Messages in this thread | | | Date | Tue, 26 Sep 2017 09:23:20 +0800 | From | zhouchengming <> | Subject | Re: [PATCH] sched/rt.c: pick and check task if double_lock_balance() unlock the rq |
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On 2017/9/26 3:40, Steven Rostedt wrote: > On Mon, 11 Sep 2017 14:51:49 +0800 > Zhou Chengming<zhouchengming1@huawei.com> wrote: > >> push_rt_task() pick the first pushable task and find an eligible >> lowest_rq, then double_lock_balance(rq, lowest_rq). So if >> double_lock_balance() unlock the rq (when double_lock_balance() return 1), >> we have to check if this task is still on the rq. >> >> The problem is that the check conditions are not sufficient: >> >> if (unlikely(task_rq(task) != rq || >> !cpumask_test_cpu(lowest_rq->cpu,&task->cpus_allowed) || >> task_running(rq, task) || >> !rt_task(task) || >> !task_on_rq_queued(task))) { >> >> cpu2 cpu1 cpu0 >> push_rt_task(rq1) >> pick task_A on rq1 >> find rq0 >> double_lock_balance(rq1, rq0) >> unlock(rq1) >> rq1 __schedule >> pick task_A run >> task_A sleep (dequeued) >> lock(rq0) >> lock(rq1) >> do_above_check(task_A) >> task_rq(task_A) == rq1 >> cpus_allowed unchanged >> task_running == false >> rt_task(task_A) == true >> try_to_wake_up(task_A) >> select_cpu = cpu3 >> enqueue(rq3, task_A) > How can this happen? The try_to_wake_up(task_A) needs to grab the rq > that task A is on, and we have that rq lock. > > /me confused. > > -- Steve
Thanks for the reply! After the task_A sleep on cpu1, the try_to_wake_up(task_A) on cpu0 select a different cpu3, so it will grab the rq3 lock, not the rq1 lock.
Thanks.
> >> task_A->on_rq = 1 >> task_on_rq_queued(task_A) >> above_check passed, return rq0 >> ... >> migrate task_A from rq1 to rq0 >> >> So we can't rely on these checks of task_A to make sure the task_A is >> still on the rq1, even though we hold the rq1->lock. This patch will >> repick the first pushable task to be sure the task is still on the rq. >> >> Signed-off-by: Zhou Chengming<zhouchengming1@huawei.com> >> > . >
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