From: Chris Redpath chris.redpath@arm.com
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 Signed-off-by: Jon Medhurst tixy@linaro.org --- kernel/sched/fair.c | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index a771fa6..01cc427 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -1460,12 +1460,9 @@ 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; }