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

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Message-ID: <CANRm+CwLaCqVKK=EoXqMtMQCGO-DDzJm7dEeV0BMn6ew86ZpOw@mail.gmail.com>
Date:	Thu, 14 Jul 2016 20:11:01 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Benjamin Segall <bsegall@...gle.com>
Cc:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	xlpang@...hat.com, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.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

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