Re: [PATCH v1 0/5] cpuidle: teo: Rework the idle state selection logic

[Doug Smythies <dsmythies@telus.net>]

On Wed, Jul 28, 2021 at 10:47 AM Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
>
> On Wednesday, July 28, 2021 3:52:51 PM CEST Rafael J. Wysocki wrote:
> > On Tue, Jul 27, 2021 at 10:06 PM Doug Smythies <dsmythies@telus.net> wrote:
> > >
> > > Hi Rafael,
> > >
> > > Further to my reply of 2021.07.04  on this, I have
> > > continued to work with and test this patch set.
> > >
> > > On 2021.06.02 11:14 Rafael J. Wysocki wrote:
> > >
> > > >This series of patches addresses some theoretical shortcoming in the
> > > > TEO (Timer Events Oriented) cpuidle governor by reworking its idle
> > > > state selection logic to some extent.
> > > >
> > > > Patches [1-2/5] are introductory cleanups and the substantial changes are
> > > > made in patches [3-4/5] (please refer to the changelogs of these two
> > > > patches for details).  The last patch only deals with documentation.
> > > >
> > > > Even though this work is mostly based on theoretical considerations, it
> > > > shows a measurable reduction of the number of cases in which the shallowest
> > > > idle state is selected while it would be more beneficial to select a deeper
> > > > one or the deepest idle state is selected while it would be more beneficial to
> > > > select a shallower one, which should be a noticeable improvement.
> > >
> > > I am concentrating in the idle state 0 and 1 area.
> > > When I disable idle state 0, the expectation is its
> > > usage will fall to idle state 1. It doesn't.
> > >
> > > Conditions:
> > > CPU: Intel(R) Core(TM) i5-10600K CPU @ 4.10GHz
> > > HWP: disabled
> > > CPU frequency scaling driver: intel_pstate, active
> > > CPU frequency scaling governor: performance.
> > > Idle configuration: As a COMETLAKE processor, with 4 idle states.
> > > Sample time for below: 1 minute.
> > > Workflow: Cross core named pipe token passing, 12 threads.
> > >
> > > Kernel 5.14-rc3: idle: teo governor
> > >
> > > All idle states enabled: PASS
> > > Processor: 97 watts
> > > Idle state 0 entries: 811151
> > > Idle state 1 entries: 140300776
> > > Idle state 2 entries: 889
> > > Idle state 3 entries: 8
> > >
> > > Idle state 0 disabled: FAIL <<<<<
> > > Processor: 96 watts
> > > Idle state 0 entries: 0
> > > Idle state 1 entries: 65599283
> > > Idle state 2 entries: 364399
> > > Idle state 3 entries: 65112651
> >
> > This looks odd.
> >
> > Thanks for the report, I'll take a look at this.
>
> I have found an issue in the code that may be responsible for the
> observed behavior and should be addressed by the appended patch (not
> tested yet).
>
> Basically, the "disabled" check in the second loop over states in
> teo_select() needs to exclude the first enabled state, because
> there are no more states to check after that.
>
> Plus the time span check needs to be done when the given state
> is about to be selected, because otherwise the function may end up
> returning a state for which the sums are too low.
>
> Thanks!
>
> ---
>  drivers/cpuidle/governors/teo.c |   26 ++++++++++++++------------
>  1 file changed, 14 insertions(+), 12 deletions(-)
>
> Index: linux-pm/drivers/cpuidle/governors/teo.c
> ===================================================================
> --- linux-pm.orig/drivers/cpuidle/governors/teo.c
> +++ linux-pm/drivers/cpuidle/governors/teo.c
> @@ -404,25 +404,27 @@ static int teo_select(struct cpuidle_dri
>                         intercept_sum += bin->intercepts;
>                         recent_sum += bin->recent;
>
> -                       if (dev->states_usage[i].disable)
> +                       if (dev->states_usage[i].disable && i > idx0)
>                                 continue;
>
>                         span_ns = teo_middle_of_bin(i, drv);
> -                       if (!teo_time_ok(span_ns)) {
> -                               /*
> -                                * The current state is too shallow, so select
> -                                * the first enabled deeper state.
> -                                */
> -                               duration_ns = last_enabled_span_ns;
> -                               idx = last_enabled_idx;
> -                               break;
> -                       }
>
>                         if ((!alt_recent || 2 * recent_sum > idx_recent_sum) &&
>                             (!alt_intercepts ||
>                              2 * intercept_sum > idx_intercept_sum)) {
> -                               idx = i;
> -                               duration_ns = span_ns;
> +                               if (!teo_time_ok(span_ns) ||
> +                                   dev->states_usage[i].disable) {
> +                                       /*
> +                                        * The current state is too shallow or
> +                                        * disabled, so select the first enabled
> +                                        * deeper state.
> +                                        */
> +                                       duration_ns = last_enabled_span_ns;
> +                                       idx = last_enabled_idx;
> +                               } else {
> +                                       idx = i;
> +                                       duration_ns = span_ns;
> +                               }
>                                 break;
>                         }

Hi Rafael,

I tried the patch and when I disabled idle state 0
got, very similar to before:

Idle state 0 disabled: FAIL
Processor: 95 watts
Idle state 0 entries: 0
Idle state 1 entries: 65,475,534
Idle state 2 entries: 333144
Idle state 3 entries: 65,247,048

However, I accidently left it for about 30 minutes
and noticed:

Idle state 0 disabled:
Processor: 83 watts
Idle state 0 entries: 0
Idle state 1 entries: 88,706,831
Idle state 2 entries: 100
Idle state 3 entries: 662

I went back to unmodified kernel 5.13-rc3 and
let it run longer with idle state 0 disabled, and
after 30 minutes it had changed but nowhere
near as much:

Idle state 0 disabled:
Processor: 87 watts
Idle state 0 entries: 0
Idle state 1 entries: 70,361,020
Idle state 2 entries: 71219
Idle state 3 entries: 27,249,975

... Doug