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SubjectRe: [PATCH 1/2] sched: reset blocked load decay_count during synchronization
Date
Chris Redpath <chris.redpath@arm.com> writes:

> If an entity happens to sleep for less than one tick duration
> the tracked load associated with that entity can be decayed by an
> unexpectedly large amount if it is later migrated to a different
> CPU. This can interfere with correct scheduling when entity load
> is used for decision making.
>
> The reason for this is that when an entity is dequeued and enqueued
> quickly, such that se.avg.decay_count and cfs_rq.decay_counter
> do not differ when that entity is enqueued again,
> __synchronize_entity_decay skips the calculation step and also skips
> clearing the decay_count. At a later time that entity may be
> migrated and its load will be decayed incorrectly.
>
> All users of this function expect decay_count to be zero'ed after
> use.
>
> Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Reviewed-by: Ben Segall <bsegall@google.com>

> ---
> kernel/sched/fair.c | 8 +++-----
> 1 file changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e8b652e..b7e5945 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2142,12 +2142,10 @@ static inline u64 __synchronize_entity_decay(struct sched_entity *se)
> u64 decays = atomic64_read(&cfs_rq->decay_counter);
>
> decays -= se->avg.decay_count;
> - if (!decays)
> - return 0;
> -
> - se->avg.load_avg_contrib = decay_load(se->avg.load_avg_contrib, decays);
> + if (decays)
> + se->avg.load_avg_contrib =
> + decay_load(se->avg.load_avg_contrib, decays);
> se->avg.decay_count = 0;
> -
> return decays;
> }


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