linux-kernel - Re: scheduler / perf stat question about CPU-migrations

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Message-ID: <20110616151836.GC23624@somewhere.redhat.com>
Date:	Thu, 16 Jun 2011 17:18:39 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Pádraig Brady <P@...igBrady.com>
Cc:	Theodore Ts'o <tytso@....edu>, linux-kernel@...r.kernel.org
Subject: Re: scheduler / perf stat question about CPU-migrations

On Thu, Jun 16, 2011 at 04:11:03PM +0100, Pádraig Brady wrote:
> On 16/06/11 16:03, Frederic Weisbecker wrote:
> > On Thu, Jun 16, 2011 at 10:46:26AM -0400, Theodore Ts'o wrote:
> >> Can someone tell me how I'm being confused?
> >>
> >> I ran the following command as root:
> >>
> >> 	perf stat schedtool -a 1 -e e2fsck -ft /dev/funarg/kbuild
> > 
> > A theory is that schedtool does:
> > 
> > if (!fork()) {
> > 	set affinity there
> > 	launch e2fsck
> > }
> 
> Well `taskset` and `chrt` from util-linux don't fork at least,
> so you could try those instead.

Even if they don't fork and change their own affinity, you'll get
migrations that happened since taskset/chrt were launched.

The only solution is too set perf affinity itself:

	schedtool -a 1 -e perf stat -- e2fsck -ft /dev/funarg/kbuild

Taskset would work too.

> 
> I also suggest protecting the command with --
> in case perf is not stopping at the first unrecognized option.
> 
> cheers,
> Pádraig.
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