linux-kernel - RE: [PATCH v4 1/6] x86,sched: Add support for frequency invariance

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Date:   Tue, 26 Nov 2019 23:32:34 -0800
From:   "Doug Smythies" <dsmythies@...us.net>
To:     "'Giovanni Gherdovich'" <ggherdovich@...e.cz>
Cc:     <x86@...nel.org>, <linux-pm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        "'Mel Gorman'" <mgorman@...hsingularity.net>,
        "'Matt Fleming'" <matt@...eblueprint.co.uk>,
        "'Viresh Kumar'" <viresh.kumar@...aro.org>,
        "'Juri Lelli'" <juri.lelli@...hat.com>,
        "'Paul Turner'" <pjt@...gle.com>,
        "'Peter Zijlstra'" <peterz@...radead.org>,
        "'Vincent Guittot'" <vincent.guittot@...aro.org>,
        "'Quentin Perret'" <qperret@...rret.net>,
        "'Dietmar Eggemann'" <dietmar.eggemann@....com>,
        "'Srinivas Pandruvada'" <srinivas.pandruvada@...ux.intel.com>,
        "'Thomas Gleixner'" <tglx@...utronix.de>,
        "'Ingo Molnar'" <mingo@...hat.com>,
        "'Borislav Petkov'" <bp@...e.de>, "'Len Brown'" <lenb@...nel.org>,
        "'Rafael J . Wysocki'" <rjw@...ysocki.net>
Subject: RE: [PATCH v4 1/6] x86,sched: Add support for frequency invariance

On 2019.11.26 07:20 Giovanni Gherdovich wrote:
> On Mon, 2019-11-25 at 21:59 -0800, Doug Smythies wrote:
>> [...]
>> The issue with the schedutil governor not working properly in the 5.4 RC series
>> appears to be hardware dependant.
>> 
>> My test computer is Intel(R) Core(TM) i7-2600K CPU @ 3.40GHz., Sandy Bridge.
>> On a temporary basis, I acquired a computer with an
>> Intel(R) Core(TM) i5-4460 CPU @ 3.20GHz, Haswell,
>> and schedutil governor behaviour with the exact same kernels is fine:
>> 
>> That "gitsource" test, "make test" 6 times, first run thrown out:
>> 
>> Kernel 5.4 intel_cpufreq/schedutil: 3411.8 seconds
>> Kernel 5.4 + gg 6 intel_cpufreq/schedutil: 1696.7 seconds
>> Ratio: 0.49
>> Recall you got a ratio of 0.49 with 5th generation, Broadwell.
>
> It's good to hear that we're getting the same performance numbers for this
> patchset on all hardware that is not a Sandy Bridge. Thanks for double
> checking, independent verification is always valuable.
> 
> Now, regarding the 5.4 regression for schedutil you see on Sandy Bridge: can
> we move this to the kernel bugzilla? Would you care to open a bug there and CC
> me to it?

O.K., I'll need another day or two to isolate further, then I'll open a bug.
I now understand considerably more, and why my bisection ended up
at a strange spot.

> If it's reproducible we should assess it and see what can be done.

On my Sandy Bridge system if the kernel configuration contains:

CONFIG_UCLAMP_TASK_GROUP=y

Then the intel_cpufreq/schedutil will respond much like the performance
governor.

If the kernel configuration contains:

# CONFIG_UCLAMP_TASK_GROUP is not set

Then the intel_cpufreq/schedutil will respond much like it used to.

On the Haswell computer, it doesn't seem to matter, and your tests
seem to confirm this.

Note: I steal my kernel configuration from the Ubuntu mainline builds,
and they changed this parameter during the 5.4-rc series.

... Doug