Hey all,

This series is a set of patches that optimize scheduler decisions around
realtime tasks and softirqs.  This series is a rebased and reworked set
of changes that have been shipping on Android devices for a number of
years, originally created to resolve audio glitches seen on devices
caused by softirqs for network or storage drivers.

Long running softirqs cause issues because they aren’t currently taken
into account when a realtime task is woken up, but they will delay
realtime tasks from running if the realtime tasks are placed on a cpu
currently running a softirq.

This can easily be seen on some devices by running cyclictest* along
with some heavy background filesystems noise:

Without the patches:
T: 0 ( 7596) P:99 I:1000 C:  59980 Min:      7 Act:   13 Avg:   29 Max: 4107
T: 1 ( 7597) P:99 I:1500 C:  39990 Min:     14 Act:   26 Avg:   36 Max: 8994
T: 2 ( 7598) P:99 I:2000 C:  29995 Min:      7 Act:   22 Avg:   35 Max: 3616
T: 3 ( 7599) P:99 I:2500 C:  23915 Min:      7 Act:   25 Avg:   49 Max: 40273
T: 4 ( 7600) P:99 I:3000 C:  19995 Min:      8 Act:   22 Avg:   38 Max: 10510
T: 5 ( 7601) P:99 I:3500 C:  17135 Min:      7 Act:   26 Avg:   39 Max: 13194
T: 6 ( 7602) P:99 I:4000 C:  14990 Min:      7 Act:   26 Avg:   40 Max: 9470
T: 7 ( 7603) P:99 I:4500 C:  13318 Min:      8 Act:   29 Avg:   44 Max: 20101

Which you can visually see in the image here:
 https://github.com/johnstultz-work/misc/raw/main/images/2022-08-09-softirq-rt-big-latency.png

Which is from the perfetto trace captured here:
 https://ui.perfetto.dev/#!/?s=33661aec8ec82c2da0a59263f36f7d72b4a2f4e7a99b28b222bd12ad872f

The first patch adds a bit of generic infrastructure to get the per-cpu
softirq_pending flag.

The second patch in the series adds logic to account for when softirqs
are running, and then conditionally based on
CONFIG_RT_SOFTIRQ_AWARE_SCHED allows rt-task placement to be done in a
way that’s aware if a current softirq might be a long-running one, to
potentially place the rt task on another free core.

The third patch in the series adds logic in __do_softirq(), also under
CONFIG_RT_SOFTIRQ_AWARE_SCHED, to defer some of the potentially long
running softirqs to ksoftirqd if a -rt task is currently running on the
cpu. This patch also includes a folded down fix that stubbs out
ksoftirqd_running() based on CONFIG_RT_SOFTIRQ_AWARE_SCHED, since in
changing to more frequently defer long running softirqs, the logic using
ksoftirqd_running will end up being too conservative and needlessly
delay shorter-running softirqs.

With these patches we see dramatic improvements in the worst case
latencies in the cyclictest* + filesystem noise test above:

With the patches
T: 0 ( 7527) P:99 I:1000 C:  59998 Min:      6 Act:   29 Avg:   35 Max: 1734
T: 1 ( 7528) P:99 I:1500 C:  40000 Min:      7 Act:   39 Avg:   35 Max: 1181
T: 2 ( 7529) P:99 I:2000 C:  30000 Min:      7 Act:   25 Avg:   25 Max: 444
T: 3 ( 7530) P:99 I:2500 C:  24000 Min:      7 Act:   34 Avg:   36 Max: 1729
T: 4 ( 7531) P:99 I:3000 C:  20000 Min:      7 Act:   36 Avg:   25 Max: 406
T: 5 ( 7532) P:99 I:3500 C:  17143 Min:      7 Act:   38 Avg:   34 Max: 1264
T: 6 ( 7533) P:99 I:4000 C:  15000 Min:      7 Act:   27 Avg:   33 Max: 2351
T: 7 ( 7534) P:99 I:4500 C:  13334 Min:      7 Act:   41 Avg:   29 Max: 2285

Since these patches have been carried along for years, and have at times
badly collided with upstream, I wanted to submit them for some initial
review, discussion and feedback so we could hopefully eventually find a
reasonable solution that might land upstream.

* Unfortunately cyclictest had a bug that causes it to always affine
threads to cpus preventing them from being migrated. So you’ll need
to update to the latest version (which includes a fix) to reproduce.

Let me know what you think!

thanks
-john

New in v5:
* Conditionalize active_softirqs handling (suggested by Alexander
  Gordeev <agordeev@linux.ibm.com>)
* Reorder rt_task_fits_cpu to have the "fast" function first
  (Suggested by Alexander Gordeev <agordeev@linux.ibm.com>)
* Fix bug I introduced in v2 condensing
  task_thread_info(task)->preempt_count to preempt_count()
  (Reported-by: Alexander Gordeev <agordeev@linux.ibm.com>)
* Tweak comment discription to remove the vauge "slow"
  descriptor of softirqs being run by ksoftirqd
  (Suggested by Alexander Gordeev <agordeev@linux.ibm.com>)
* Switch to using CONFIG_RT_SOFTIRQ_AWARE_SCHED (suggested by
  Joel Fernandes <joel@joelfernandes.org>)
* Simplify cpu_busy_with_softirqs() logic as pointed out by
  Alexander Gordeev <agordeev@linux.ibm.com>
* Switch to using IS_ENABLED rather then defining my own macro
  (suggsted by Joel Fernandes <joel@joelfernandes.org>)

Cc: John Dias <joaodias@google.com>
Cc: Connor O'Brien <connoro@google.com>
Cc: Rick Yiu <rickyiu@google.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Qais Yousef <qyousef@google.com>
Cc: Chris Redpath <chris.redpath@arm.com>
Cc: Abhijeet Dharmapurikar <adharmap@quicinc.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: kernel-team@android.com

Connor O'Brien (1):
  sched: Avoid placing RT threads on cores handling long softirqs

John Stultz (1):
  softirq: Add generic accessor to percpu softirq_pending data

Pavankumar Kondeti (1):
  softirq: defer softirq processing to ksoftirqd if CPU is busy with RT

 arch/s390/include/asm/hardirq.h |  6 ++++
 include/linux/interrupt.h       | 20 ++++++++++++++
 init/Kconfig                    | 10 +++++++
 kernel/sched/rt.c               | 49 +++++++++++++++++++++++++++------
 kernel/softirq.c                | 46 +++++++++++++++++++++++++++++--
 5 files changed, 120 insertions(+), 11 deletions(-)

-- 
2.38.1.431.g37b22c650d-goog