2016-07-13 1:25 GMT+08:00 <bseg...@google.com>: > Konstantin Khlebnikov <khlebni...@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 <xp...@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 >>>>>>> <khlebni...@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
