On 03/03/16 16:28, Peter Zijlstra wrote:
> On Thu, Mar 03, 2016 at 04:38:17PM +0100, Peter Zijlstra wrote:
>> On Thu, Mar 03, 2016 at 03:01:15PM +0100, Vincent Guittot wrote:
>>>> In case a more formal derivation of this formula is needed, it is
>>>> based on the following 3 assumptions:
>>>>
>>>> (1) Performance is a linear function of frequency.
>>>> (2) Required performance is a linear function of the utilization ratio
>>>> x = util/max as provided by the scheduler (0 <= x <= 1).
>>>
>>> Just to mention that the utilization that you are using, varies with
>>> the frequency which add another variable in your equation
>>
>> Right, x86 hasn't implemented arch_scale_freq_capacity(), so the
>> utilization values we use are all over the map. If we lower freq, the
>> util will go up, which would result in us bumping the freq again, etc..
>
> Something like the completely untested below should maybe work.
>
> Rafael?
>
[...]
> +void arch_scale_freq_tick(void)
> +{
> + u64 aperf, mperf;
> + u64 acnt, mcnt;
> +
> + if (!static_cpu_has(X86_FEATURE_APERFMPERF))
> + return;
> +
> + aperf = rdmsrl(MSR_IA32_APERF);
> + mperf = rdmsrl(MSR_IA32_APERF);
> +
> + acnt = aperf - this_cpu_read(arch_prev_aperf);
> + mcnt = mperf - this_cpu_read(arch_prev_mperf);
> +
> + this_cpu_write(arch_prev_aperf, aperf);
> + this_cpu_write(arch_prev_mperf, mperf);
> +
> + this_cpu_write(arch_cpu_freq, div64_u64(acnt * SCHED_CAPACITY_SCALE, mcnt));
Wasn't there the problem that this ratio goes to zero if the cpu is idle
in the old power estimation approach on x86?
[...]
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