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

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