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

[Srinivas Pandruvada]

On 02/10/2016 03:11 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:
> > > 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).
> This patch set solves a long standing issue with the intel_pstate driver.
> 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).
>
> For an idle system, this patch set seems to change the maximum duration from
> 4 seconds to 0.5 seconds for most CPUs. However, when using v1 of patches 1
> and 2 of 3 and v5 of 3 of 3, sometimes the durations (time between passes of
> the intel-pstate driver for a given CPU) of upwards of 120 seconds were observed.
> When patches 1, 2, and 3 of 3 v6 were used, the maximum observed durations of an
> idle system were on the order of 500 milliseconds for most CPUs, but CPU 6
> sometimes went to 3.5 seconds and CPU 7 sometimes went to 4 seconds (small
> sample space, I'll consider to run an overnight test for a much much larger
> sample space). Note 4 seconds, is O.K., and what it was before, I'm just noting
> it is all.
>
> I have a bunch of graphs, if anyone wants to see the supporting data.
>
> 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.

- Srinivas
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