Re: [patch v8 4/9] sched: fix slept time double counting in enqueue entity

From: Paul Turner
Date: Mon Jun 17 2013 - 07:51:46 EST


On Fri, Jun 7, 2013 at 12:20 AM, Alex Shi <alex.shi@xxxxxxxxx> wrote:
> The wakeuped migrated task will __synchronize_entity_decay(se); in
> migrate_task_fair, then it needs to set
> `se->avg.last_runnable_update -= (-se->avg.decay_count) << 20'
> before update_entity_load_avg, in order to avoid slept time is updated
> twice for se.avg.load_avg_contrib in both __syncchronize and
> update_entity_load_avg.
>
> but if the slept task is waked up from self cpu, it miss the
> last_runnable_update before update_entity_load_avg(se, 0, 1), then the
> slept time was used twice in both functions.
> So we need to remove the double slept time counting.

Good catch.

>
> Signed-off-by: Alex Shi <alex.shi@xxxxxxxxx>
> ---
> kernel/sched/fair.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1fc30b9..42c7be0 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1570,7 +1570,8 @@ static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
> }
> wakeup = 0;
> } else {
> - __synchronize_entity_decay(se);
> + se->avg.last_runnable_update += __synchronize_entity_decay(se)
> + << 20;

Can you add something like:

+ /*
+ * Task re-woke on same cpu (or else
migrate_task_rq_fair()
+ * would have made count negative); we must be careful
to avoid
+ * double-accounting blocked time after synchronizing
decays.
+ */


Reviewed-by: Paul Turner <pjt@xxxxxxxxxx>
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