linux-kernel - [RFC PATCH 2/3] cpuidle: ladder: Tune promotion/demotion threshold

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Date:   Sun,  6 Nov 2022 01:42:24 +0800
From:   Zhang Rui <rui.zhang@...el.com>
To:     rjw@...ysocki.net, daniel.lezcano@...aro.org
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH 2/3] cpuidle: ladder: Tune promotion/demotion threshold

After fixing the bogus comparison between u64 and s64, the ladder
governor stops making promotion decisions errornously.

However, after this, it is found that the ladder governor demotes much
easier than promotes.

Below is captured using turbostat after a 30 seconds runtime idle,

Without previous patch,
Busy%	IRQ	POLL	C1	C1E	C3	C6	C7s	C8	C9	C10	CPU%c1	CPU%c3	CPU%c6	CPU%c7	PkgWatt
0.30	2373	0	0	0	4	9	25	122	326	2857	0.36	0.04	0.57	98.73	1.48

With previous patch,
Busy%	IRQ	POLL	C1	C1E	C3	C6	C7s	C8	C9	C10	CPU%c1	CPU%c3	CPU%c6	CPU%c7	PkgWatt
0.42	3071	0	771	838	447	327	336	382	299	344	34.18	16.21	17.69	31.51	2.00

And this is caused by the imbalanced PROMOTION_COUNT/DEMOTION_COUNT.

With this patch,
Busy%	IRQ	POLL	C1	C1E	C3	C6	C7s	C8	C9	C10	CPU%c1	CPU%c3	CPU%c6	CPU%c7	PkgWatt
0.39	2436	0	1	72	177	51	194	243	799	1883	0.50	0.32	0.35	98.45	1.53

Note that this is an experimental patch to illustrate the problem,
and it is checked with idle scenario only for now.
I will try to evaluate with more scenarios, and if someone can help
evaluate with more scenarios at the same time and provide data for the
benefit with different PROMOTION_COUNT/DEMOTION_COUNT values, that
would be great.

Signed-off-by: Zhang Rui <rui.zhang@...el.com>
---
 drivers/cpuidle/governors/ladder.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/cpuidle/governors/ladder.c b/drivers/cpuidle/governors/ladder.c
index fb61118aef37..4b47aa0a4da9 100644
--- a/drivers/cpuidle/governors/ladder.c
+++ b/drivers/cpuidle/governors/ladder.c
@@ -20,8 +20,8 @@
 #include <asm/io.h>
 #include <linux/uaccess.h>

-#define PROMOTION_COUNT 4
-#define DEMOTION_COUNT 1
+#define PROMOTION_COUNT 2
+#define DEMOTION_COUNT 4

 struct ladder_device_state {
 	struct {
-- 
2.25.1