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From: Chengming Zhou <zhouchengming@bytedance.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@redhat.com, acme@kernel.org, mark.rutland@arm.com,
	alexander.shishkin@linux.intel.com, jolsa@kernel.org,
	namhyung@kernel.org, eranian@google.com,
	linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	duanxiongchun@bytedance.com, songmuchun@bytedance.com
Subject: Re: [External] Re: [PATCH v2 1/6] perf/core: Fix incosistency between cgroup sched_out and sched_in
Date: Tue, 22 Mar 2022 21:38:21 +0800	[thread overview]
Message-ID: <b55676c3-07da-f4f1-e4c8-252cd9d4fac2@bytedance.com> (raw)
In-Reply-To: <YjnIKCIRV+ePJVCN@hirez.programming.kicks-ass.net>

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.

  reply	other threads:[~2022-03-22 13:38 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-03-22 12:08 [PATCH v2 0/6] perf/core: Fixes and cleanup for cgroup events Chengming Zhou
2022-03-22 12:08 ` [PATCH v2 1/6] perf/core: Fix incosistency between cgroup sched_out and sched_in Chengming Zhou
2022-03-22 12:59   ` Peter Zijlstra
2022-03-22 13:38     ` Chengming Zhou [this message]
2022-03-22 14:54       ` [External] " Peter Zijlstra
2022-03-22 15:16         ` Chengming Zhou
2022-03-22 15:28           ` Chengming Zhou
2022-03-22 22:06             ` Namhyung Kim
2022-03-23  8:11             ` Peter Zijlstra
2022-03-22 12:08 ` [PATCH v2 2/6] perf/core: Introduce percpu perf_cgroup Chengming Zhou
2022-03-22 13:01   ` Peter Zijlstra
2022-03-22 16:33     ` [External] " Chengming Zhou
2022-03-23  8:13       ` Peter Zijlstra
2022-03-23 12:58         ` Chengming Zhou
2022-03-22 22:21     ` Namhyung Kim
2022-03-22 22:18   ` Namhyung Kim
2022-03-23  1:27     ` [Phishing Risk] [External] " Chengming Zhou
2022-03-23 12:51   ` Peter Zijlstra
2022-03-23 13:07     ` [External] " Chengming Zhou
2022-03-23 13:17       ` Peter Zijlstra
2022-03-23 13:37         ` Chengming Zhou
2022-03-23 14:05           ` Peter Zijlstra
2022-03-23 15:44             ` Chengming Zhou
2022-03-22 12:08 ` [PATCH v2 3/6] perf/core: Don't pass task around when ctx sched in Chengming Zhou
2022-03-22 13:01   ` Peter Zijlstra
2022-03-22 12:08 ` [PATCH v2 4/6] perf/core: Use stable cpuctx->cgrp when update perf cgroup time Chengming Zhou
2022-03-22 13:03   ` Peter Zijlstra
2022-03-22 12:08 ` [PATCH v2 5/6] perf/core: Always set cpuctx cgrp when enable cgroup event Chengming Zhou
2022-03-22 12:08 ` [PATCH v2 6/6] perf/core: Don't need event_filter_match when merge_sched_in() Chengming Zhou

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