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

[Tim Chen]

On Wed, 2025-11-12 at 12:21 +0100, Peter Zijlstra wrote:
> On Wed, Nov 12, 2025 at 04:39:43PM +0530, Shrikanth Hegde wrote:
> > 
> > 
> 
> > > So perhaps this is the better option -- or did I overlook something with
> > > should_we_balance? It doesn't look like that will make a different
> > > decision on the retry.
> > > 
> > 
> > I think in newidle balance, these checks are there in swb to bail of load balance.
> > redo logic catches it right?
> 
> Urgh, my brain still thinks we're not serializing on newidle. Perhaps I
> should make this 2 patches, one moving the serializing and one adding it
> to newidle.
> 
> > env->dst_rq lock is taken only in attach_tasks, meanwhile, if the wakeup happened,
> > pending would be set. is irq enabled or remote CPU can set ttwu_pending on this rq?
> > 
> >         if (env->idle == CPU_NEWLY_IDLE) {
> >                 if (env->dst_rq->nr_running > 0 || env->dst_rq->ttwu_pending)
> >                         return 0;
> >                 return 1;
> >         }
> 
> Right, that could get tickled.

How about something like the following on top of v4 patch?
This will avoid releasing the lock and take care of the NEWLY_IDLE case.

Tim

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 43c5ec039633..26179f4b77f6 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11772,14 +11772,13 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
 		.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;
@@ -11916,9 +11915,9 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
 			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);
-
+				if (env->idle == CPU_NEWLY_IDLE &&
+				    (env->dst_running > 0 || env->dst_rq->ttwu_pending))
+					goto out;
 				goto redo;
 			}
 			goto out_all_pinned;