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Date: Tue, 8 Sep 2015 16:04:49 +0800 From: Wanpeng Li <wanpeng.li@...mail.com> To: Byungchul Park <byungchul.park@....com> CC: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org, yuyang.du@...el.com Subject: Re: [PATCH] sched: fix lose fair sleeper bonus in switch_to_fair() On 9/8/15 3:57 PM, Byungchul Park wrote: > On Tue, Sep 08, 2015 at 03:30:08PM +0800, Wanpeng Li wrote: >> On 9/8/15 3:11 PM, Byungchul Park wrote: >>> On Tue, Sep 08, 2015 at 02:42:52PM +0800, Wanpeng Li wrote: >>>> On 9/8/15 2:32 PM, Byungchul Park wrote: >>>>> On Tue, Sep 08, 2015 at 03:14:26PM +0900, Byungchul Park wrote: >>>>>> On Tue, Sep 08, 2015 at 01:38:08PM +0800, Wanpeng Li wrote: >>>>>>> On 9/8/15 1:28 PM, Byungchul Park wrote: >>>>>>>> On Tue, Sep 08, 2015 at 11:46:01AM +0800, Wanpeng Li wrote: >>>>>>>>> On 9/7/15 10:02 PM, Peter Zijlstra wrote: >>>>>>>>>> Please always Cc at least the person who wrote the lines you modify. >>>>>>>>>> >>>>>>>>>> On Mon, Sep 07, 2015 at 05:45:20PM +0800, Wanpeng Li wrote: >>>>>>>>>>> The sleeper task will be normalized when moved from fair_sched_class, in >>>>>>>>>>> order that vruntime will be adjusted either the task is running or sleeping >>>>>>>>>>> when moved back. The nomalization in switch_to_fair for sleep task will >>>>>>>>>>> result in lose fair sleeper bonus in place_entity() once the vruntime - >>>>>>>>>>> cfs_rq->min_vruntime is big when moved from fair_sched_class. >>>>>>>>>>> >>>>>>>>>>> This patch fix it by adjusting vruntime just during migrating as original >>>>>>>>>>> codes since the vruntime of the task has usually NOT been normalized in >>>>>>>>>>> this case. >>>>>>>>>> Sorry, I cannot follow that at all. Maybe its me being sleep deprived, >>>>>>>>>> but could you try that again? >>>>>>>>> When changing away from the fair class while sleeping, relative >>>>>>>>> vruntime is calculated to handle the case sleep when moved from >>>>>>>>> fair_sched_class and running when moved to fair_sched_class. The >>>>>>>> i don't think relative vruntime is calculated to handle the special case >>>>>>>> you mentioned. i think the calculation is necessary for all cases detaching >>>>>>> Please refer why the relative vruntime caculation is introduced to >>>>>>> switched_from_fair(): https://lkml.org/lkml/2011/1/17/129 >>>>>> hello, >>>>>> >>>>>> it is just a bug caused by not calculating a relative vruntime when >>>>>> detached a task from cfs_rq, which is necessary though. >>>>>> >>>>>>>> a task from a cfs_rq. >>>>>>>> >>>>>>>>> absolute vruntime will be calculated in enqueue_entity() either the >>>>>>>>> task is running or sleeping when moved back. The fair sleeper bonus >>>>>>>> i think absolute vruntime is calculated in enqueue_entuty() only when the >>>>>>>> task is on rq. therefore in the case that the task is not on rq, >>>>>>>> switched_to_fair() has to calculate the absolute vruntime instread. >>>>>>> Absolute vruntime is caculated in place_entity() which is called by >>>>>>> enqueue_entity() for DEQUEUE_SLEEP task. >>>>>> as you may know, place_entity() is not for calculating an absolute >>>>>> vruntime though.. anyway the important thing here is that, when a >>>>>> sleeping task is moved back to fair class, enqueue_entity() for >>>>>> DEQUEUE_SLEEP task won't be called. >>>>> you may talk about calling enqueue_entity() when the task is woken up, >>>>> not just when it is moved back. right? >>>> Exactly. >>>> >>>>> even if yes, i think place_entity() should not be used directly for >>>>> calculating an absolute vruntime. it should be called after non/normalizing >>>>> operations. >>>> The se->vruntime += cfs_rq->min_vruntime(in your switched_to_fair()) >>>> which means that se->vruntime is bigger than cfs_rq->min_vruntime, >>> it is not always true since se->vruntime can have a negative value (even >>> though it is a unsigned type.. i think it can be another problem) by >>> se->vruntime -= cfs_rq->min_vruntime in detach_task_cfs_rq(). >> Yeah, it can be negative. >> >>>> however, fair sleeper bonus is min_vuntime - sysctl_sched_latency/2, >>>> which means that max_vruntime() will select the absolute vruntime >>>> which is caculated in your switched_to_fair() as the se->vruntime, >>> since se->vruntime can have a negative value, max_vruntime() may select >>> the fair sleeper bonused value. >>> >>> by the way, this logic is unchanged by my patch. which part of my patch >>> changed this kind of logic? >> However, if se->vruntime -= cfs_rq->min_vruntime is positive, the >> behavior is different after your patch. e.g. se->vruntime(the >> relative vruntime in switched_to_fair()) < min_vruntime - >> sysctl_sched_latency/2 >> >> before your patch: >> >> se->vruntime = min_vruntime - sysctl_sched_latency/2 (place_entity()) > my patch is based on ff277d4 commit at tip/sched/core. > > there's no change between before and after. > > check it please. > > and this logic seems to be no problem to me. :( Your logic will lose fair sleeper bonus in the scenario which I pointed out. > >> after your patch: >> >> se->vruntime += cfs->min_vruntime (switched_to_fair()) >> se->vruntime = se->vruntime (place_entity()) >> >> >> Regards, >> Wanpeng Li >> >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in >> the body of a message to majordomo@...r.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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