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

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Message-ID: <20130815183949.GW24092@twins.programming.kicks-ass.net>
Date:	Thu, 15 Aug 2013 20:39:49 +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 Thu, Aug 15, 2013 at 11:23:53AM -0700, Paul Turner wrote:
> On Thu, Aug 15, 2013 at 10:39 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > 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.
> 
> This should be ok: it's accounted like rq->nr_running, not cfs_rq->nr_running.
> Specifically: both only account tasks; group-entities do not contribute.

Ah, ok. I should have looked at the code I guess... :-)

> The fact that this distinction exists, despite the very similar names
> is unfortunate.
> We could consider renaming to h_nr_{running_,}tasks for clarity.
> The same applies to rq->nr_running, although that would involve more churn.

Yah.. that would clarify, although longer variable names will also get
us into more line-breaks I'm sure.

Lets keep it as is. Maybe a comment somewhere would be enough.
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