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

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Date:	Mon, 11 Jul 2016 20:12:24 +0800
From:	Xunlei Pang <xpang@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>, xlpang@...hat.com,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:	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

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.

Regards,
Xunlei