Re: [RFC/RFT] [PATCH 02/10] cpufreq: intel_pstate: Conditional frequency invariant accounting

Juri Lelli
2018-05-17T15:04:24Z

On 17/05/18 12:59, Juri Lelli wrote:
> On 16/05/18 18:31, Juri Lelli wrote:
> > On 16/05/18 17:47, Peter Zijlstra wrote:
> > > On Wed, May 16, 2018 at 05:19:25PM +0200, Juri Lelli wrote:
> > >
> > > > Anyway, FWIW I started testing this on a E5-2609 v3 and I'm not seeing
> > > > hackbench regressions so far (running with schedutil governor).
> > >
> > > https://en.wikipedia.org/wiki/Haswell_(microarchitecture)#Server_processors
> > >
> > > Lists the E5 2609 v3 as not having turbo at all, which is basically a
> > > best case scenario for this patch.
> > >
> > > As I wrote earlier today; when turbo exists, like say the 2699, then
> > > when we're busy we'll run at U=2.3/3.6 ~ .64, which might confuse
> > > things.
> >
> > Indeed. I was mostly trying to see if adding this to the tick might
> > introduce noticeable overhead.
>
> Blindly testing on an i5-5200U (2.2/2.7 GHz) gave the following
>
> # perf bench sched messaging --pipe --thread --group 2 --loop 20000
>
> count mean std min 50% 95% 99% max
> hostname kernel
> i5-5200U test_after 30.0 13.843433 0.590605 12.369 13.810 14.85635 15.08205 15.127
> test_before 30.0 13.571167 0.999798 12.228 13.302 15.57805 16.40029 16.690
>
> It might be interesting to see what happens when using a single CPU
> only?
>
> Also, I will look at how the util signals look when a single CPU is
> busy..

And this is showing where the problem is (as you were saying [1]):

https://gist.github.com/jlelli/f5438221186e5ed3660194e4f645fe93

Just look at the plots (and ignore setup).

First one (pid:4483) shows a single task busy running on a single CPU,
which seems to be able to sustain turbo for 5 sec. So task util reaches
~1024.

Second one (pid:4283) shows the same task, but running together with
other 3 tasks (each one pinned to a different CPU). In this case util
saturates at ~943, which is due to the fact that max freq is still
considered to be the turbo one. :/

[1] https://marc.info/?l=linux-kernel&m=152646464017810&w=2