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

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Message-ID: <56B29662.5040507@linux.intel.com>
Date:	Wed, 3 Feb 2016 16:08:02 -0800
From:	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>
Cc:	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 0/3] cpufreq: Replace timers with utilization update
 callbacks



On 02/03/2016 02:20 PM, Rafael J. Wysocki wrote:
> On Friday, January 29, 2016 11:52:15 PM Rafael J. Wysocki wrote:
>> Hi,
>>
>> The following patch series introduces a mechanism allowing the cpufreq core
>> and "setpolicy" drivers to provide utilization update callbacks to be invoked
>> by the scheduler on utilization changes.  Those callbacks can be used to run
>> the sampling and frequency adjustments code (intel_pstate) or to schedule the
>> execution of that code in process context (cpufreq core) instead of per-CPU
>> deferrable timers used in cpufreq today (which Thomas complained about during
>> the last Kernel Summit).
>>
>> [1/3] Introduce a mechanism for calling into cpufreq from the scheduler and
>>        registering callbacks to be executed from there.
>>
>> [2/3] Modify intel_pstate to use the mechanism introduced by [1/3] instead
>>        of per-CPU deferrable timers to do its work.
>>
>> This isn't entirely straightforward as the scheduler context running those
>> callbacks is really special.  Among other things it can only use raw
>> spinlocks and cannot invoke wake_up_process() directly.  Also, calling
>> ktime_get() from there may be too expensive on some systems.  All that has to
>> be taken into account, but even then the change allows some lines of code to be
>> cut from the driver.
>>
>> Some performance and energy consumption measurements have been carried out with
>> an earlier version of this patch and it looks like the changes lead to a
>> slightly better performing system that consumes slightly less energy at the
>> same time overall.
>>
>> [3/3] Modify the cpufreq core to use the mechanism introduced by [1/3] instead
>>        of per-CPU deferrable timers to queue up the execution of governor work.
>>
>> Again, this isn't really straightforward for the above reasons, but still the
>> code size is reduced a bit by the changes.
>>
>> I'm still unsure about the energy consumption and performance impact of [3/3]
>> as earlier versions of it led to inconsistent results (most likely due to bugs
>> in them that hopefully have been fixed in this version).  In particular, the
>> additional irq_work may turn out to be problematic, but more optimizations are
>> possible on top of this one even if it makes things worse by itself.
>>
>> For example, it should be possible to move the execution of state selection
>> code into the utilization update callback itself, at least in principle, for
>> all governors.  The P-state/OPP adjustment may need to be run from process
>> context still, but for the drivers that can do it without sleeping it should
>> be possible to move that into the utilization update callback as well.
>>
>> The patches are on top of 4.5-rc1 and have been tested on a couple of x86
>> machines.
> Well, no responses here, so I'm inclined to believe that this series is fine
> by everybody (at least by everybody in the CC).
>
> I can wait for a few days more, but new material is starting to pile up on top
> of these patches and I'll simply need to move forward at one point.
Based on the test results for intel_pstate and acpi_cpufreq, I don't see 
any problem in applying these patches.

Thanks,
Srinivas
> Thanks,
> Rafael
>
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