From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752160AbcGNWtQ (ORCPT ); Thu, 14 Jul 2016 18:49:16 -0400 Received: from mail-pa0-f41.google.com ([209.85.220.41]:32978 "EHLO mail-pa0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751512AbcGNWtN (ORCPT ); Thu, 14 Jul 2016 18:49:13 -0400 From: bsegall@google.com To: Wanpeng Li Cc: Konstantin Khlebnikov , xlpang@redhat.com, Peter Zijlstra , Ingo Molnar , "linux-kernel\@vger.kernel.org" , stable@vger.kernel.org Subject: Re: [PATCH] sched/fair: do not announce throttled next buddy in dequeue_task_fair References: <146608183552.21905.15924473394414832071.stgit@buzz> <57835735.6020906@redhat.com> <57835BFD.90201@redhat.com> <57838D28.4090003@redhat.com> <5783907A.6030609@yandex-team.ru> Date: Thu, 14 Jul 2016 15:49:10 -0700 In-Reply-To: (Wanpeng Li's message of "Fri, 15 Jul 2016 06:30:22 +0800") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Wanpeng Li writes: > 2016-07-15 1:54 GMT+08:00 : >> Wanpeng Li writes: >> >>> 2016-07-14 1:06 GMT+08:00 : >>>> Wanpeng Li writes: >>>> >>>>> 2016-07-13 1:25 GMT+08:00 : >>>>>> Konstantin Khlebnikov writes: >>>>>> >>>>>>> On 11.07.2016 15:12, Xunlei Pang wrote: >>>>>>>> On 2016/07/11 at 17:54, Wanpeng Li wrote: >>>>>>>>> Hi Konstantin, Xunlei, >>>>>>>>> 2016-07-11 16:42 GMT+08:00 Xunlei Pang : >>>>>>>>>> On 2016/07/11 at 16:22, Xunlei Pang wrote: >>>>>>>>>>> On 2016/07/11 at 15:25, Wanpeng Li wrote: >>>>>>>>>>>> 2016-06-16 20:57 GMT+08:00 Konstantin Khlebnikov : >>>>>>>>>>>>> Hierarchy could be already throttled at this point. Throttled next >>>>>>>>>>>>> buddy could trigger null pointer dereference in pick_next_task_fair(). >>>>>>>>>>>> There is cfs_rq->next check in pick_next_entity(), so how can null >>>>>>>>>>>> pointer dereference happen? >>>>>>>>>>> I guess it's the following code leading to a NULL se returned: >>>>>>>>>> s/NULL/empty-entity cfs_rq se/ >>>>>>>>>> >>>>>>>>>>> pick_next_entity(): >>>>>>>>>>> if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) >>>>>>>>> ^^^^^^^^^^^^^ >>>>>>>>> I think this will return false. >>>>>>>> >>>>>>>> With the wrong throttled_hierarchy(), I think this can happen. But after we have the >>>>>>>> corrected throttled_hierarchy() patch, I can't see how it is possible. >>>>>>>> >>>>>>>> dequeue_task_fair(): >>>>>>>> if (task_sleep && parent_entity(se)) >>>>>>>> set_next_buddy(parent_entity(se)); >>>>>>>> >>>>>>>> How does dequeue_task_fair() with DEQUEUE_SLEEP set(true task_sleep) happen to a throttled hierarchy? >>>>>>>> IOW, a task belongs to a throttled hierarchy is running? >>>>>>>> >>>>>>>> Maybe Konstantin knows the reason. >>>>>>> >>>>>>> This function (dequeue_task_fair) check throttling but at point it could skip several >>>>>>> levels and announce as next buddy actually throttled entry. >>>>>>> Probably this bug hadn't happened but this's really hard to prove that this is impossible. >>>>>>> ->set_curr_task(), PI-boost or some tricky migration in balancer could break this easily. >>>>>> >>>>>> sched_setscheduler can call put_prev_task, which then can cause a >>>>>> throttle outside of __schedule(), then the task blocks normally and >>>>>> deactivate_task(DEQUEUE_SLEEP) happens and you lose. >>>>> >>>>> The cfs_rq_throttled() check in dequeue_task_fair() will capture the >>>>> cfs_rq which is throttled in sched_setscheduler::put_prev_task path, >>>>> so nothing lost, where I miss? >>>>> >>>>> Regards, >>>>> Wanpeng Li >>>> >>>> The cfs_rq_throttled() checks there are done bottom-up, so they will >>>> trigger too late. a/b/t, where t is descheduling and a is throttled can >>>> still cause a set_next_buddy(b); >>> >>> throttle cfs_rq is up-bottom, so when a is throttled, b and c are not >>> yet, then task_sleep && se && !throttled_hierarchy(cfs_rq) still can't >>> prevent a set_next_buddy(b). >>> >>> Regards, >>> Wanpeng Li >> >> They don't race or anything, everything's under rq->lock. >> throttled_hierarchy will register properly, the issue is that a parent >> is the one cfs_rq_throttled(), not the current cfs_rq, and >> set_next_buddy will set cfs_rq->next to an se that is !on_rq. > > Why b is !on_rq after throttle a? > > Regards, > Wanpeng Li a is !on_rq (because of throttle), but set_next_buddy will set ->next up the entire tree.