From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753170AbbIHHaT (ORCPT ); Tue, 8 Sep 2015 03:30:19 -0400 Received: from blu004-omc1s28.hotmail.com ([65.55.116.39]:59512 "EHLO BLU004-OMC1S28.hotmail.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751803AbbIHHaQ (ORCPT ); Tue, 8 Sep 2015 03:30:16 -0400 X-TMN: [pYhp8jj5XOGPafESwNG1e+jfY50Z7Cn2] X-Originating-Email: [wanpeng.li@hotmail.com] Message-ID: Subject: Re: [PATCH] sched: fix lose fair sleeper bonus in switch_to_fair() To: Byungchul Park References: <20150907140221.GZ3644@twins.programming.kicks-ass.net> <20150908052817.GD6455@byungchulpark-X58A-UD3R> <20150908061426.GE6455@byungchulpark-X58A-UD3R> <20150908063240.GF6455@byungchulpark-X58A-UD3R> <20150908071139.GI6455@byungchulpark-X58A-UD3R> CC: Peter Zijlstra , Ingo Molnar , linux-kernel@vger.kernel.org, yuyang.du@intel.com From: Wanpeng Li Date: Tue, 8 Sep 2015 15:30:08 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: <20150908071139.GI6455@byungchulpark-X58A-UD3R> Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit X-OriginalArrivalTime: 08 Sep 2015 07:30:13.0620 (UTC) FILETIME=[335E4B40:01D0EA08] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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()) after your patch: se->vruntime += cfs->min_vruntime (switched_to_fair()) se->vruntime = se->vruntime (place_entity()) Regards, Wanpeng Li