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

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Date:	Thu, 4 Feb 2016 10:51:16 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Steve Muckle <steve.muckle@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/3] cpufreq: Replace timers with utilization update
 callbacks

Hi Rafael,

On 03/02/16 23:20, 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 did intend to test and review this series, but then other patches
required attention as well and I didn't find time to have a look at
these. Sorry about that. Also, if I can speak for him, I think that
Steve is OOO this week.

> 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.
> 

Unfortunately, I can't promise anything at the moment, but, if I find
some time, I'll run some tests (BTW, do you have alredy something that I
can put to run on my boxes?). I guess I can eventually do that after
this gets merged as well.

Best,

- Juri