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 01:32:23PM +0530, Srikar Dronamraju wrote:
> >  	group = sched_balance_find_src_group(&env);
> >  	if (!group) {
> >  		schedstat_inc(sd->lb_nobusyg[idle]);
> > @@ -11892,6 +11916,9 @@ 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);
> > +
> 
> One nit:
> While the current code is good, would conditionally resetting the
> need_unlock just after resetting the atomic variable better than
> unconditional reset that we do now?

Right, I had the same thought when grabbed the patch yesterday, but
ignored it.

But perhaps something like so?

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11717,14 +11717,20 @@ 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]);
 
+	if (0) {
 redo:
-	need_unlock = false;
+		if (need_unlock) {
+			atomic_set_release(&sched_balance_running, 0);
+			need_unlock = false;
+		}
+	}
+
 	if (!should_we_balance(&env)) {
 		*continue_balancing = 0;
 		goto out_balanced;
@@ -11861,9 +11867,6 @@ 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;


---

The other option is something like this, but Linus hated on this pattern
when we started with things.

---
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11692,6 +11692,8 @@ static void update_lb_imbalance_stat(str
  */
 static atomic_t sched_balance_running = ATOMIC_INIT(0);
 
+DEFINE_FREE(balance_unlock, atomic_t *, if (_T) atomic_set_release(_T, 0));
+
 /*
  * Check this_cpu to ensure it is balanced within domain. Attempt to move
  * tasks if there is an imbalance.
@@ -11717,24 +11719,23 @@ static int sched_balance_rq(int this_cpu
 		.fbq_type	= all,
 		.tasks		= LIST_HEAD_INIT(env.tasks),
 	};
-	bool need_unlock;
 
 	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;
+		return 0;
 	}
 
+	atomic_t *lock __free(balance_unlock) = NULL;
 	if (sd->flags & SD_SERIALIZE) {
-		if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1)) {
-			goto out_balanced;
-		}
-		need_unlock = true;
+		if (!atomic_try_cmpxchg_acquire(&sched_balance_running, 0, 1))
+			return 0;
+
+		lock = &sched_balance_running;
 	}
 
 	group = sched_balance_find_src_group(&env);
@@ -11861,9 +11862,6 @@ 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;
@@ -11980,9 +11978,6 @@ static int sched_balance_rq(int this_cpu
 	    sd->balance_interval < sd->max_interval)
 		sd->balance_interval *= 2;
 out:
-	if (need_unlock)
-		atomic_set_release(&sched_balance_running, 0);
-
 	return ld_moved;
 }