Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks | Date | Wed, 03 Feb 2016 23:20:19 +0100 |
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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.
Thanks, Rafael
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