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

[PATCH] 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 tasks. Since the values
of non-fair tasks can be stale.

__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") uses cur 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>
---
 kernel/sched/fair.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0feec172fd37..6347ae4cb419 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11016,7 +11016,9 @@ 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)
+		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] sched/cache: Ignore non-fair current in alb_break_llc()

[Tim Chen]

On Tue, 2026-10-20 at 19:17 +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 tasks. Since the values
> of non-fair tasks can be stale.
> 
> __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") uses cur 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>
> ---
>  kernel/sched/fair.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0feec172fd37..6347ae4cb419 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11016,7 +11016,9 @@ 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)
> +		if (cur && cur->sched_class != &fair_sched_class)

Thanks. util was already 0 for a non-fair curr, but cur was still
passed to task_misfits_asym_cpu(), which would evaluate
task_fits_cpu() on a non-CFS task. Since nr_pref_llc_running and
h_nr_runnable only count CFS tasks, curr can be e.g. an RT task here
with a CFS task queued behind it, and active balance would only ever
detach a CFS task anyway. So the fix makes sense.

Suggest that you put a comment here:

                  /*
                   * 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);

Could you update the changelog to say explicitly that the issue is the
non-fair curr being fed to task_misfits_asym_cpu(), not util?

Tim

> +			cur = NULL;
> +		if (cur)
>  			util = task_util(cur);
>  
>  		if (task_misfits_asym_cpu(env, cur) ||