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

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Date:	Fri, 27 Mar 2015 09:17:04 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Mike Galbraith <efault@....de>,
	"nicolas.pitre@...aro.org" <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 26 March 2015 at 18:38, Morten Rasmussen <morten.rasmussen@....com> wrote:
> On Wed, Mar 25, 2015 at 06:08:42PM +0000, Vincent Guittot wrote:
>> 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
>
> Another potential solution is to stay with weak functions but move the
> multiplication and shift into the arch_scale_*() functions by passing
> the value we want to scale into the arch_scale_*() function. That way we
> can completely avoid multiplication and shift in the default case (no
> arch_scale*() implementations, which is better than what we have today.

the sched_rt_avg_update only uses the mul with
arch_scale_freq_capacity because the shift by SCHED_CAPACITY_SHIFT has
been factorized in scale_rt_capacity

>
> The only downside is that for frequency invariance we need three
> arch_scale_freq_capacity() calls instead of two.
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