> > )
> >
> > The hard-lockup moslty occurs when running the Apache2 benchmarks
> > with
> > ab (Apache HTTP benchmarking tool) on the patched kernel. However,
> > this
> > problem is not exclusive to the mentioned benchmark and only occurs
> > while the SHARED_RUNQ feature is enabled. Disabling SHARED_RUNQ
> > feature
> > prevents the occurrence of the lockup.
> >
> > ab (Apache HTTP benchmarking tool):
> > https://httpd.apache.org/docs/2.4/programs/ab.html
> >
> > Hardlockup with Patched Kernel:
> > ------------------------------
> >
> > [ 3289.727912][ C123] rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
> > [ 3289.727943][ C123] rcu: 124-...0: (1 GPs behind) idle=f174/1/0x4000000000000000 softirq=12283/12289 fqs=732
> > [ 3289.727976][ C123] rcu: (detected by 123, t=2103 jiffies, g=127061, q=5517 ncpus=128)
> > [ 3289.728008][ C123] Sending NMI from CPU 123 to CPUs 124:
> > [ 3295.182378][ C123] CPU 124 didn't respond to backtrace IPI, inspecting paca.
> > [ 3295.182403][ C123] irq_soft_mask: 0x01 in_mce: 0 in_nmi: 0 current: 15 (ksoftirqd/124)
> > [ 3295.182421][ C123] Back trace of paca->saved_r1 (0xc000000de13e79b0) (possibly stale):
> > [ 3295.182437][ C123] Call Trace:
> > [ 3295.182456][ C123] [c000000de13e79b0] [c000000de13e7a70] 0xc000000de13e7a70 (unreliable)
> > [ 3295.182477][ C123] [c000000de13e7ac0] [0000000000000008] 0x8
> > [ 3295.182500][ C123] [c000000de13e7b70] [c000000de13e7c98] 0xc000000de13e7c98
> > [ 3295.182519][ C123] [c000000de13e7ba0] [c0000000001da8bc] move_queued_task+0x14c/0x280
> > [ 3295.182557][ C123] [c000000de13e7c30] [c0000000001f22d8] newidle_balance+0x648/0x940
> > [ 3295.182602][ C123] [c000000de13e7d30] [c0000000001f26ac] pick_next_task_fair+0x7c/0x680
> > [ 3295.182647][ C123] [c000000de13e7dd0] [c0000000010f175c] __schedule+0x15c/0x1040
> > [ 3295.182675][ C123] [c000000de13e7ec0] [c0000000010f26b4] schedule+0x74/0x140
> > [ 3295.182694][ C123] [c000000de13e7f30] [c0000000001c4994] smpboot_thread_fn+0x244/0x250
> > [ 3295.182731][ C123] [c000000de13e7f90] [c0000000001bc6e8] kthread+0x138/0x140
> > [ 3295.182769][ C123] [c000000de13e7fe0] [c00000000000ded8] start_kernel_thread+0x14/0x18
> > [ 3295.182806][ C123] rcu: rcu_sched kthread starved for 544 jiffies! g127061 f0x0 RCU_GP_DOING_FQS(6) ->state=0x0 ->cpu=66
> > [ 3295.182845][ C123] rcu: Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior.
> > [ 3295.182878][ C123] rcu: RCU grace-period kthread stack dump:
> >
> > -----------------------------------------
> >
> > [ 3943.438625][ C112] watchdog: CPU 112 self-detected hard LOCKUP @ _raw_spin_lock_irqsave+0x4c/0xc0
> > [ 3943.438631][ C112] watchdog: CPU 112 TB:115060212303626, last heartbeat TB:115054309631589 (11528ms ago)
> > [ 3943.438673][ C112] CPU: 112 PID: 2090 Comm: kworker/112:2 Tainted: G W L 6.5.0-rc2-00028-g7475adccd76b #51
> > [ 3943.438676][ C112] Hardware name: 8335-GTW POWER9 (raw) 0x4e1203 opal:skiboot-v6.5.3-35-g1851b2a06 PowerNV
> > [ 3943.438678][ C112] Workqueue: 0x0 (events)
> > [ 3943.438682][ C112] NIP: c0000000010ff01c LR: c0000000001d1064 CTR: c0000000001e8580
> > [ 3943.438684][ C112] REGS: c000007fffb6bd60 TRAP: 0900 Tainted: G W L (6.5.0-rc2-00028-g7475adccd76b)
> > [ 3943.438686][ C112] MSR: 9000000000009033 <SF,HV,EE,ME,IR,DR,RI,LE> CR: 24082222 XER: 00000000
> > [ 3943.438693][ C112] CFAR: 0000000000000000 IRQMASK: 1
