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

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Date:	Wed, 13 Jul 2016 10:06:32 -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:

> 2016-07-13 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);