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

[Andreas Herrmann <aherrmann@suse.com>]

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.

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.


Regards,

Andreas