Re: PROBLEM: Persistent unfair sharing of a processor by auto groupsin 3.11-rc2 (has twice regressed)

[Paul Turner]

On Fri, Jul 26, 2013 at 2:03 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
>
> OK, so I have the below; however on a second look, Paul, shouldn't that
> update_cfs_shares() call be in entity_tick(), right after calling
> update_cfs_rq_blocked_load(). Because placing it in
> update_cfs_rq_blocked_load() means its now called twice on the
> enqueue/dequeue paths through:
>
>   {en,de}queue_entity()
>     {en,de}queue_entity_load_avg()
>       update_cfs_rq_blocked_load()
>         update_cfs_shares()

Yes, I agree: placing it directly in entity_tick() would be better.

[ In f269ae046 the calls to update_cfs_rq_blocked_load() were amortized
and the separate update in {en,de}queue_entity_load_avg() were
removed. ]


>
>
>
> ---
> Subject: sched: Ensure update_cfs_shares() is called for parents of continuously-running tasks
> From: Max Hailperin <max@xxxxxxxxxxxx>
>
> We typically update a task_group's shares within the dequeue/enqueue
> path.  However, continuously running tasks sharing a CPU are not
> subject to these updates as they are only put/picked.  Unfortunately,
> when we reverted f269ae046 (in 17bc14b7), we lost the augmenting
> periodic update that was supposed to account for this; resulting in a
> potential loss of fairness.
>
> To fix this, re-introduce the explicit update in
> update_cfs_rq_blocked_load() [called via entity_tick()].
>
> Cc: stable@xxxxxxxxxx
> Reviewed-by: Paul Turner <pjt@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c |    1 +
>  1 file changed, 1 insertion(+)
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1531,6 +1531,7 @@ static void update_cfs_rq_blocked_load(s
>         }
>
>         __update_cfs_rq_tg_load_contrib(cfs_rq, force_update);
> +       update_cfs_shares(cfs_rq);
>  }
>
>  static inline void update_rq_runnable_avg(struct rq *rq, int runnable)
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