From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752023AbcGMB7X (ORCPT ); Tue, 12 Jul 2016 21:59:23 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49818 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751490AbcGMB7M (ORCPT ); Tue, 12 Jul 2016 21:59:12 -0400 Reply-To: xlpang@redhat.com 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> To: Wanpeng Li , Benjamin Segall Cc: Konstantin Khlebnikov , xlpang@redhat.com, Peter Zijlstra , Ingo Molnar , "linux-kernel@vger.kernel.org" , stable@vger.kernel.org From: Xunlei Pang Message-ID: <5785A045.60505@redhat.com> Date: Wed, 13 Jul 2016 09:58:29 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Wed, 13 Jul 2016 01:58:33 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2016/07/13 at 09:50, Wanpeng Li wrote: > 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? cfs_rq_throttled() returns false for child cgroups in the throttled hierarchy, so throttled_hierarchy() should be relied on in such cases. Regards, Xunlei