On Tue, Jul 04, 2017 at 11:12:34AM +0200, Vincent Guittot wrote: > On 4 July 2017 at 10:34, Peter Zijlstra <pet...@infradead.org> wrote: > > 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. > > In fact, the delta between dequeue_entity_load_avg() and > update_blocked_averages() is not 0 on my platform (hikey) but can be > longer than 60us (at lowest frequency with only 1 task group level)
But but but, how can that happen? Should it not all be under the same rq->lock and thus have only a single update_rq_clock() and thus be at the same 'instant' ?
