linux-kernel - RE: [RFC/RFT][PATCH v2] cpufreq: intel_pstate: Proportional algorithm for Atom

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Date:   Wed, 5 Oct 2016 16:58:40 -0700
From:   "Doug Smythies" <dsmythies@...us.net>
To:     "'Rafael J. Wysocki'" <rjw@...ysocki.net>
Cc:     "'Linux Kernel Mailing List'" <linux-kernel@...r.kernel.org>,
        "'Srinivas Pandruvada'" <srinivas.pandruvada@...ux.intel.com>,
        "'Linux PM list'" <linux-pm@...r.kernel.org>
Subject: RE: [RFC/RFT][PATCH v2] cpufreq: intel_pstate: Proportional algorithm for Atom

Hi Rafael,

It doesn't compile for me. See further down.

On 2016.10.05 06:15 Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> The PID algorithm used by the intel_pstate driver tends to drive
> performance to the minimum for workloads with utilization below the
> setpoint, which is undesirable, so replace it with a modified
> "proportional" algorithm on Atom.
>
> The new algorithm will set the new P-state to be 1.25 times the
> available maximum times the (frequency-invariant) utilization during
> the previous sampling period except when the target P-state computed
> this way is lower than the average P-state during the previous
> sampling period.  In the latter case, it will increase the target by
> 50% of the difference between it and the average P-state to prevent
> performance from dropping down too fast in some cases.
>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>
> It is better to compare the average P-state to the target (than to
> compare the average perf ratio to the utilization), because that takes
> turbo into account more accurately.
>
> Plus if the target is below the min, it is better to compare the min
> to the average instead of comparing the target to it.
>
> ---
> drivers/cpufreq/intel_pstate.c |   22 +++++++++++++++++++++-
> 1 file changed, 21 insertions(+), 1 deletion(-)
>
> Index: linux-pm/drivers/cpufreq/intel_pstate.c
> ===================================================================
> --- linux-pm.orig/drivers/cpufreq/intel_pstate.c
> +++ linux-pm/drivers/cpufreq/intel_pstate.c
> @@ -1232,6 +1232,7 @@ static inline int32_t get_target_pstate_
>  {
> 	struct sample *sample = &cpu->sample;
> 	int32_t busy_frac, boost;
> +	int target, avg_pstate;
> 
> 	busy_frac = div_fp(sample->mperf, sample->tsc);
> 
> @@ -1242,7 +1243,26 @@ static inline int32_t get_target_pstate_
>  		busy_frac = boost;
> 
> 	sample->busy_scaled = busy_frac * 100;
> -	return get_avg_pstate(cpu) - pid_calc(&cpu->pid, sample->busy_scaled);
> +
> +	target = limits->no_turbo || limits->turbo_disabled :
	                                                    ^
	                                                   ^^^
For proper conditional expression syntax, shouldn't that be a "?" ?

I can not get this patch to compile,
but if I change that ":" to a "?" (as in the previous version of this patch)
it compiles.

> +			cpu->pstate.max_pstate : cpu->pstate.turbo_pstate;
> +	target += target >> 2;
> +	target = mul_fp(target, busy_frac);
> +	if (target < cpu->pstate.min_pstate)
> +		target = cpu->pstate.min_pstate;
> +
> +	/*
> +	 * If the average P-state during the previous cycle was higher than the
> +	 * current target, add 50% of the difference to the target to reduce
> +	 * possible performance oscillations and offset possible performance
> +	 * loss related to moving the workload from one CPU to another within
> +	 * a package/module.
> +	 */
> +	avg_pstate = get_avg_pstate(cpu);
> +	if (avg_pstate > target)
> +		target += (avg_pstate - target) >> 1;
> +
> +	return target;
>  }
> 
>  static inline int32_t get_target_pstate_use_performance(struct cpudata *cpu)
>
> --