[PATCH] sched/fair : fix reordering of enqueue_task_fair

[PATCH] sched/fair : fix reordering of enqueue_task_fair

[Vincent Guittot]

Even when a cgroup is throttled, the group se of a child cgroup can still
be enqueued and its gse->on_rq stays true. When a task is enqueued on such
child, we still have to update the load_avg and increase h_nr_running of
the throttled cfs. Nevertheless, the 1st for_each_sched_entity loop is
skipped because of gse->on_rq == true and the 2nd loop because the cfs is
throttled whereas we have to update both load_avg with the old
h_nr_running and increment h_nr_running in such case.

Note that the update of load_avg will effectively happen only once in order
to sync with the throttled time. Next call for updating load_avg will stop
early because the clock stays unchanged.

Fixes: 6d4d22468dae ("sched/fair: Reorder enqueue/dequeue_task_fair path")
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fcc968669aea..22d827fc46c3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5432,7 +5432,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 		cfs_rq = cfs_rq_of(se);
 
 		/* end evaluation on encountering a throttled cfs_rq */
-		if (cfs_rq_throttled(cfs_rq))
+		if (!se->on_rq && cfs_rq_throttled(cfs_rq))
 			goto enqueue_throttle;
 
 		update_load_avg(cfs_rq, se, UPDATE_TG);
-- 
2.17.1

Re: [PATCH] sched/fair : fix reordering of enqueue_task_fair

[Vincent Guittot]

On Wed, 4 Mar 2020 at 16:17, Vincent Guittot <vincent.guittot@linaro.org> wrote:
>
> Even when a cgroup is throttled, the group se of a child cgroup can still
> be enqueued and its gse->on_rq stays true. When a task is enqueued on such
> child, we still have to update the load_avg and increase h_nr_running of
> the throttled cfs. Nevertheless, the 1st for_each_sched_entity loop is
> skipped because of gse->on_rq == true and the 2nd loop because the cfs is
> throttled whereas we have to update both load_avg with the old
> h_nr_running and increment h_nr_running in such case.
>
> Note that the update of load_avg will effectively happen only once in order
> to sync with the throttled time. Next call for updating load_avg will stop
> early because the clock stays unchanged.
>
> Fixes: 6d4d22468dae ("sched/fair: Reorder enqueue/dequeue_task_fair path")
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---

Please ignore this patch, it's not enough to fix all cases


>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fcc968669aea..22d827fc46c3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5432,7 +5432,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>                 cfs_rq = cfs_rq_of(se);
>
>                 /* end evaluation on encountering a throttled cfs_rq */
> -               if (cfs_rq_throttled(cfs_rq))
> +               if (!se->on_rq && cfs_rq_throttled(cfs_rq))
>                         goto enqueue_throttle;
>
>                 update_load_avg(cfs_rq, se, UPDATE_TG);
> --
> 2.17.1
>