On 26.07.2014 23:39, Oleg Nesterov wrote: > Hi Kirill, > > I'll try to read this series later, just one silly question for now. > > On 07/26, Kirill Tkhai wrote: >> >> Patch [2/5] is main in the series. It introduces new state: ONRQ_MIGRATING >> and teaches scheduler to understand it (we need a little changes >> predominantly >> in try_to_wake_up()). This will be used in the following way: >> >> (we are changing task's rq) >> >> raw_spin_lock(&src_rq->lock); >> dequeue_task(src_rq, p, 0); >> p->on_rq = ONRQ_MIGRATING; >> set_task_cpu(p, dst_cpu); >> raw_spin_unlock(&src_rq->lock); >> >> raw_spin_lock(&dst_rq->lock); >> p->on_rq = ONRQ_QUEUED; >> enqueue_task(dst_rq, p, 0); >> raw_spin_unlock(&dst_rq->lock); > > Hmm. And what if the code above doesn't hold p->pi_lock (4/5) and, say, > __sched_setscheduler() does fair_sched_class->rt_sched_class transition > in between? > > ONRQ_MIGRATING helps to avoid the wrong dequeue + enqueue, but I am not > sure about check_class_changed(). > > Say, switched_from_fair() will use dst_rq even if p was never queued on > this rq... This only affects the .decay_count logic, perhaps this is fine, > I simply do not know what this code does.
You're right. We have to check for "task_migrating" in switched_from_fair(). One more place is switched_from_dl(). > What about switched_to_rt() ? we lose the push_rt_task() logic... Hmm, > which I can't understand too ;) > > And we also lose ENQUEUE_HEAD in this case, but this looks fine. > > In short: could you confirm there are no problems here? This will be the reason of some RT/DL imbalance. We need a method how to avoid this. Maybe, it would be good to call something like check_class_changed() at the end of migration process. We just need to save task's class before migration and compare with the class after migration (for [3/5], __migrate_task()). For [4/5] and [5/5] the class is always fair_sched_class. Thanks for the comments. I'll think how to fix this in a good way, and update the series. Kirill -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
