Re: [PATCH] sched/fair: do not announce throttled next buddy in dequeue_task_fair

From: Wanpeng Li
Date: Tue Jul 12 2016 - 22:15:23 EST

2016-07-13 9:58 GMT+08:00 Xunlei Pang <xpang@xxxxxxxxxx>:
> On 2016/07/13 at 09:50, Wanpeng Li wrote:
>> 2016-07-13 1:25 GMT+08:00 <bsegall@xxxxxxxxxx>:
>>> Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> 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@xxxxxxxxxx>:
>>>>>>> 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@xxxxxxxxxxxxxx>:
>>>>>>>>>> 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?
> cfs_rq_throttled() returns false for child cgroups in the throttled hierarchy, so
> throttled_hierarchy() should be relied on in such cases.

Yes, so what's lost in bsegall's reply?

Wanpeng Li