linux-kernel - [PATCH v2 2/3] cpuidle: teo: Avoid stopping the tick unnecessarily when bailing out

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Date:   Fri, 28 Jul 2023 12:00:46 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Linux PM <linux-pm@...r.kernel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        Kajetan Puchalski <kajetan.puchalski@....com>
Subject: [PATCH v2 2/3] cpuidle: teo: Avoid stopping the tick unnecessarily when bailing out

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

When teo_select() is going to return early in some special cases, make
it avoid stopping the tick if the idle state to be returned is shallow.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
---
 drivers/cpuidle/governors/teo.c |   50 +++++++++++++++++++++++++---------------
 1 file changed, 32 insertions(+), 18 deletions(-)

Index: linux-pm/drivers/cpuidle/governors/teo.c
===================================================================
--- linux-pm.orig/drivers/cpuidle/governors/teo.c
+++ linux-pm/drivers/cpuidle/governors/teo.c
@@ -462,9 +462,9 @@ static int teo_select(struct cpuidle_dri
 	/* Avoid unnecessary overhead. */
 	if (idx < 0) {
 		idx = 0; /* No states enabled, must use 0. */
-		goto end;
+		goto bail_out;
 	} else if (idx == idx0) {
-		goto end;
+		goto bail_out;
 	}
 
 	/*
@@ -547,8 +547,10 @@ static int teo_select(struct cpuidle_dri
 	 * If there is a latency constraint, it may be necessary to select an
 	 * idle state shallower than the current candidate one.
 	 */
-	if (idx > constraint_idx)
+	if (idx > constraint_idx) {
 		idx = constraint_idx;
+		goto bail_out;
+	}
 
 	/*
 	 * If the CPU is being utilized over the threshold, choose a shallower
@@ -569,23 +571,35 @@ static int teo_select(struct cpuidle_dri
 
 end:
 	/*
-	 * Don't stop the tick if the selected state is a polling one or if the
-	 * expected idle duration is shorter than the tick period length.
+	 * Allow the tick to be stopped unless the selected state is a polling
+	 * one or the expected idle duration is shorter than the tick period
+	 * length.
 	 */
-	if (((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) ||
-	    duration_ns < TICK_NSEC) && !tick_nohz_tick_stopped()) {
-		*stop_tick = false;
+	if ((!(drv->states[idx].flags & CPUIDLE_FLAG_POLLING) &&
+	    duration_ns >= TICK_NSEC) || tick_nohz_tick_stopped())
+		return idx;
 
-		/*
-		 * The tick is not going to be stopped, so if the target
-		 * residency of the state to be returned is not within the time
-		 * till the closest timer including the tick, try to correct
-		 * that.
-		 */
-		if (idx > idx0 &&
-		    drv->states[idx].target_residency_ns > delta_tick)
-			idx = teo_find_shallower_state(drv, dev, idx, delta_tick, false);
-	}
+retain_tick:
+	*stop_tick = false;
+
+	/*
+	 * The tick is not going to be stopped, so if the target residency of
+	 * the state to be returned is not within the time till the closest
+	 * timer including the tick, try to correct that.
+	 */
+	if (idx > idx0 &&
+	    drv->states[idx].target_residency_ns > delta_tick)
+		idx = teo_find_shallower_state(drv, dev, idx, delta_tick, false);
+
+	return idx;
+
+bail_out:
+	/*
+	 * Do not allow the tick to be stopped if the selected state is shallow
+	 * enough.
+	 */
+	if (drv->states[idx].target_residency_ns < TICK_NSEC)
+		goto retain_tick;
 
 	return idx;
 }