From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751262AbcGNMLG (ORCPT ); Thu, 14 Jul 2016 08:11:06 -0400 Received: from mail-oi0-f67.google.com ([209.85.218.67]:33515 "EHLO mail-oi0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750966AbcGNMLD (ORCPT ); Thu, 14 Jul 2016 08:11:03 -0400 MIME-Version: 1.0 In-Reply-To: References: <146608183552.21905.15924473394414832071.stgit@buzz> <57835735.6020906@redhat.com> <57835BFD.90201@redhat.com> <57838D28.4090003@redhat.com> <5783907A.6030609@yandex-team.ru> From: Wanpeng Li Date: Thu, 14 Jul 2016 20:11:01 +0800 Message-ID: Subject: Re: [PATCH] sched/fair: do not announce throttled next buddy in dequeue_task_fair To: Benjamin Segall Cc: Konstantin Khlebnikov , xlpang@redhat.com, Peter Zijlstra , Ingo Molnar , "linux-kernel@vger.kernel.org" , stable@vger.kernel.org Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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