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SubjectRe: [PATCH v2] sched/fair: Correctly insert cfs_rq's to list on unthrottle
On Thu, 3 Jun 2021 at 13:41, Odin Ugedal <odin@uged.al> wrote:
>
> This fixes an issue where fairness is decreased since cfs_rq's can
> end up not being decayed properly. For two sibling control groups with
> the same priority, this can often lead to a load ratio of 99/1 (!!).
>
> This happen because when a cfs_rq is throttled, all the descendant cfs_rq's
> will be removed from the leaf list. When they initial cfs_rq is
> unthrottled, it will currently only re add descendant cfs_rq's if they
> have one or more entities enqueued. This is not a perfect heuristic.
>
> Insted, we insert all cfs_rq's that contain one or more enqueued
> entities, or contributes to the load of the task group.
>
> Can often lead to sutiations like this for equally weighted control
> groups:
>
> $ ps u -C stress
> USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
> root 10009 88.8 0.0 3676 100 pts/1 R+ 11:04 0:13 stress --cpu 1
> root 10023 3.0 0.0 3676 104 pts/1 R+ 11:04 0:00 stress --cpu 1
>
> Fixes: 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
> Signed-off-by: Odin Ugedal <odin@uged.al>
> ---
>
> Original thread: https://lore.kernel.org/lkml/20210518125202.78658-3-odin@uged.al/
> Changes since v1:
> - Replaced cfs_rq field with using tg_load_avg_contrib
> - Went from 3 to 1 pathces; one is merged and one is replaced
> by a new patchset.
>
> kernel/sched/fair.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 794c2cb945f8..0f1b39ca5ca8 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4719,8 +4719,11 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
> cfs_rq->throttled_clock_task_time += rq_clock_task(rq) -
> cfs_rq->throttled_clock_task;
>
> - /* Add cfs_rq with already running entity in the list */
> - if (cfs_rq->nr_running >= 1)
> + /*
> + * Add cfs_rq with tg load avg contribution or one or more
> + * already running entities to the list
> + */
> + if (cfs_rq->tg_load_avg_contrib || cfs_rq->nr_running)

Out of curiosity, why did you decide to use
cfs_rq->tg_load_avg_contrib instead of !cfs_rq_is_decayed(cfs_rq)
which is used to delete the cfs_rq from the list when updating blocked
load ?

> list_add_leaf_cfs_rq(cfs_rq);
> }
>
> --
> 2.31.1
>

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