On Tue, Apr 05, 2016 at 06:00:40PM +0100, Dietmar Eggemann wrote:
@@ -2893,8 +2906,12 @@ static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s se->avg.last_update_time = cfs_rq->avg.last_update_time; cfs_rq->avg.load_avg += se->avg.load_avg; cfs_rq->avg.load_sum += se->avg.load_sum;
cfs_rq->avg.util_avg += se->avg.util_avg;
cfs_rq->avg.util_sum += se->avg.util_sum;
if (!entity_is_task(se))
return;
rq_of(cfs_rq)->cfs.avg.util_avg += se->avg.util_avg;
rq_of(cfs_rq)->cfs.avg.util_sum += se->avg.util_sum;
To me it seems that you cannot be sure that the rq_of(cfs_rq)->cfs.avg time stamp is aligned with se->avg time stamp, which is necessary before you can add/subtract two geometric series without introducing an error.
attach_entity_load_avg() is called (through a couple of other functions) from the for_each_sched_entity() loop in enqueue_task_fair() which works its way towards the root cfs_rq, i.e. rq_of(cfs_rq)->cfs. So in the loop iteration where you attach the task sched_entity, we haven't yet visited and updated rq_of(cfs_rq)->cfs.avg.
If you just add the task contribution and discover later that there is a time delta when you update rq_of(cfs_rq)->cfs.avg you end up decaying the task contribution which was already up-to-date and its util contribution to rq_of(cfs_rq)->cfs.avg ends up being smaller than it should be.
Am I missing something?