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

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Message-ID: <CAKfTPtCE3gy_656EfcrfjZ-15CTxAqk91O6qR1Jz3BCABvDNCg@mail.gmail.com>
Date:	Tue, 7 Jan 2014 15:11:22 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.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>,
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	"corbet@....net" <corbet@....net>,
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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 7 January 2014 14:22, Peter Zijlstra <peterz@...radead.org> wrote:
> 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.

That needs to be studied more deeply but that could be a way to have a
larger picture

Another point is that we are using runnable and blocked load average
which are the sum of load_avg_contrib of tasks but we are not using
the runnable_avg_sum of the cpus which is not the now picture but a
average of the past running time (without taking into account task
weight)

Vincent
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