linux-kernel - Re: [PATCH v2 1/6] perf/core: Fix incosistency between cgroup sched_out and sched

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Message-ID: <YjnIKCIRV+ePJVCN@hirez.programming.kicks-ass.net>
Date:   Tue, 22 Mar 2022 13:59:20 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Chengming Zhou <zhouchengming@...edance.com>
Cc:     mingo@...hat.com, acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
        namhyung@...nel.org, eranian@...gle.com,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        duanxiongchun@...edance.com, songmuchun@...edance.com
Subject: Re: [PATCH v2 1/6] perf/core: Fix incosistency between cgroup
 sched_out and sched_in

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.