Re: [PATCH 1/1] cpufreq: pcc-cpufreq: Re-introduce deadband effect to reduce number of frequency changes

[Andreas Herrmann]

On Mon, Aug 29, 2016 at 11:31:53AM +0530, Viresh Kumar wrote:
> On 19-08-16, 14:21, Andreas Herrmann wrote:
> > 
> > Commit 6393d6a102 (cpufreq: ondemand: Eliminate the deadband effect)
> > introduced a performance regression for systems using pcc-cpufreq and
> > ondemand governor. This is measurable with different workloads. E.g.
> > wall-clock time for kernel compilation significantly increased.
> > 
> > The elimination of the deadband effect significantly increased the
> > number of frequency changes with pcc-cpufreq.
> > 
> > Instead of reverting commit 6393d6a102 I suggest to add a workaround
> > in pcc-cpufreq to re-introduce the deadband effect for this driver
> > only - to restore the old performance behaviour with pcc-cpufreq with
> > ondemand governor.
> > 
> > Following some performance numbers for similar kernel compilations to
> > illustrate the effect of commit 6393d6a102 and the proposed fix.
> > 
> > Following typical numbers of kernel compilation tests with varying number of
> > compile jobs:
> > 
> >                      v4.8.0-rc2               4.8.0-rc2-pcc-cpufreq-deadband
> >  # of jobst   user     sys   elapsed   CPU     user     sys   elapsed   CPU
> >        2     440.39  116.49  4:33.35   203%   404.85  109.10  4:10.35   205%
> >        4     436.87  133.39  2:22.88   399%   381.83  128.00  2:06.84   401%
> >        8     475.49  157.68  1:22.24   769%   344.36  149.08  1:04.29   767%
> >       16     620.69  188.33  0:54.74  1477%   374.60  157.40  0:36.76  1447%
> >       32     815.79  209.58  0:37.22  2754%   490.46  160.22  0:24.87  2616%
> >       64     394.13   60.55  0:13.54  3355%   386.54   60.33  0:12.79  3493%
> >      120     398.24   61.55  0:14.60  3148%   390.44   61.19  0:13.07  3453%
> > 
> > (HP ProLiant DL580 Gen8 system, 60 CPUs @ 2.80GHz)
> > 
> > Link: http://marc.info/?l=linux-pm&m=147160912625600
> > Signed-off-by: Andreas Herrmann <aherrmann@xxxxxxxx>
> > ---
> >  drivers/cpufreq/pcc-cpufreq.c | 20 ++++++++++++++++++++
> >  1 file changed, 20 insertions(+)
> > 
> > If this change is accepted maybe it's a good idea to tag it also for
> > stable kernels, e.g. starting with v4.4.

> I am _really_ worried about such hacks in drivers to negate the effect of a
> patch, that was actually good.

> Did you try to increase the sampling period of ondemand governor to see if that
> helps without this patch.

With an older kernel I've modified transition_latency of the driver
which in turn is used to calculate the sampling rate.

I started with the value return as "nominal latency" for PCC.  This
was 300000 ns on the test system and made things worse.  I've tested
other values as well unitl I've found a local optimium at 45000ns but
performance was lower in comparison to when I've applied my hack.

> Also, it is important to understand why is the performance going
> down, while the original commit should have made it better.

My understanding is that the original commit was tested with certain
combinations of hardware and cpufreq-drivers and the claim was that
for those (two?) tested combinations performance increased and power
consumption was lower. So I am not so sure what to expect from all
other cpufreq-driver/hardware combinations.

> Is it only about more transitions ?

I think this is the main issue.

In an older kernel version I activated/added debug output in
__cpufreq_driver_target(). Of course this creates a huge amount of
messages. But with original patch reverted it was like:

[   29.489677] cpufreq: target for CPU 0: 1760000 kHz (1200000 kHz), relation 2, requested 1760000 kHz
[   29.570364] cpufreq: target for CPU 0: 1216000 kHz (1760000 kHz), relation 2, requested 1216000 kHz
[   29.571055] cpufreq: target for CPU 1: 1200000 kHz (1148000 kHz), relation 0, requested 1200000 kHz
[   29.571483] cpufreq: target for CPU 1: 1200000 kHz (1200000 kHz), relation 2, requested 1200000 kHz
[   29.572042] cpufreq: target for CPU 2: 1200000 kHz (1064000 kHz), relation 0, requested 1200000 kHz
[   29.572503] cpufreq: target for CPU 2: 1200000 kHz (1200000 kHz), relation 2, requested 1200000 kHz

a lot of stuff, but system could still handle it and booted to the
prompt.

With the original patch applied the system was really flooded and
eventually became unresponsive:

** 459 printk messages dropped ** [   29.838689] cpufreq: target for CPU 43: 1408000 kHz (2384000 kHz), relation 2, requested 1408000 kHz
** 480 printk messages dropped ** [   29.993849] cpufreq: target for CPU 54: 1200000 kHz (1248000 kHz), relation 2, requested 1200000 kHz
** 413 printk messages dropped ** [   30.113921] cpufreq: target for CPU 59: 2064000 kHz (1248000 kHz), relation 2, requested 2064000 kHz
** 437 printk messages dropped ** [   30.245846] cpufreq: target for CPU 21: 1296000 kHz (1296000 kHz), relation 2, requested 1296000 kHz
** 435 printk messages dropped ** [   30.397748] cpufreq: target for CPU 13: 1280000 kHz (2640000 kHz), relation 2, requested 1280000 kHz
** 480 printk messages dropped ** [   30.541846] cpufreq: target for CPU 58: 2112000 kHz (1632000 kHz), relation 2, requested 2112000 kHz


Of course, ideas to further debug this are welcome, or suggestions
to fix the issue in another way.


Thanks,

Andreas