linux-kernel - Re: [PATCH v10 04/11] sched: Make sched entity usage tracking scale-invariant

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Date:	Wed, 25 Mar 2015 19:08:42 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Mike Galbraith <efault@....de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>
Subject: Re: [PATCH v10 04/11] sched: Make sched entity usage tracking scale-invariant

On 25 March 2015 at 18:33, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Mar 24, 2015 at 11:00:57AM +0100, Vincent Guittot wrote:
>> On 23 March 2015 at 14:19, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Fri, Feb 27, 2015 at 04:54:07PM +0100, Vincent Guittot wrote:
>> >
>> >> +     unsigned long scale_freq = arch_scale_freq_capacity(NULL, cpu);
>> >
>> >> +                     sa->running_avg_sum += delta_w * scale_freq
>> >> +                             >> SCHED_CAPACITY_SHIFT;
>> >
>> > so the only thing that could be improved is somehow making this
>> > multiplication go away when the arch doesn't implement the function.
>> >
>> > But I'm not sure how to do that without #ifdef.
>> >
>> > Maybe a little something like so then... that should make the compiler
>> > get rid of those multiplications unless the arch needs them.
>>
>> yes, it removes useless multiplication when not used by an arch.
>> It also adds a constraint on the arch side which have to define
>> arch_scale_freq_capacity like below:
>>
>> #define arch_scale_freq_capacity xxx_arch_scale_freq_capacity
>> with xxx_arch_scale_freq_capacity an architecture specific function
>
> Yeah, but it not being weak should make that a compile time warn/fail,
> which should be pretty easy to deal with.
>
>> If it sounds acceptable i can update the patch with your proposal ?
>
> I'll stick it to the end, I just wanted to float to patch to see if
> people had better solutions.

ok. all other methods that i have tried, was removing the optimization
when default arch_scale_freq_capacity was used
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