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

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Date:	Wed, 05 Sep 2012 14:36:56 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com, mingo@...nel.org, hpa@...or.com,
	linux-kernel@...r.kernel.org, rakib.mullick@...il.com,
	tglx@...utronix.de
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched: Fix load avg vs cpu-hotplug

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.

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