> > [ 3943.438693][ C112] GPR00: c0000000001d1064 c000000e16d1fb20 c0000000014e8200 c000000e092fed3c
> > [ 3943.438693][ C112] GPR04: c000000e16d1fc58 c000000e092fe3c8 00000000000000e1 fffffffffffe0000
> > [ 3943.438693][ C112] GPR08: 0000000000000000 00000000000000e1 0000000000000000 c00000000299ccd8
> > [ 3943.438693][ C112] GPR12: 0000000024088222 c000007ffffb8300 c0000000001bc5b8 c000000deb46f740
> > [ 3943.438693][ C112] GPR16: 0000000000000008 c000000e092fe280 0000000000000001 c000007ffedd7b00
> > [ 3943.438693][ C112] GPR20: 0000000000000001 c0000000029a1280 0000000000000000 0000000000000001
> > [ 3943.438693][ C112] GPR24: 0000000000000000 c000000e092fed3c c000000e16d1fdf0 c00000000299ccd8
> > [ 3943.438693][ C112] GPR28: c000000e16d1fc58 c0000000021fbf00 c000007ffee6bf00 0000000000000001
> > [ 3943.438722][ C112] NIP [c0000000010ff01c] _raw_spin_lock_irqsave+0x4c/0xc0
> > [ 3943.438725][ C112] LR [c0000000001d1064] task_rq_lock+0x64/0x1b0
> > [ 3943.438727][ C112] Call Trace:
> > [ 3943.438728][ C112] [c000000e16d1fb20] [c000000e16d1fb60] 0xc000000e16d1fb60 (unreliable)
> > [ 3943.438731][ C112] [c000000e16d1fb50] [c000000e16d1fbf0] 0xc000000e16d1fbf0
> > [ 3943.438733][ C112] [c000000e16d1fbf0] [c0000000001f214c] newidle_balance+0x4bc/0x940
> > [ 3943.438737][ C112] [c000000e16d1fcf0] [c0000000001f26ac] pick_next_task_fair+0x7c/0x680
> > [ 3943.438739][ C112] [c000000e16d1fd90] [c0000000010f175c] __schedule+0x15c/0x1040
> > [ 3943.438743][ C112] [c000000e16d1fe80] [c0000000010f26b4] schedule+0x74/0x140
> > [ 3943.438747][ C112] [c000000e16d1fef0] [c0000000001afd44] worker_thread+0x134/0x580
> > [ 3943.438749][ C112] [c000000e16d1ff90] [c0000000001bc6e8] kthread+0x138/0x140
> > [ 3943.438753][ C112] [c000000e16d1ffe0] [c00000000000ded8] start_kernel_thread+0x14/0x18
> > [ 3943.438756][ C112] Code: 63e90001 992d0932 a12d0008 3ce0fffe 5529083c 61290001 7d001
> >
> > -----------------------------------------
> >
> > System configuration:
> > --------------------
> >
> > # lscpu
> > Architecture: ppc64le
> > Byte Order: Little Endian
> > CPU(s): 128
> > On-line CPU(s) list: 0-127
> > Thread(s) per core: 4
> > Core(s) per socket: 16
> > Socket(s): 2
> > NUMA node(s): 8
> > Model: 2.3 (pvr 004e 1203)
> > Model name: POWER9 (raw), altivec supported
> > Frequency boost: enabled
> > CPU max MHz: 3800.0000
> > CPU min MHz: 2300.0000
> > L1d cache: 1 MiB
> > L1i cache: 1 MiB
> > NUMA node0 CPU(s): 64-127
> > NUMA node8 CPU(s): 0-63
> > NUMA node250 CPU(s):
> > NUMA node251 CPU(s):
> > NUMA node252 CPU(s):
> > NUMA node253 CPU(s):
> > NUMA node254 CPU(s):
> > NUMA node255 CPU(s):
> >
> > # uname -r
> > 6.5.0-rc2-00028-g7475adccd76b
> >
> > # cat /sys/kernel/debug/sched/features
> > GENTLE_FAIR_SLEEPERS START_DEBIT NO_NEXT_BUDDY LAST_BUDDY
> > CACHE_HOT_BUDDY WAKEUP_PREEMPTION NO_HRTICK NO_HRTICK_DL NO_DOUBLE_TICK
> > NONTASK_CAPACITY TTWU_QUEUE NO_SIS_PROP SIS_UTIL NO_WARN_DOUBLE_CLOCK
> > RT_PUSH_IPI NO_RT_RUNTIME_SHARE NO_LB_MIN ATTACH_AGE_LOAD WA_IDLE
> > WA_WEIGHT WA_BIAS UTIL_EST UTIL_EST_FASTUP NO_LATENCY_WARN ALT_PERIOD
> > BASE_SLICE HZ_BW SHARED_RUNQ
> >
> > -----------------------------------------
> >
> > Please let me know if I've missed anything here. I'll continue
> > investigating and share any additional information I find.
> >
> > Thanks and Regards,
> > Aboorva
> >
© 2016 - 2025 Red Hat, Inc.