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Subject: Re: [RFC PATCH 0/2] perf_events: add support for per-cpu
 per-cgroup monitoring (v3)
From: Peter Zijlstra <peterz@infradead.org>
To: Stephane Eranian <eranian@google.com>
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, paulus@samba.org,
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        Balbir Singh <balbir@linux.vnet.ibm.com>
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Date: Tue, 21 Sep 2010 16:03:21 +0200
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On Tue, 2010-09-21 at 15:38 +0200, Stephane Eranian wrote:
> > Hmm, indeed. One thing we can do about that is move perf into the
> > cgroup, create the counter (disabled) using self to identify the cgroup,
> > move perf back to where it came from, and enable the counter.
> >
> Yes, that's another possibility. I wonder if there are any non-obvious
> difficulties with this approach.

Yes, there is, but I think we can fix it. The problem with moving perf
itself around is that perf is not a fully dormant process and can thus
interact with the cgroup state.

If we were to fork a child that's simply sitting idle in waitpid() (or
any other blocking syscall) we can move that around cgroup without
affecting the cgroup itself.

>  Is it as simple as:
>    FILE *fp;
>    fp = fopen("/dev/cgroup/test/tasks", "w");
>    fprintf(fp, "%d", gettid());
>    close(fp): 

Except I've never in my life mounted a cgroup filesystem in /dev/ :-)