Re: [PATCH] sched: fix sched_entity avg statistics update

From: Chris Redpath
Date: Wed Jan 22 2014 - 05:10:25 EST

On 21/01/14 16:12, Vincent Guittot wrote:
With the current implementation, the load average statistics of a sched entity
change according to other activity on the CPU even if this activity is done
between the running window of the sched entity and have no influence on the
running duration of the task.

When a task wakes up on the same CPU, we currently update last_runnable_update
with the return of __synchronize_entity_decay without updating the
runnable_avg_sum and runnable_avg_period accordingly. In fact, we have to sync
the load_contrib of the se with the rq's blocked_load_contrib before removing
it from the latter (with __synchronize_entity_decay) but we must keep
last_runnable_update unchanged for updating runnable_avg_sum/period during the
next update_entity_load_avg.

Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
kernel/sched/fair.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e64b079..5b0ef90 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2370,8 +2370,7 @@ static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
* would have made count negative); we must be careful to avoid
* double-accounting blocked time after synchronizing decays.
- se->avg.last_runnable_update += __synchronize_entity_decay(se)
- << 20;
+ __synchronize_entity_decay(se);

/* migrated tasks did not contribute to our blocked load */

I've noticed this problem too. It becomes more apparent if you are closely inspecting load signals and comparing against ideal signals generated from task runtime traces. IMO it should be fixed.

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