linux-kernel - [PATCH v4] cpuidle: menu: Use residency threshold in polling state override decisions

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Message-ID: <20251006013954.17972-1-aboorvad@linux.ibm.com>
Date: Mon,  6 Oct 2025 07:09:54 +0530
From: Aboorva Devarajan <aboorvad@...ux.ibm.com>
To: rafael@...nel.org, christian.loehle@....com, daniel.lezcano@...aro.org
Cc: aboorvad@...ux.ibm.com, gautam@...ux.ibm.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v4] cpuidle: menu: Use residency threshold in polling state override decisions

On virtualized PowerPC (pseries) systems, where only one polling state
(Snooze) and one deep state (CEDE) are available, selecting CEDE when
the predicted idle duration is less than the target residency of CEDE
state can hurt performance. In such cases, the entry/exit overhead of
CEDE outweighs the power savings, leading to unnecessary state
transitions and higher latency.

Menu governor currently contains a special-case rule that prioritizes
the first non-polling state over polling, even when its target residency
is much longer than the predicted idle duration. On PowerPC/pseries,
where the gap between the polling state (Snooze) and the first non-polling
state (CEDE) is large, this behavior causes performance regressions.

This patch refines the special case by adding an extra requirement:
the first non-polling state can only be chosen if its
target residency is below the defined RESIDENCY_THRESHOLD_NS. If
this condition is not satisfied, polling is allowed instead, avoiding
suboptimal non-polling state entries.

This change is limited to the single special-case rule for the first
non-polling state. The general non-polling state selection logic in the
menu governor remains unchanged.

Performance improvement observed with pgbench on PowerPC (pseries)
system:
+---------------------------+------------+------------+------------+
| Metric                    | Baseline   | Patched    | Change (%) |
+---------------------------+------------+------------+------------+
| Transactions/sec (TPS)    | 495,210    | 536,982    | +8.45%     |
| Avg latency (ms)          | 0.163      | 0.150      | -7.98%     |
+---------------------------+------------+------------+------------+
CPUIdle state usage:
+--------------+--------------+-------------+
| Metric       | Baseline     | Patched     |
+--------------+--------------+-------------+
| Total usage  | 12,735,820   | 13,918,442  |
| Above usage  | 11,401,520   | 1,598,210   |
| Below usage  | 20,145       | 702,395     |
+--------------+--------------+-------------+

Above/Total and Below/Total usage percentages:
+------------------------+-----------+---------+
| Metric                 | Baseline  | Patched |
+------------------------+-----------+---------+
| Above % (Above/Total)  | 89.56%    | 11.49%  |
| Below % (Below/Total)  | 0.16%     | 5.05%   |
| Total cpuidle miss (%) | 89.72%    | 16.54%  |
+------------------------+-----------+---------+

The results indicate that restricting CEDE selection to cases where
its residency matches the predicted idle time reduces mispredictions,
lowers unnecessary state transitions, and improves overall throughput.

Reviewed-by: Christian Loehle <christian.loehle@....com>
Signed-off-by: Aboorva Devarajan <aboorvad@...ux.ibm.com>
---

v3: https://lore.kernel.org/all/20250908075443.208570-1-aboorvad@linux.ibm.com/

v3 -> v4

- Rebased onto the linux-pm/pm branch.
- Updated commit message and comments based on review feedback.
- Reordered condition checks as recommended in review.
- Added Reviewed-by tag from Christian.

---
 drivers/cpuidle/governors/menu.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
index 4d9aa5ce31f0..6a98a724442e 100644
--- a/drivers/cpuidle/governors/menu.c
+++ b/drivers/cpuidle/governors/menu.c
@@ -320,10 +320,12 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 		}

 		/*
-		 * Use a physical idle state, not busy polling, unless a timer
-		 * is going to trigger soon enough.
+		 * Use a physical idle state instead of busy polling as long as
+		 * its target residency is below the residency threshold and the
+		 * next timer doesn't expire soon.
 		 */
 		if ((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) &&
+		    s->target_residency_ns < RESIDENCY_THRESHOLD_NS &&
 		    s->target_residency_ns <= data->next_timer_ns) {
 			predicted_ns = s->target_residency_ns;
 			idx = i;
-- 
2.47.1