[v1] sched/balancing: Misc updates & cleanups

[PATCH 01/10] sched/balancing: Switch the 'DEFINE_SPINLOCK(balancing)' spinlock into an 'atomic_t sched_balance_running' flag

Posted by Ingo Molnar 1 year, 11 months ago

The 'balancing' spinlock added in:

  08c183f31bdb ("[PATCH] sched: add option to serialize load balancing")

... is taken when the SD_SERIALIZE flag is set in a domain, but in reality it
is a glorified global atomic flag serializing the load-balancing of
those domains.

It doesn't have any explicit locking semantics per se: we just
spin_trylock() it.

Turn it into a ... global atomic flag. This makes it more
clear what is going on here, and reduces overhead and code
size a bit:

  # kernel/sched/fair.o: [x86-64 defconfig]

     text	   data	    bss	    dec	    hex	filename
    60730	   2721	    104	  63555	   f843	fair.o.before
    60718	   2721	    104	  63543	   f837	fair.o.after

Also document the flag a bit.

No change in functionality intended.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6a16129f9a5c..2ef89b36aed1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11633,7 +11633,20 @@ static int active_load_balance_cpu_stop(void *data)
 	return 0;
 }
 
-static DEFINE_SPINLOCK(balancing);
+/*
+ * This flag serializes load-balancing passes over large domains
+ * (above the NODE topology level) - only one load-balancing instance
+ * may run at a time, to reduce overhead on very large systems with
+ * lots of CPUs and large NUMA distances.
+ *
+ * - Note that load-balancing passes triggered while another one
+ *   is executing are skipped and not re-tried.
+ *
+ * - Also note that this does not serialize rebalance_domains()
+ *   execution, as non-SD_SERIALIZE domains will still be
+ *   load-balanced in parallel.
+ */
+static atomic_t sched_balance_running = ATOMIC_INIT(0);
 
 /*
  * Scale the max load_balance interval with the number of CPUs in the system.
@@ -11711,7 +11724,7 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
 
 		need_serialize = sd->flags & SD_SERIALIZE;
 		if (need_serialize) {
-			if (!spin_trylock(&balancing))
+			if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))
 				goto out;
 		}
 
@@ -11729,7 +11742,7 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
 			interval = get_sd_balance_interval(sd, busy);
 		}
 		if (need_serialize)
-			spin_unlock(&balancing);
+			atomic_set_release(&sched_balance_running, 0);
 out:
 		if (time_after(next_balance, sd->last_balance + interval)) {
 			next_balance = sd->last_balance + interval;
-- 
2.40.1

Re: [PATCH 01/10] sched/balancing: Switch the 'DEFINE_SPINLOCK(balancing)' spinlock into an 'atomic_t sched_balance_running' flag

Posted by Shrikanth Hegde 1 year, 10 months ago


On 3/8/24 4:28 PM, Ingo Molnar wrote:
> The 'balancing' spinlock added in:
> 

> Also document the flag a bit.
> 
> No change in functionality intended.
> 

> -static DEFINE_SPINLOCK(balancing);
> +/*
> + * This flag serializes load-balancing passes over large domains
> + * (above the NODE topology level) - only one load-balancing instance
> + * may run at a time, to reduce overhead on very large systems with
> + * lots of CPUs and large NUMA distances.
> + *
> + * - Note that load-balancing passes triggered while another one
> + *   is executing are skipped and not re-tried.
> + *
> + * - Also note that this does not serialize rebalance_domains()

nit: please change rebalance_domains to sched_balance_domains. 

> + *   execution, as non-SD_SERIALIZE domains will still be
> + *   load-balanced in parallel.
> + */
> +static atomic_t sched_balance_running = ATOMIC_INIT(0);
>  
>  /*
>   * Scale the max load_balance interval with the number of CPUs in the system.
> @@ -11711,7 +11724,7 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
>  
>  		need_serialize = sd->flags & SD_SERIALIZE;
>  		if (need_serialize) {

[tip: sched/core] sched/balancing: Switch the 'DEFINE_SPINLOCK(balancing)' spinlock into an 'atomic_t sched_balance_running' flag

Posted by tip-bot2 for Ingo Molnar 1 year, 11 months ago

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     214c1b7f13954559cf09d5d04b934bf32ba4d618
Gitweb:        https://git.kernel.org/tip/214c1b7f13954559cf09d5d04b934bf32ba4d618
Author:        Ingo Molnar <mingo@kernel.org>
AuthorDate:    Fri, 08 Mar 2024 11:58:52 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Tue, 12 Mar 2024 11:03:39 +01:00

sched/balancing: Switch the 'DEFINE_SPINLOCK(balancing)' spinlock into an 'atomic_t sched_balance_running' flag

The 'balancing' spinlock added in:

  08c183f31bdb ("[PATCH] sched: add option to serialize load balancing")

... is taken when the SD_SERIALIZE flag is set in a domain, but in reality it
is a glorified global atomic flag serializing the load-balancing of
those domains.

It doesn't have any explicit locking semantics per se: we just
spin_trylock() it.

Turn it into a ... global atomic flag. This makes it more
clear what is going on here, and reduces overhead and code
size a bit:

  # kernel/sched/fair.o: [x86-64 defconfig]

     text	   data	    bss	    dec	    hex	filename
    60730	   2721	    104	  63555	   f843	fair.o.before
    60718	   2721	    104	  63543	   f837	fair.o.after

Also document the flag a bit.

No change in functionality intended.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://lore.kernel.org/r/20240308105901.1096078-2-mingo@kernel.org
---
 kernel/sched/fair.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6a16129..2ef89b3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11633,7 +11633,20 @@ out_unlock:
 	return 0;
 }
 
-static DEFINE_SPINLOCK(balancing);
+/*
+ * This flag serializes load-balancing passes over large domains
+ * (above the NODE topology level) - only one load-balancing instance
+ * may run at a time, to reduce overhead on very large systems with
+ * lots of CPUs and large NUMA distances.
+ *
+ * - Note that load-balancing passes triggered while another one
+ *   is executing are skipped and not re-tried.
+ *
+ * - Also note that this does not serialize rebalance_domains()
+ *   execution, as non-SD_SERIALIZE domains will still be
+ *   load-balanced in parallel.
+ */
+static atomic_t sched_balance_running = ATOMIC_INIT(0);
 
 /*
  * Scale the max load_balance interval with the number of CPUs in the system.
@@ -11711,7 +11724,7 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
 
 		need_serialize = sd->flags & SD_SERIALIZE;
 		if (need_serialize) {
-			if (!spin_trylock(&balancing))
+			if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))
 				goto out;
 		}
 
@@ -11729,7 +11742,7 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
 			interval = get_sd_balance_interval(sd, busy);
 		}
 		if (need_serialize)
-			spin_unlock(&balancing);
+			atomic_set_release(&sched_balance_running, 0);
 out:
 		if (time_after(next_balance, sd->last_balance + interval)) {
 			next_balance = sd->last_balance + interval;