Messages in this thread | | | From | Vincent Guittot <> | Date | Tue, 4 Jul 2017 11:57:12 +0200 | Subject | Re: [PATCH v2] sched/pelt: fix false running accounting |
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On 4 July 2017 at 11:44, Peter Zijlstra <peterz@infradead.org> wrote: > On Tue, Jul 04, 2017 at 11:12:34AM +0200, Vincent Guittot wrote: >> On 4 July 2017 at 10:34, Peter Zijlstra <peterz@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' ?
idle_balance() unlock rq->lock before calling update_blocked_averages And update_blocked_averages() starts by calling update_rq_clock()
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