linux-kernel - Re: false nr_running check in load balance?

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Date:	Thu, 15 Aug 2013 19:39:53 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Turner <pjt@...gle.com>
Cc:	Lei Wen <adrian.wenl@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Lei Wen <leiwen@...vell.com>
Subject: Re: false nr_running check in load balance?

On Tue, Aug 13, 2013 at 01:08:17AM -0700, Paul Turner wrote:
> On Tue, Aug 13, 2013 at 12:38 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Tue, Aug 13, 2013 at 12:45:12PM +0800, Lei Wen wrote:
> >> > Not quite right; I think you need busiest->cfs.h_nr_running.
> >> > cfs.nr_running is the number of entries running in this 'group'. If
> >> > you've got nested groups like:
> >> >
> >> >  'root'
> >> >    \
> >> >    'A'
> >> >    / \
> >> >   t1 t2
> >> >
> >> > root.nr_running := 1 'A', even though you've got multiple running tasks.

One thing though; doesn't h_nr_running over count the number of tasks?
That is, doesn't it count the runnable entities so the above case would
give root.h_nr_running := 3, where we would only have 2 runnable tasks.

Double check this and be careful when doing the conversion.
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