linux-kernel - Re: [PATCH v6 0/3] cpufreq: Replace timers with utilization update callbacks

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Date:	Fri, 12 Feb 2016 00:28:25 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Doug Smythies <dsmythies@...us.net>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Juri Lelli <juri.lelli@....com>,
	Steve Muckle <steve.muckle@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v6 0/3] cpufreq: Replace timers with utilization update callbacks

Hi Doug,

On Thu, Feb 11, 2016 at 11:50 PM, Doug Smythies <dsmythies@...us.net> wrote:
> On 2016.02.10 15:18 Rafael J. Wysocki wrote:
>> On Wednesday, February 10, 2016 03:11:43 PM Doug Smythies wrote:
>>> On 2016.02.10 07:17 Rafael J. Wysocki wrote:
>>>> On Friday, January 29, 2016 11:52:15 PM Rafael J. Wysocki wrote:
>>>>>
>>> This patch set solves a long standing issue with the intel_pstate driver.
>
>> Good to hear that, thanks!
>
>>> The issue began with the introduction of the "duration" method for deciding
>>> if the CPU had been idle for a long time resulting in forcing the
>>> target pstate downwards. Often this was the correct action, but sometimes this
>>> was the wrong thing to do, because the cpu was actually very busy, but just so
>>> happened to be idle on jiffy boundaries (perhaps similar to what Steve Muckle
>>> was referring to on another branch of this thread).
>
>>> I have a bunch of graphs, if anyone wants to see the supporting data.
>
>> It would be good to see how the data with and without the patchset compare
>> to each other if you have that.
>
> Please see:
> double u double u double u dot smythies dot com /~doug/linux/intel_pstate/rjw_patch_set/index.html

Thanks for the data.

> Specific duration tests graphs are posted, and also a bunch of idle tests graphs are posted.
> The references section includes links to all raw and post processed data.
>
> Note that on my 2 hour idle tests, I had a few 300 second durations
> on CPU 6 with the v5 patch set.
> (likely what Steve Muckle was referring to.)
> Such long durations did not occur in v6 or v7 2 hour idle tests.

OK, that suggests that using rq_lock(rq) in patch [1/3] is a win.

> Very interesting patterns in the 2 hour idle tests durations for
> individual CPUs.
>
> On 2016.02.10 22:03 Srinivas Pandruvada wrote:
>
>>> My test computer has an older model i7 (Intel(R) Core(TM) i7-2600K CPU @ 3.40GHz)
>> Thanks Doug. If you have specific workloads, please compare performance.
>
> My work so far has been testing functionality, with unrealistic workloads specifically
> designed to exaggerate issues, in this case the duration problem.
>
> I'll look at some real world workload scenarios.
>
> What I do have from my 2 hour idle tests is the of total number of passes through
> the intel_pstate driver:
>
> Control sample: Kernel 4.3-rc3: 37949 passes.
> Kernel 4.3-rc3 + rjw 3 patch set v5: 180355 passes
> Kernel 4.3-rc3 + rjw 3 patch set v6: 201307 passes
> Kernel 4.3-rc3 + rjw 3 patch set v7: 203619 passes

That reflects how things work with the changes.  The driver is called
more often now and has to decide whether or not to take a sample.

It would be interesting to see how many of those were samples that
were actually taken if you can instrument that.

> While I should have, I did not run turbostat to get idle energy and/or power.
> However, a 1 hour idle test with turbostat gave (Package Joules):
> Control sample: Kernel 4.3-rc3: 13788 J or 3.83 Watts
> Kernel 4.3-rc3 + rjw 3 patch set v7: 13929 J or 3.87 Watts

So it shows a slight increase in energy consumption with your
workloads.  It is not as much as to make me worry in any way, but I'm
wondering if performance is better too as a result (and how much
better if so).

Thanks,
Rafael