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

[Vincent Guittot]

On 27 March 2015 at 09:17, Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
> On 26 March 2015 at 18:38, Morten Rasmussen <morten.rasmussen@xxxxxxx> wrote:
>> On Wed, Mar 25, 2015 at 06:08:42PM +0000, Vincent Guittot wrote:
>>> On 25 March 2015 at 18:33, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>> > On Tue, Mar 24, 2015 at 11:00:57AM +0100, Vincent Guittot wrote:
>>> >> On 23 March 2015 at 14:19, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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

when arch_scale_freq_capacity is not defined by an arch, the mul is
optimized in a lsl 10

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