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

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Date:   Wed, 28 Jul 2021 19:47:46 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Doug Smythies <dsmythies@...us.net>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v1 0/5] cpuidle: teo: Rework the idle state selection logic

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@...us.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;
 			}