Messages in this thread | | | Date | Tue, 22 Mar 2022 21:38:21 +0800 | Subject | Re: [External] Re: [PATCH v2 1/6] perf/core: Fix incosistency between cgroup sched_out and sched_in | From | Chengming Zhou <> |
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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.
Thanks.
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