linux-kernel - Re: [RFC PATCH 0/2] perf_events: add support for per-cpu per-cgroup monitoring (v3)

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Date:	Tue, 21 Sep 2010 16:03:21 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, fweisbec@...il.com,
	perfmon2-devel@...ts.sf.net, eranian@...il.com,
	robert.richter@....com, acme@...hat.com,
	Paul Menage <menage@...gle.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 0/2] perf_events: add support for per-cpu
 per-cgroup monitoring (v3)

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/ :-)
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