Messages in this thread | | | Date | Tue, 22 Mar 2022 15:54:20 +0100 | From | Peter Zijlstra <> | Subject | Re: [External] Re: [PATCH v2 1/6] perf/core: Fix incosistency between cgroup sched_out and sched_in |
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On Tue, Mar 22, 2022 at 09:38:21PM +0800, Chengming Zhou wrote: > On 2022/3/22 8:59 下午, Peter Zijlstra wrote: > > On Tue, Mar 22, 2022 at 08:08:29PM +0800, Chengming Zhou wrote: > >> There is a race problem that can trigger WARN_ON_ONCE(cpuctx->cgrp) > >> in perf_cgroup_switch(). > >> > >> CPU1 CPU2 > >> (in context_switch) (attach running task) > >> perf_cgroup_sched_out(prev, next) > >> cgrp1 == cgrp2 is True > >> next->cgroups = cgrp3 > >> perf_cgroup_attach() > >> perf_cgroup_sched_in(prev, next) > >> cgrp1 == cgrp3 is False > >> > >> The commit a8d757ef076f ("perf events: Fix slow and broken cgroup > >> context switch code") would save cpuctx switch out/in when the > >> perf_cgroup of "prev" and "next" are the same. > >> > >> But perf_cgroup of task can change in concurrent with context_switch. > > > > Can you clarify? IIRC then a task changes cgroup it goes throught the > > whole ->attach() dance, and that serializes against the context switch > > code. > > > > task->cgroups changed before perf_cgroup_attach(), and is not serialized > against the context switch, since task->cgroups can be changed without > rq lock held. (cgroup v1 or cgroup v2 with PSI disabled) > > So perf_cgroup_from_task() in perf_cgroup_switch() may see the old or > new perf_cgroup when do context switch.
__schedule() local_irq_disable(); <--- IRQ disable rq_lock();
...
context_switch() prepare_task_switch() perf_event_task_sched_out() __perf_event_task_sched_out() perf_cgroup_sched_out();
switch_to() finish_task_switch() perf_event_task_sched_in() __perf_event_task_sched_in() perf_cgroup_sched_in(); finish_lock_switch() raw_spin_irq_unlock_irq(); <--- IRQ enable
vs
perf_event_cgrp_subsys.attach = perf_cgroup_attach() cgroup_taskset_for_each() task_function_call(task, __perf_cgroup_move) <--- sends IPI
Please explain how this can interleave.
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