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

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Date:   Wed, 23 Mar 2022 20:58:03 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 2022/3/23 4:13 下午, Peter Zijlstra wrote:
> 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.

Sorry for the confusion ;-)
I will draw a better picture including more function names in the next version.

Thanks.