linux-kernel - [RFT][PATCH v1 5/5] cpuidle: menu: Avoid discarding useful information

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Message-ID: <7770672.EvYhyI6sBW@rjwysocki.net>
Date: Thu, 06 Feb 2025 15:29:05 +0100
From: "Rafael J. Wysocki" <rjw@...ysocki.net>
To: Linux PM <linux-pm@...r.kernel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
 Daniel Lezcano <daniel.lezcano@...aro.org>,
 Christian Loehle <christian.loehle@....com>,
 Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
 Aboorva Devarajan <aboorvad@...ux.ibm.com>
Subject:
 [RFT][PATCH v1 5/5] cpuidle: menu: Avoid discarding useful information

From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

When giving up on making a high-confidence prediction,
get_typical_interval() always returns UINT_MAX which means that the
next idle interval prediction will be based entirely on the time till
the next timer.  However, the information represented by the most
recent intervals may not be completely useless in those cases.

Namely, the largest recent idle interval is an upper bound on the
recently observed idle duration, so it is reasonable to assume that
the next idle duration is unlikely to exceed it.  Moreover, this is
still true after eliminating the suspected outliers if the sample
set still under consideration is at least as large as 50% of the
maximum sample set size.

Accordingly, make get_typical_interval() return the current maximum
recent interval value in that case instead of UINT_MAX.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
---
 drivers/cpuidle/governors/menu.c |   13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

--- a/drivers/cpuidle/governors/menu.c
+++ b/drivers/cpuidle/governors/menu.c
@@ -190,8 +190,19 @@
 	 * This can deal with workloads that have long pauses interspersed
 	 * with sporadic activity with a bunch of short pauses.
 	 */
-	if ((divisor * 4) <= INTERVALS * 3)
+	if (divisor * 4 <= INTERVALS * 3) {
+		/*
+		 * If there are sufficiently many data points still under
+		 * consideration after the outliers have been eliminated,
+		 * returning without a prediction would be a mistake because it
+		 * is likely that the next interval will not exceed the current
+		 * maximum, so return the latter in that case.
+		 */
+		if (divisor >= INTERVALS / 2)
+			return max;
+
 		return UINT_MAX;
+	}

 	/* Update the thresholds for the next round. */
 	if (avg - min > max - avg)