Re: [RFC/RFT][PATCH] cpufreq: intel_pstate: Work in passive mode with HWP enabled

["Rafael J. Wysocki" <rafael@kernel.org>]

On Tue, May 26, 2020 at 5:51 PM Doug Smythies <dsmythies@telus.net> wrote:
>
> On 2020.05.26 01:19 Rafael J. Wysocki wrote:
> >  to On Mon, May 25, 2020 at 10:57 PM Francisco Jerez
> > > "Rafael J. Wysocki" <rafael@kernel.org> writes:
> > > > On Mon, May 25, 2020 at 3:39 AM Francisco Jerez
> > >
> > > Why not HWP_MIN_PERF?  That would leave the HWP quite some room for
> > > maneuvering (the whole HWP_MIN_PERF-HWP_MAX_PERF P-state range, it's not
> > > like P-state selection would be entirely driven by the kernel), while
> > > avoiding every failure scenario I was describing in my previous reply.
>
> I have re-done my tests.
> The problem that I observe seems specific to hwp itself
> and not this patch and it's use in passive mode.
> I see the exact same thing with intel_pstate/powersave.
> [1] detail A.
>
>
> Test: still simple single threaded load sweep,
> at 347 hertz work/sleep frequency.
> What do I see?
>
> Unexpected frequency drops at around 70% load.
> Example, from trace:
>
> First, the thing has been going for awhile at 4.8 GHz.
>
> Old epp ; new epp ; freq GHz; load % ; duration mS
> 80        ; 82      ; 4.57    ; 61.94  ; 20.001
> 82      ; 80    ; 4.57    ; 62.47  ; 40.003
> 80      ; 44      ; 3.73    ;   68.63  ; 62.009  <<<< What? Why freq down? Why long duration?
> 44      ;  0      ; 1.96    ; 100.23 ; 19.995  <<<< Even lower freq. load overruns.
>  0      ; 73      ; 4.56    ; 82.93  ; 40.07   <<<< O.K. recovered, but damage done.
> 73      ; 46      ; 2.36    ;   79.19  ; 20.94   <<< now things oscillate a little.
> 46      ; 0       ; 1.9884  ;   100.24 ; 20.99
>  0      ; 75      ; 4.5624  ;   82.1   ; 41.002  <<< Event ends. Next event in 487 milliseconds.
>
> Observation: Events are often, but not always, preceded by a longer than normal duration.
> However, long durations are also not out of the ordinary in passive mode.
>
> And yes, the above trace was with DELAY_HWP 20,000, but I do have trace examples
> with it at 5,000. This was just a particularly good example.
>
> Observation (from looking at a lot of trace data): There are phase delays
> between the two systems, intel_cpufreq and hwp, and sometimes they seem to
> oscillate a little and fight each other. There maybe some problematic
> work/sleep frequencies where the oscillation builds into a full blown
> resonance.
>
> Why does hwp drop the frequency?
>
> This system is otherwise fairly idle,
> so maybe because the pll drops down during the non work periods.
>
> Maybe HWP thinks the system is idle and drops the frequency.
> I can eliminate the overruns by disabling deep idle states such
> that the PLL vote is never relinquished, but it's not a fair test.
>
> Note that the above response can be "tuned".
> If we take the conversation algorithm from target frequency to EPP
> and introduce and offset, the above can be improved.
>
> At what cost? More sluggishness, for a large positive offset.
> So, the overruns just move from the steady state side of the task to
> when the task starts. I did not find if there is a "sweet spot"
> between offset and system response, and I do not think there is value
> added in trying.
>
> Note: With original settings, I rarely observe a problem with the step
> function response to a new task.
>
> >
> > Actually, I have been thinking about the HWP min as an alternative
> > that may be worth evaluating.
> >
> > However, I would rather set the HWP min to something like 80% if the
> > cpufreq request.
>
> Yes, this is a good idea and should not suffer from the two servo systems
> fighting each other.

OK, thanks for the feedback!

I am about to post this patch.

> I got 0 overruns, verses 2240 overruns with no min limitation (100 second test).
>
> As for INTEL_CPUFREQ_TRANSITION_DELAY_HWP, I'll probably use
> 10 milliseconds moving forward, because that is what I am most
> familiar with from years ago work on the this driver. But, I did
> not observe any issue with 5 milliseconds.

I'm going to use INTEL_CPUFREQ_TRANSITION_DELAY_HWP in the new patch
for now then.

> [1] http://www.smythies.com/~doug/linux/intel_pstate/passive-hwp/passive-hwp-but-active-powersave.png

Thanks!