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

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Message-ID: <20150325173309.GS21418@twins.programming.kicks-ass.net>
Date:	Wed, 25 Mar 2015 18:33:09 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.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 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.
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