Re: [PATCH v2] sched/pelt: fix false running accounting

From: Peter Zijlstra
Date: Tue Jul 04 2017 - 04:34:14 EST

On Tue, Jul 04, 2017 at 09:27:07AM +0200, Peter Zijlstra wrote:
> On Sat, Jul 01, 2017 at 07:06:13AM +0200, Vincent Guittot wrote:
> > The running state is a subset of runnable state which means that running
> > can't be set if runnable (weight) is cleared. There are corner cases
> > where the current sched_entity has been already dequeued but cfs_rq->curr
> > has not been updated yet and still points to the dequeued sched_entity.
> > If ___update_load_avg is called at that time, weight will be 0 and running
> > will be set which is not possible.
> >
> > This case happens during pick_next_task_fair() when a cfs_rq becomes idles.
> > The current sched_entity has been dequeued so se->on_rq is cleared and
> > cfs_rq->weight is null. But cfs_rq->curr still points to se (it will be
> > cleared when picking the idle thread). Because the cfs_rq becomes idle,
> > idle_balance() is called and ends up to call update_blocked_averages()
> > with these wrong running and runnable states.
> >
> > Add a test in ___update_load_avg to correct the running state in this case.
> Cute, however did you find that ?

Hmm,.. could you give a little more detail?

Because if ->on_rq=0, we'll have done dequeue_task() which will have
done update_curr() with ->on_rq, weight and ->running consistently.

Then the above, inconsistent update should not happen, because delta=0.