linux-kernel - Re: [External] Re: [PATCH v2 2/6] perf/core: Introduce percpu perf

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Message-ID: <YjrWxnnIZvn3PWoX@hirez.programming.kicks-ass.net>
Date:   Wed, 23 Mar 2022 09:13:58 +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: [External] Re: [PATCH v2 2/6] perf/core: Introduce percpu
 perf_cgroup

On Wed, Mar 23, 2022 at 12:33:51AM +0800, Chengming Zhou wrote:
> On 2022/3/22 9:01 下午, Peter Zijlstra wrote:
> > On Tue, Mar 22, 2022 at 08:08:30PM +0800, Chengming Zhou wrote:
> >> Although we don't have incosistency problem any more, we can
> >> have other problem like:
> >>
> >> CPU1					CPU2
> >> (in context_switch)			(attach running task)
> >> 					prev->cgroups = cgrp2
> >> perf_cgroup_sched_switch(prev, next)
> >> 	cgrp2 == cgrp2 is True
> >>
> > 
> > Again, I'm not following, how can you attach to a running task from
> > another CPU ?
> 
> Hi Peter, I make a little testcase which can reproduce the race
> problem, on system with PSI disabled. Because when PSI enabled,
> cgroup_move_task() will hold rq lock to assign task->cgroups.

No, the problem is that you're talking about cgroup attach while I'm
thinking of attaching a event to a task. And your picture has nothing to
clarify.

Those pictures of yours could really do with a few more function names
in them, otherwise it's absolutely unclear what code is running where.