linux-kernel - Re: [PATCH] sched/fair: do not announce throttled next buddy in dequeue_task

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Date:	Thu, 14 Jul 2016 15:49:10 -0700
From:	bsegall@...gle.com
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	xlpang@...hat.com, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	stable@...r.kernel.org
Subject: Re: [PATCH] sched/fair: do not announce throttled next buddy in dequeue_task_fair

Wanpeng Li <kernellwp@...il.com> writes:

> 2016-07-15 1:54 GMT+08:00  <bsegall@...gle.com>:
>> Wanpeng Li <kernellwp@...il.com> writes:
>>
>>> 2016-07-14 1:06 GMT+08:00  <bsegall@...gle.com>:
>>>> Wanpeng Li <kernellwp@...il.com> writes:
>>>>
>>>>> 2016-07-13 1:25 GMT+08:00  <bsegall@...gle.com>:
>>>>>> Konstantin Khlebnikov <khlebnikov@...dex-team.ru> 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 <xpang@...hat.com>:
>>>>>>>>>> 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 <khlebnikov@...dex-team.ru>:
>>>>>>>>>>>>> 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.