2016-06-17 20:01 GMT+08:00 Peter Zijlstra <pet...@infradead.org>: > Vincent and Yuyang found another few scenarios in which entity > tracking goes wobbly. > > The scenarios are basically due to the fact that new tasks are not > immediately attached and thereby differ from the normal situation -- a > task is always attached to a cfs_rq load average (such that it > includes its blocked contribution) and are explicitly > detached/attached on migration to another cfs_rq. > > Scenario 1: switch to fair class > > p->sched_class = fair_class; > if (queued) > enqueue_task(p); > ... > enqueue_entity() > enqueue_entity_load_avg() > migrated = !sa->last_update_time (true) > if (migrated) > attach_entity_load_avg() > check_class_changed() > switched_from() (!fair) > switched_to() (fair) > switched_to_fair() > attach_entity_load_avg() > > If @p is a new task that hasn't been fair before, it will have > !last_update_time and, per the above, end up in > attach_entity_load_avg() _twice_. > > Scenario 2: change between cgroups > > sched_move_group(p) > if (queued) > dequeue_task() > task_move_group_fair() > detach_task_cfs_rq() > detach_entity_load_avg() > set_task_rq() > attach_task_cfs_rq() > attach_entity_load_avg() > if (queued) > enqueue_task(); > ... > enqueue_entity() > enqueue_entity_load_avg() > migrated = !sa->last_update_time (true) > if (migrated) > attach_entity_load_avg() > > Similar as with scenario 1, if @p is a new task, it will have > !load_update_time and we'll end up in attach_entity_load_avg() > _twice_. > > Furthermore, notice how we do a detach_entity_load_avg() on something > that wasn't attached to begin with. > > As stated above; the problem is that the new task isn't yet attached > to the load tracking and thereby violates the invariant assumption. > > This patch remedies this by ensuring a new task is indeed properly > attached to the load tracking on creation, through > post_init_entity_util_avg(). > > Of course, this isn't entirely as straight forward as one might think, > since the task is hashed before we call wake_up_new_task() and thus
What's the meaning of "the task is hashed before we call wake_up_new_task()"? Regards, Wanpeng Li
