linux-kernel - Re: [RFC] sched: CPU topology try

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Message-ID: <20140107132220.GZ31570@twins.programming.kicks-ass.net>
Date:	Tue, 7 Jan 2014 14:22:20 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Dietmar Eggemann <dietmar.eggemann@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"pjt@...gle.com" <pjt@...gle.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	"cmetcalf@...era.com" <cmetcalf@...era.com>,
	"tony.luck@...el.com" <tony.luck@...el.com>,
	"alex.shi@...aro.org" <alex.shi@...aro.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"corbet@....net" <corbet@....net>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"len.brown@...el.com" <len.brown@...el.com>,
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	"schwidefsky@...ibm.com" <schwidefsky@...ibm.com>,
	"heiko.carstens@...ibm.com" <heiko.carstens@...ibm.com>
Subject: Re: [RFC] sched: CPU topology try

On Tue, Jan 07, 2014 at 09:32:04AM +0100, Vincent Guittot wrote:
> On 6 January 2014 17:31, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Mon, Jan 06, 2014 at 02:41:31PM +0100, Vincent Guittot wrote:
> >> IMHO, these settings will disappear sooner or later, as an example the
> >> idle/busy _idx are going to be removed by Alex's patch.
> >
> > Well I'm still entirely unconvinced by them..
> >
> > removing the cpu_load array makes sense, but I'm starting to doubt the
> > removal of the _idx things.. I think we want to retain them in some
> > form, it simply makes sense to look at longer term averages when looking
> > at larger CPU groups.
> >
> > So maybe we can express the things in log_2(group-span) or so, but we
> > need a working replacement for the cpu_load array. Ideally some
> > expression involving the blocked load.
> 
> Using the blocked load can surely give benefit in the load balance
> because it gives a view of potential load on a core but it still decay
> with the same speed than runnable load average so it doesn't solve the
> issue for longer term average. One way is to have a runnable average
> load with longer time window

Ah, another way of looking at it is that the avg without blocked
component is a 'now' picture. It is the load we are concerned with right
now.

The more blocked we add the further out we look; with the obvious limit
of the entire averaging period.

So the avg that is runnable is right now, t_0; the avg that is runnable +
blocked is t_0 + p, where p is the avg period over which we expect the
blocked contribution to appear.

So something like:

  avg = runnable + p(i) * blocked; where p(i) \e [0,1]

could maybe be used to replace the cpu_load array and still represent
the concept of looking at a bigger picture for larger sets. Leaving open
the details of the map p.
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