Re: [PATCH 4/4] sched,fair: Fix PELT integrity for new tasks

[Wanpeng Li <kernellwp@gmail.com>]

2016-06-17 20:01 GMT+08:00 Peter Zijlstra <peterz@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