linux-kernel - Re: [tip:sched/core] sched: Fix load avg vs cpu-hotplug

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Message-ID: <20120905132939.GA3480@gmail.com>
Date:	Wed, 5 Sep 2012 15:29:39 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	paulmck@...ux.vnet.ibm.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, rakib.mullick@...il.com,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched: Fix load avg vs cpu-hotplug


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, 2012-09-04 at 11:43 -0700, tip-bot for Peter Zijlstra wrote:
> > Commit-ID:  f319da0c6894fcf55e21320e40506418a2aad629
> > Gitweb:     http://git.kernel.org/tip/f319da0c6894fcf55e21320e40506418a2aad629
> > Author:     Peter Zijlstra <peterz@...radead.org>
> > AuthorDate: Mon, 20 Aug 2012 11:26:57 +0200
> > Committer:  Ingo Molnar <mingo@...nel.org>
> > CommitDate: Tue, 4 Sep 2012 14:30:18 +0200
> > 
> > sched: Fix load avg vs cpu-hotplug
> > 
> > Rabik and Paul reported two different issues related to the same few
> > lines of code.
> > 
> > Rabik's issue is that the nr_uninterruptible migration code is wrong in
> > that he sees artifacts due to this (Rabik please do expand in more
> > detail).
> > 
> > Paul's issue is that this code as it stands relies on us using
> > stop_machine() for unplug, we all would like to remove this assumption
> > so that eventually we can remove this stop_machine() usage altogether.
> > 
> > The only reason we'd have to migrate nr_uninterruptible is so that we
> > could use for_each_online_cpu() loops in favour of
> > for_each_possible_cpu() loops, however since nr_uninterruptible() is the
> > only such loop and its using possible lets not bother at all.
> > 
> > The problem Rabik sees is (probably) caused by the fact that by
> > migrating nr_uninterruptible we screw rq->calc_load_active for both rqs
> > involved.
> > 
> > So don't bother with fancy migration schemes (meaning we now have to
> > keep using for_each_possible_cpu()) and instead fold any nr_active delta
> > after we migrate all tasks away to make sure we don't have any skewed
> > nr_active accounting.
> 
> Oh argh.. this patch isn't actually right.. I actually removed 
> it from my series but forgot to update the tarball.

Sigh.

> Ingo can you still make it go away or should I do a delta?

Please do a delta.

Thanks,

	Ingo
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