linux-kernel - Re: [PATCH v2 06/13] sched: Store maximum per-cpu capacity in root domain

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Date:	Thu, 14 Jul 2016 15:25:36 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Dietmar Eggemann <dietmar.eggemann@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Yuyang Du <yuyang.du@...el.com>, mgalbraith@...e.de,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 06/13] sched: Store maximum per-cpu capacity in root domain

On 13 July 2016 at 18:37, Morten Rasmussen <morten.rasmussen@....com> wrote:
> On Wed, Jul 13, 2016 at 02:48:24PM +0100, Dietmar Eggemann wrote:
>> On 13/07/16 13:40, Vincent Guittot wrote:
>> > On 22 June 2016 at 19:03, Morten Rasmussen <morten.rasmussen@....com> wrote:
>> >> From: Dietmar Eggemann <dietmar.eggemann@....com>
>> >>
>> >> To be able to compare the capacity of the target cpu with the highest
>> >> available cpu capacity, store the maximum per-cpu capacity in the root
>> >> domain.
>> >
>> > I thought that the capacity of all CPUS were built so the highest
>> > capacity of the CPU of the system is 1024  for big LITTLE system . So
>> > this patch doesn't seem necessary for big.LITTLE system
>>
>> The asymmetric cpu capacity support currently only has an effect on arm
>> big.LITTLE (32bit) using the existing 'struct cpu_efficiency
>> table_efficiency[]' based approach.
>
> True for this patch set, but longer term and if you use the preview
> branch mentioned in the cover letter Vincent is right. The idea is that
> the highest capacity anywhere should be 1024.
>
> If we fix the arch/arm/kernel/topology.c code at the same time we could
> kill this patch.
>
> However, even further down the road we might need it (or something
> similar) anyway due to the thermal framework. At some point we would
> like to adjust the max capacity based any OPP constraints imposed by the
> thermal framework. In extreme cases big cpus might be capped so hard
> that they effectively have smaller capacity than little. I don't think
> it makes sense to re-normalize everything to the highest available
> capacity to ensure that there is always a cpu with capacity = 1024 in
> the system, instead we must be able to cope with scenarios where max
> capacity is smaller than 1024.

Yes we will have to found a solution for thermal mitigation but i
don't know if a rd->max_cpu_capacity would the best solution
>
> Also, for SMT max capacity is less than 1024 already. No?

Yes, it is. I haven't looked in details but i think that we could use
a capacity of 1024 for SMT with changes that have been done on how to
evaluate if a sched_group is overloaded or not.

> But we may be able to cater for this in wake_cap() somehow. I can have a
> look if Vincent doesn't like this patch.

IMO, rd->max_cpu_capacity field doesn't seem to be required for now .

Vincent

>
> Cheers,
> Morten