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    SubjectRe: [PATCH] sched/fair: do not announce throttled next buddy in dequeue_task_fair
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    On 2016/07/13 at 09:50, Wanpeng Li wrote:
    > 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?

    cfs_rq_throttled() returns false for child cgroups in the throttled hierarchy, so
    throttled_hierarchy() should be relied on in such cases.

    Regards,
    Xunlei

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