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

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Message-ID: <alpine.LFD.2.00.0907171441020.11571@localhost.localdomain>
Date:	Fri, 17 Jul 2009 14:48:43 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rafael Wysocki <rjw@...k.pl>, Ingo Molnar <mingo@...e.hu>,
	Nathan Lynch <ntl@...ox.com>,
	Nigel Cunningham <nigel@...onice.net>, stable@...nel.org,
	containers@...ts.linux-foundation.org,
	linux-pm@...ts.linux-foundation.org,
	Matt Helsley <matthltc@...ibm.com>
Subject: Re: [patch 2/2] sched: fix nr_uninterruptible accounting of frozen
 tasks really

On Fri, 17 Jul 2009, Peter Zijlstra wrote:

> On Fri, 2009-07-17 at 12:25 +0000, Thomas Gleixner wrote:
> > plain text document attachment (freezer-fix-accounting-for-real.patch)
> > commit e3c8ca8336 (sched: do not count frozen tasks toward load) broke
> > the nr_uninterruptible accounting on freeze/thaw. On freeze the task
> > is excluded from accounting with a check for (task->flags &
> > PF_FROZEN), but that flag is cleared before the task is thawed. So
> > while we prevent that the freezing task with state
> > TASK_UNINTERRUPTIBLE is accounted to nr_uninterruptible we decrement
> > nr_uninterruptible on thaw.
> > 
> > Use a separate flag which is handled by the freezing task itself. Set
> > it before calling the scheduler with TASK_UNINTERRUPTIBLE state and
> > clear it after we return from frozen state.
> 
> Right, so I'm wondering why we don't fully revert e3c8ca8336 to begin
> with.

Fine with me, but it seems that the cgroup folks have some luser space
stuff looking at proc/loadavg which goes berserk when loadavg
increases rapidly due to freezing. OTOH that stuff seems to be
oblivious to the fact that the commit in question brings loadavg
irreversibly to 0 when you do enough freeze/thaw cycles.

So either we revert or apply the fix, which keeps the accounting
straight and solves the freezer loadavg thing. No strong opinion on
that.

Thanks,

	tglx
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