2016-07-15 1:54 GMT+08:00 <bseg...@google.com>: > Wanpeng Li <kernel...@gmail.com> writes: > >> 2016-07-14 1:06 GMT+08:00 <bseg...@google.com>: >>> Wanpeng Li <kernel...@gmail.com> writes: >>> >>>> 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 >>> >>> 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 > > They don't race or anything, everything's under rq->lock. > throttled_hierarchy will register properly, the issue is that a parent > is the one cfs_rq_throttled(), not the current cfs_rq, and > set_next_buddy will set cfs_rq->next to an se that is !on_rq.
Why b is !on_rq after throttle a? Regards, Wanpeng Li
