Re: [PATCH v4] sched/fair: Skip sched_balance_running cmpxchg when balance is not due

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Nov 12, 2025 at 04:55:48PM +0530, Srikar Dronamraju wrote:

> If the CPU that was doing the balance was not the first CPU of the domain
> span, but it was doing the balance since the first CPU was busy, and the
> first CPU now happens to be idle at redo, the scheduler would have chosen the
> first CPU to do the balance. However it will now choose the CPU that had the atomic..
> 
> I think this is better because 
> - The first CPU may have tried just before this CPU dropped the atomic and
>   hence we may miss the balance opportunity.
> - The first CPU and the other CPU may not be sharing cache and hence there
>   may be a cache-miss, which we are avoiding by doing this.

I'm not sure I understand what you're arguing for. Are you saying it
would be better to retain the lock where possible?


--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11717,23 +11717,22 @@ static int sched_balance_rq(int this_cpu
 		.fbq_type	= all,
 		.tasks		= LIST_HEAD_INIT(env.tasks),
 	};
-	bool need_unlock;
+	bool need_unlock = false;
 
 	cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask);
 
 	schedstat_inc(sd->lb_count[idle]);
 
 redo:
-	need_unlock = false;
 	if (!should_we_balance(&env)) {
 		*continue_balancing = 0;
 		goto out_balanced;
 	}
 
-	if (sd->flags & SD_SERIALIZE) {
-		if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1)) {
+	if (!need_unlock && sd->flags & SD_SERIALIZE) {
+		if (!atomic_try_cmpxchg_acquire(&sched_balance_running, 0, 1))
 			goto out_balanced;
-		}
+
 		need_unlock = true;
 	}
 
@@ -11861,9 +11860,6 @@ static int sched_balance_rq(int this_cpu
 			if (!cpumask_subset(cpus, env.dst_grpmask)) {
 				env.loop = 0;
 				env.loop_break = SCHED_NR_MIGRATE_BREAK;
-				if (need_unlock)
-					atomic_set_release(&sched_balance_running, 0);
-
 				goto redo;
 			}
 			goto out_all_pinned;