[PATCH v2] sched/cache: Ignore non-fair current in alb_break_llc()

[PATCH v2] sched/cache: Ignore non-fair current in alb_break_llc()

[Kayra Cizmeci]

Commit d6b9afab44e23 ("sched/cache: Fix
checking active load balance by only considering the CFS task")
stopped using task_util() for non-fair cur.

__active_load_balance_cpu_stop() calls detach_one_task(), that only looks
to cfs_tasks list. So if the rq->curr is not fair, doing a decision
according to it is wrong, since we only migrate tasks that is fair.

Commit f0d243a96f26 ("sched/fair: Avoid creating misfits during
cache-aware balancing") passes cur to task_misfits_asym_cpu()
without checking if the cur is in fair class or not. Fix this.

Fixes: f0d243a96f26 ("sched/fair: Avoid creating misfits during cache-aware balancing")
Signed-off-by: Kayra Cizmeci <kayracizmeci@gmail.com>
---
Changes v1->v2:
* Add suggested comment (Tim Chen :>)
* Change the changelog so that it says the issue is passing a
  non-fair cur to task_misfits_asym_cpu()

This versions not compile-tested. Tho only change is this comment,
so it should be fine. Right? ;-).

 kernel/sched/fair.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0feec172fd37..9d7fd6cec376 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11016,7 +11016,14 @@ alb_break_llc(struct lb_env *env)
 			return true;
 
 		cur = rcu_dereference_all(env->src_rq->curr);
-		if (cur && cur->sched_class == &fair_sched_class)
+		/*
+		 * Active balance only detaches CFS tasks, so a non-fair
+		 * curr is never migrated. Ignore it and avoid using its
+		 * stale util in the misfit and LLC checks.
+		 */
+		if (cur && cur->sched_class != &fair_sched_class)
+			cur = NULL;
+		if (cur)
 			util = task_util(cur);
 
 		if (task_misfits_asym_cpu(env, cur) ||
-- 
2.53.0

Re: [PATCH v2] sched/cache: Ignore non-fair current in alb_break_llc()

[Tim Chen]

On Tue, 2026-10-20 at 21:54 +0300, Kayra Cizmeci wrote:
> Commit d6b9afab44e23 ("sched/cache: Fix
> checking active load balance by only considering the CFS task")
> stopped using task_util() for non-fair cur.
> 
> __active_load_balance_cpu_stop() calls detach_one_task(), that only looks
> to cfs_tasks list. So if the rq->curr is not fair, doing a decision
> according to it is wrong, since we only migrate tasks that is fair.
> 
> Commit f0d243a96f26 ("sched/fair: Avoid creating misfits during
> cache-aware balancing") passes cur to task_misfits_asym_cpu()
> without checking if the cur is in fair class or not. Fix this.

Sent out a suggested update to the commit log of v1 before I saw v2.

This updated log and patch looks okay to me.

Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>

Tim

> 
> Fixes: f0d243a96f26 ("sched/fair: Avoid creating misfits during cache-aware balancing")
> Signed-off-by: Kayra Cizmeci <kayracizmeci@gmail.com>
> ---
> Changes v1->v2:
> * Add suggested comment (Tim Chen :>)
> * Change the changelog so that it says the issue is passing a
>   non-fair cur to task_misfits_asym_cpu()
> 
> This versions not compile-tested. Tho only change is this comment,
> so it should be fine. Right? ;-).
> 
>  kernel/sched/fair.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0feec172fd37..9d7fd6cec376 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11016,7 +11016,14 @@ alb_break_llc(struct lb_env *env)
>  			return true;
>  
>  		cur = rcu_dereference_all(env->src_rq->curr);
> -		if (cur && cur->sched_class == &fair_sched_class)
> +		/*
> +		 * Active balance only detaches CFS tasks, so a non-fair
> +		 * curr is never migrated. Ignore it and avoid using its
> +		 * stale util in the misfit and LLC checks.
> +		 */
> +		if (cur && cur->sched_class != &fair_sched_class)
> +			cur = NULL;
> +		if (cur)
>  			util = task_util(cur);
>  
>  		if (task_misfits_asym_cpu(env, cur) ||