linux-kernel - Re: [PATCH 3/3] cpufreq: intel_pstate: Provide resolve

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Date:   Sat, 10 Jun 2017 02:26:27 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>, linaro-kernel@...ts.linaro.org,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <Juri.Lelli@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Patrick Bellasi <patrick.bellasi@....com>,
        john.ettedgui@...il.com,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH 3/3] cpufreq: intel_pstate: Provide resolve_freq() to fix regression

On Fri, Jun 9, 2017 at 3:15 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> When the schedutil governor calls cpufreq_driver_resolve_freq() for the
> intel_pstate (in passive mode) driver, it simply returns the requested
> frequency as there is no ->resolve_freq() callback provided.
>
> The result is that get_next_freq() doesn't get a chance to know the
> frequency which will be set eventually and we can hit a potential
> regression as explained in the following paragraph.
>
> For example, consider the possible range of frequencies as 900 MHz, 1
> GHz, 1.1 GHz, and 1.2 GHz. If the current frequency is 1.1 GHz and the
> next frequency (based on current utilization) is 1 GHz, then the
> schedutil governor will try to set the average of these as the next
> frequency (i.e. 1.05 GHz).
>
> Because we always try to find the lowest frequency greater than equal to
> the target frequency, the intel_pstate driver will end up setting the
> frequency as 1.1 GHz.
>
> Though the sg_policy->next_freq field gets updated with the average
> frequency only. And so we will finally select the min frequency when the
> next_freq is 1 more than the min frequency as the average then will be
> equal to the min frequency. But that will also take lots of iterations
> of the schedutil update callbacks.
>
> Fix that by providing a resolve_freq() callback.
>
> Tested on desktop with Intel Skylake processors.
>
> Fixes: 39b64aa1c007 ("cpufreq: schedutil: Reduce frequencies slower")
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
>  drivers/cpufreq/intel_pstate.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> index 029a93bfb558..e177352180c3 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -2213,6 +2213,19 @@ static int intel_cpufreq_target(struct cpufreq_policy *policy,
>         return 0;
>  }
>
> +unsigned int intel_cpufreq_resolve_freq(struct cpufreq_policy *policy,
> +                                       unsigned int target_freq)

Should be defined as static?

Thanks,
Joel