Date:   Mon, 19 Sep 2016 18:16:11 +0200
From:   Andreas Herrmann <aherrmann@...e.com>
To:     Stratos Karafotis <skarafotis@...il.com>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Stratos Karafotis <stratosk@...aphore.gr>,
        Thomas Renninger <trenn@...e.com>
Subject: Re: [PATCH 1/1] cpufreq: pcc-cpufreq: Re-introduce deadband effect
 to reduce number of frequency changes

On Fri, Sep 16, 2016 at 09:58:42PM +0300, Stratos Karafotis wrote:
> Hi,
> 
> [ I 'm resending this message, because I think some recipients didn't receive
> it. ]
> 
> On 16/09/2016 12:47 μμ, Andreas Herrmann wrote:
> > On Wed, Sep 07, 2016 at 10:32:01AM +0530, Viresh Kumar wrote:
> >> On 01-09-16, 15:21, Andreas Herrmann wrote:
> >>> On Mon, Aug 29, 2016 at 11:31:53AM +0530, Viresh Kumar wrote:
> >
> >>>> 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.
> >
> >> Naah, that isn't what I was looking for, sorry 
> >
> >> To explain it a bit more, this is what the patch did.
> >
> >> Suppose, your platform supports frequencies: F1 (lowest), F2, F3, F4,
> >> F5, F6 and F7 (highest). The cpufreq governor (ondemand) based on a
> >> sampling rate and system load tries to change the frequency of the
> >> underlying hardware and select one of those.
> >
> >> Before the original patch came in, F2 and F3 were never getting
> >> selected and the system was stuck in F1 for a long time.
> >
> > I think this is not a general statement. Such a behaviour is not
> > common to all systems. Before commit 6393d6a target frequency was
> > based on
> >
> >    freq_next = load * policy->cpuinfo.max_freq / 100;
> >
> > F2 would have been selected if
> >
> >   load = F2 * 100 / F7
> >
> > If F2 was not seen it can mean
> >
> > (1) either the load value was not hit in practice during monitoring of
> >     a certain workload
> >
> > (2) or the calculated load value (in integer representation) would
> >     select F1 or F3 (there is no corresponding integer value that
> >     would select F2)
> >
> > E.g. for the Intel i7-3770 system mentioned in commit message for
> > 6393d6a I think a load value of 49 should have selected 1700000 which
> > is not shown in the provided frequency table.
> 
> I think this is not true, because before the specific patch the relation
> of frequency selection was CPUFREQ_RELATION_L. This is the reason
> that a load value of 49 with a freq_next 1666490 would have a
> target frequency 1600000.

Hmm...
CPUFREQ_RELATION_L should select "lowest frequency at or above target"
being 1700000 in this case. Otherwise (if it would select "highest
frequency at or below target") this would imply that load values of
50, 51, 52 should select 1700000 which would contradict what was
written in commit message of 6393d6a1.

In any case probability of seeing such load values and thus selecting
a frequency of 1700000 is quite low. So I fully understand why the
patch was introduced.

> > What essentially changed was how load values are mapped to target
> > frequencies. For the HP system (min_freq=1200000, max_freq=2800000)
> > that I used in my tests, the old code would create following mapping:
> >
> > load | freq_next | used target frequency
> > ________________________________________
> > 0      0            1200000
> > 10     280000       1200000
> > 20     560000       1200000
> > 30     840000       1200000
> > 40     1120000      1200000
> > 42     1176000      1200000
> > 43     1204000      1204000
> > 50     1400000      1400000
> > 60     1680000      1680000
> > 70     1960000      1960000
> > 80     2240000      2240000
> > 90     2520000      2520000
> > 100    2800000      2800000
> >
> > The new code (introduced with commit 6393d6a) changed the mapping as
> > follows:
> >
> > load | freq_next | used target frequency
> > ________________________________________
> > 0      1200000      1200000
> > 10     1360000    1360000
> > 20     1520000    1520000
> > 30     1680000    1680000
> > 40     1840000    1840000
> > 42     1872000    1872000
> > 43     1888000    1888000
> > 50     2000000    2000000
> > 60     2160000    2160000
> > 70     2320000    2320000
> > 80     2480000    2480000
> > 90     2640000    2640000
> > 100    2800000    2800000
> >
> > My patch creates a third mapping. It basically ensures that up to a
> > load value of 42 the minimum frequency is used.
> >
> >> Which will decrease the performance for that period of time as we
> >> should have switched to a higher frequency really.
> >
> > I am not sure whether it's really useful for all systems using
> > ondemand governor to increase frequency above min_freq even if load is
> > just above 0. Of course expectation is that performance will be equal
> > or better than before. But how overall power consumption changes
> > depends on the hardware and its power saving capabilites.
> >
> >   ---8<---
> >
> >>> 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.
> >
> >> It was principally the right thing to do IMO. And I don't think any
> >> other hardware should get affected badly. At the max, the tuning needs
> >> to be made a bit better.
> >
> >   ---8<---
> >
> > It seems that the decision how to best map load values to target
> > frequencies is kind of hardware specific.
> >
> > Maybe a solution to this is that the cpufreq driver should be able to
> > provide a mapping function to overwrite the current default
> > calculation.
> >
> 
> I'm not familiar with ppc-cpufreq drive but maybe patch 6393d6 just
> uncovered an "issue" that was already existed but only on higher loads.
>
> Because, with or without patch 6393d6, if the specific CPU doesn't
> use a frequency table, there will many frequency transitions in
> higher loads too. I believe, though, that the side effect it's smaller
> in higher frequencies because CPUs tend to work on lowest and highest
> frequencies.

Might be. I didn't test this specifically.

> What about a patch in ppc-cpufreq driver that permits frequency
> changes only in specific steps and not in arbitrary values?

Which steps would you use? What scheme would be universal usable for
all affected system using this driver?

I had played with an approach to only make use of min_freq and
max_freq which eventually didn't result in better performance
in comparison to code before commit 6393d6.


Andreas

> Regards,
> Stratos

  ---8<---

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