Re: [External] Re: [PATCH v2 1/6] perf/core: Fix incosistency between cgroup sched_out and sched_in

[Chengming Zhou <zhouchengming@bytedance.com>]

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.