On Thu, Jul 02, 2020 at 10:36:27PM +0100, Mel Gorman wrote: > > commit c6e7bd7afaeb3af55ffac122828035f1c01d1d7b (refs/bisect/bad) > > Author: Peter Zijlstra <[email protected]>
> Peter, I'm not supremely confident about this but could it be because > "p->sched_contributes_to_load = !!task_contributes_to_load(p)" potentially > happens while a task is still being dequeued? In the final stages of a > task switch we have > > prev_state = prev->state; > vtime_task_switch(prev); > perf_event_task_sched_in(prev, current); > finish_task(prev); > > finish_task is when p->on_cpu is cleared after the state is updated. > With the patch, we potentially update sched_contributes_to_load while > p->state is transient so if the check below is true and ttwu_queue_wakelist > is used then sched_contributes_to_load was based on a transient value > and potentially wrong. I'm not seeing it. Once a task hits schedule(), p->state doesn't change, except through wakeup. And while dequeue depends on p->state, it doesn't change it. At this point in ttwu() we know p->on_rq == 0, which implies dequeue has started, which means we've (at least) stopped executing the task -- we started or finished schedule(). Let me stare at this more...
