linux-kernel - Re: [patch 2/2] sched: fix nr_uninterruptible accounting of frozen tasks really

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Date:	Sat, 18 Jul 2009 14:56:31 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Nathan Lynch <ntl@...ox.com>
Cc:	Matt Helsley <matthltc@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rafael Wysocki <rjw@...k.pl>, Ingo Molnar <mingo@...e.hu>,
	Nigel Cunningham <nigel@...onice.net>, stable@...nel.org,
	containers@...ts.linux-foundation.org,
	linux-pm@...ts.linux-foundation.org
Subject: Re: [patch 2/2] sched: fix nr_uninterruptible accounting of frozen
 tasks really

On Fri, 2009-07-17 at 15:55 -0500, Nathan Lynch wrote:
> On Fri, 2009-07-17 at 18:47 +0200, Peter Zijlstra wrote:
> > On Fri, 2009-07-17 at 08:22 -0700, Matt Helsley wrote:
> > 
> > > The job scheduler in question does not use FROZEN as a transient state and
> > > does not use checkpoint/restart at all since c/r is still a work in progress.
> 
> Right, the job scheduler uses the cgroup freezer as a mechanism to
> preempt a low priority job for a higher priority job.  (It had used
> SIGSTOP in the past.)  So in this scenario a frozen cgroup may remain in
> that state for a while.  Load average is consulted as a measure of
> system utilization.

I think that this is an utterly broken use for it, if you want something
like that make a signal cgroup or something and deliver SIGSTOP to all
of them.

In other words, why is the freezer any better than the SIGSTOP approach?

> > > Even when used for power management it seems wrong to count frozen tasks
> > > towards the loadavg since they aren't using CPU time or waiting for IO.
> > 
> > You're abusing it for _WHAT_?
> 
> I think Matt was referring to system-wide suspend/resume/hibernate, not
> a behavior of the job scheduler, if that's your concern.

I understood he referred to the crazy use-case you mentioned above, IMHO
frozen should be a temporary state used for things like
snapshot/migrate.

I'm still very tempted to plain simply revert that original patch.

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