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

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Date:   Mon, 30 Jul 2018 11:33:25 +0300
From:   Eero Tamminen <eero.t.tamminen@...el.com>
To:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Francisco Jerez <currojerez@...eup.net>, 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,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
Subject: Re: [PATCH 4/4] cpufreq: intel_pstate: enable boost for Skylake Xeon

Hi,

On 28.07.2018 17:14, Srinivas Pandruvada wrote:
> On Fri, 2018-07-27 at 22:34 -0700, Francisco Jerez wrote:
>> 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].
> Which SKX platform is targeted to graphics?

Patch that Chris pointed out is this:
+static const struct x86_cpu_id intel_pstate_hwp_boost_ids[] = {
+       ICPU(INTEL_FAM6_SKYLAKE_X, core_funcs),
+       ICPU(INTEL_FAM6_SKYLAKE_DESKTOP, core_funcs),
+       {}
+};

The regressing platforms in our test system were:
- SKL 6600K i5 / GT2
- SKL 6770HQ i7 / GT4e

SKL-U i5 / GT3e device wasn't impacted, so I assume U devices don't 
match INTEL_FAM6_SKYLAKE_DESKTOP.



	- Eero

>>   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
> No. You didn't check this series. We are not using EPP at all.
> The boost mechanism used here is not boost to max.
> 
> Thanks,
> Srinivas
> 
>> 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