linux-kernel - Re: [RFC][PATCH 7/7] cpufreq: intel_pstate: Change P-state selection algorithm for Core

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Date:   Fri, 19 Aug 2016 16:47:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Doug Smythies <dsmythies@...us.net>
Cc:     "'Rafael J. Wysocki'" <rjw@...ysocki.net>,
        'Srinivas Pandruvada' <srinivas.pandruvada@...ux.intel.com>,
        'Viresh Kumar' <viresh.kumar@...aro.org>,
        'Linux Kernel Mailing List' <linux-kernel@...r.kernel.org>,
        'Steve Muckle' <steve.muckle@...aro.org>,
        'Juri Lelli' <juri.lelli@....com>,
        'Ingo Molnar' <mingo@...nel.org>,
        'Linux PM list' <linux-pm@...r.kernel.org>
Subject: Re: [RFC][PATCH 7/7] cpufreq: intel_pstate: Change P-state selection
 algorithm for Core

On Sat, Aug 13, 2016 at 08:59:01AM -0700, Doug Smythies wrote:
> My previous replies (and see below) have suggested that some filtering
> is needed on the target pstate, otherwise, and dependant on the type of
> workload, it tends to oscillate.
> 
> I added the IIR (Infinite Impulse Response) filter that I have suggested in the past:

One question though; why is this filter intel_pstate specific? Should we
not do this in generic code?

> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> index c43ef55..262ec5f 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -1313,7 +1318,74 @@ static inline int32_t get_target_pstate_default(struct cpudata *cpu)
>         cpu->iowait_boost >>= 1;
> 
>         pstate = cpu->pstate.turbo_pstate;

> +       unfiltered_target = (pstate + (pstate >> 2)) * busy_frac;

> +       duration_ns = cpu->sample.time - cpu->last_sample_time;
> +
> +       scaled_gain = div_u64(int_tofp(duration_ns) *
> +               int_tofp(pid_params.p_gain_pct), int_tofp(pid_params.sample_rate_ns));

Drop int_to_fp() on one of the dividend terms and in the divisor. Same
end result since they divide away against one another but reduces the
risk of overflow.

Also, sample_rate_ns, really!? A rate is in [1/s], should that thing be
called period_ns ?

> +       if (scaled_gain > int_tofp(100))
> +               scaled_gain = int_tofp(100);

> +       if (scaled_gain < int_tofp(pid_params.p_gain_pct))
> +               scaled_gain = int_tofp(pid_params.p_gain_pct);
> +
> +       /*
> +        * Bandwidth limit the output. For now, re-task p_gain_pct for this purpose.
> +        * Use a smple IIR (Infinite Impulse Response) filter.
> +        */
> +       cpu->sample.target = div_u64((int_tofp(100) - scaled_gain) *
> +                       cpu->sample.target + scaled_gain *
> +                       unfiltered_target, int_tofp(100));


Really hard to read that stuff, maybe cure with a comment:

	/*
	 *       g = dt*p / period
	 *
	 * target' = (1 - g)*target  +  g*input
	 */

> +
> +       return fp_toint(cpu->sample.target + (1 << (FRAC_BITS-1)));
>  }