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SubjectRe: [PATCH] sched/fair: do not announce throttled next buddy in dequeue_task_fair
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Wanpeng Li <kernellwp@gmail.com> writes:

> 2016-07-14 1:06 GMT+08:00 <bsegall@google.com>:
>> Wanpeng Li <kernellwp@gmail.com> writes:
>>
>>> 2016-07-13 1:25 GMT+08:00 <bsegall@google.com>:
>>>> Konstantin Khlebnikov <khlebnikov@yandex-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@redhat.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@yandex-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.

In the other order (set_next_buddy then throttle), throttle_cfs_rq will
call dequeue which will clear the problematic buddy.

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