linux-kernel - Re: [PATCH 4/4] cpufreq: intel_pstate: enable boost for Skylake Xeon

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Message-ID: <87bmarhqk4.fsf@riseup.net>
Date:   Fri, 27 Jul 2018 22:34:03 -0700
From:   Francisco Jerez <currojerez@...eup.net>
To:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        lenb@...nel.org, rjw@...ysocki.net, mgorman@...hsingularity.net
Cc:     peterz@...radead.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, juri.lelli@...hat.com,
        viresh.kumar@...aro.org, ggherdovich@...e.cz,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Eero Tamminen <eero.t.tamminen@...el.com>
Subject: Re: [PATCH 4/4] cpufreq: intel_pstate: enable boost for Skylake Xeon

Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com> writes:

> Enable HWP boost on Skylake server and workstations.
>

Please revert this series, it led to significant energy usage and
graphics performance regressions [1].  The reasons are roughly the ones
we discussed by e-mail off-list last April: This causes the intel_pstate
driver to decrease the EPP to zero when the workload blocks on IO
frequently enough, which for the regressing benchmarks detailed in [1]
is a symptom of the workload being heavily IO-bound, which means they
won't benefit at all from the EPP boost since they aren't significantly
CPU-bound, and they will suffer a decrease in parallelism due to the
active CPU core using a larger fraction of the TDP in order to achieve
the same work, causing the GPU to have a lower power budget available,
leading to a decrease in system performance.

You may want to give a shot to my previous suggestion of using [2] in
order to detect whether the system is IO-bound, which you can use as an
indicator that the optimization implemented in this series cannot
possibly improve performance and can be expected to hurt energy usage.

Thanks.

[1] https://bugs.freedesktop.org/show_bug.cgi?id=107410
[2] https://patchwork.kernel.org/patch/10312259/

> Reported-by: Mel Gorman <mgorman@...hsingularity.net>
> Tested-by: Giovanni Gherdovich <ggherdovich@...e.cz>
> Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
> ---
>  drivers/cpufreq/intel_pstate.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> index 70bf63bb4e0e..01c8da1f99db 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -1794,6 +1794,12 @@ static const struct x86_cpu_id intel_pstate_cpu_ee_disable_ids[] = {
>  	{}
>  };
>  
> +static const struct x86_cpu_id intel_pstate_hwp_boost_ids[] __initconst = {
> +	ICPU(INTEL_FAM6_SKYLAKE_X, core_funcs),
> +	ICPU(INTEL_FAM6_SKYLAKE_DESKTOP, core_funcs),
> +	{}
> +};
> +
>  static int intel_pstate_init_cpu(unsigned int cpunum)
>  {
>  	struct cpudata *cpu;
> @@ -1824,6 +1830,10 @@ static int intel_pstate_init_cpu(unsigned int cpunum)
>  			intel_pstate_disable_ee(cpunum);
>  
>  		intel_pstate_hwp_enable(cpu);
> +
> +		id = x86_match_cpu(intel_pstate_hwp_boost_ids);
> +		if (id)
> +			hwp_boost = true;
>  	}
>  
>  	intel_pstate_get_cpu_pstates(cpu);
> -- 
> 2.13.6

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