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From:   "Rafael J. Wysocki" <rjw@rjwysocki.net>
To:     Linux PM <linux-pm@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Anna-Maria Behnsen <anna-maria@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Kajetan Puchalski <kajetan.puchalski@arm.com>
Subject: [RFT][PATCH v2 2/3] cpuidle: teo: Skip tick_nohz_get_sleep_length() call in some cases
Date:   Thu, 03 Aug 2023 23:09:18 +0200
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From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Make teo_select() avoid calling tick_nohz_get_sleep_length() if the
candidate idle state to return is state 0 or if state 0 is a polling
one and the target residency of the current candidate one is below
a certain threshold, in which cases it may be assumed that the CPU will
be woken up immediately by a non-timer wakeup source and the timers
are not likely to matter.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---

v1 -> v2: No changes

---
 drivers/cpuidle/governors/teo.c |   22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

Index: linux-pm/drivers/cpuidle/governors/teo.c
===================================================================
--- linux-pm.orig/drivers/cpuidle/governors/teo.c
+++ linux-pm/drivers/cpuidle/governors/teo.c
@@ -166,6 +166,12 @@
  */
 #define NR_RECENT	9
 
+/*
+ * Idle state target residency threshold used for deciding whether or not to
+ * check the time till the closest expected timer event.
+ */
+#define RESIDENCY_THRESHOLD_NS	(15 * NSEC_PER_USEC)
+
 /**
  * struct teo_bin - Metrics used by the TEO cpuidle governor.
  * @intercepts: The "intercepts" metric.
@@ -542,6 +548,22 @@ static int teo_select(struct cpuidle_dri
 			idx = i;
 	}
 
+	/*
+	 * Skip the timers check if state 0 is the current candidate one,
+	 * because an immediate non-timer wakeup is expected in that case.
+	 */
+	if (!idx)
+		goto out_tick;
+
+	/*
+	 * If state 0 is a polling one, check if the target residency of
+	 * the current candidate state is low enough and skip the timers
+	 * check in that case too.
+	 */
+	if ((drv->states[0].flags & CPUIDLE_FLAG_POLLING) &&
+	    drv->states[idx].target_residency_ns < RESIDENCY_THRESHOLD_NS)
+		goto out_tick;
+
 	duration_ns = tick_nohz_get_sleep_length(&delta_tick);
 	cpu_data->sleep_length_ns = duration_ns;