From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751498AbcGMRGq (ORCPT ); Wed, 13 Jul 2016 13:06:46 -0400 Received: from mail-pa0-f53.google.com ([209.85.220.53]:35459 "EHLO mail-pa0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750865AbcGMRGh (ORCPT ); Wed, 13 Jul 2016 13:06:37 -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: Wed, 13 Jul 2016 10:06:32 -0700 In-Reply-To: (Wanpeng Li's message of "Wed, 13 Jul 2016 09:50:26 +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: > 2016-07-13 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);