On Tue, Apr 05, 2016 at 09:24:24PM +0200, luca abeni wrote: > On Tue, 5 Apr 2016 20:02:52 +0200 > Peter Zijlstra <[email protected]> wrote: > > > On Tue, Apr 05, 2016 at 07:56:57PM +0200, luca abeni wrote: > > > > > > > + migrate_active = hrtimer_active(&p->dl.inactive_timer); > > > > > + if (migrate_active) > > > > > + sub_running_bw(&p->dl, &rq->dl); > > > > > + raw_spin_unlock(&rq->lock); > > > > > > > > At this point task_rq() is still the above rq, so if the inactive timer > > > > hits here it will lock this rq and subtract the running bw here _again_, > > > > right? > > > I think it will see the task state as TASK_RUNNING, so it will do nothing. > > > Or it will cancelled later when the task is enqueued... I'll double check > > > this. > > > > Right, so this is select_task_rq_dl(), we run this in wakeups, before > > TASK_RUNNING. > > Sigh... I knew I was missing something here... :( > So, I think the solution here is to use double_lock_balance() (or something > like that) to take both the rq locks so that the inactive timer handler cannot > run between sub_running_bw() and add_running_bw()... I'll try this.
I'm not sure that'll fix it, because after you unlock both again, we can hit after, and there task_rq() will still be the first rq, not the second. So we again subtract twice from the old rq. Only after __set_task_cpu()'s store to task_thread_info(p)->cpu will the timer hit the new rq. And you cannot hold a lock over that..
