[PATCH v3 1/3] cpuidle: teo: Update idle duration estimate when choosing shallower state

[Rafael J. Wysocki]

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

The TEO governor takes CPU utilization into account by refining idle state
selection when the utilization is above a certain threshold.  This is done by
choosing an idle state shallower than the previously selected one.

However, when doing this, the idle duration estimate needs to be
adjusted so as to prevent the scheduler tick from being stopped when the
candidate idle state is shallow, which may lead to excessive energy
usage if the CPU is not woken up quickly enough going forward.
Moreover, if the scheduler tick has been stopped already and the new
idle duration estimate is too small, the replacement candidate state
cannot be used.

Modify the relevant code to take the above observations into account.

Fixes: 9ce0f7c4bc64 ("cpuidle: teo: Introduce util-awareness")
Link: https://lore.kernel.org/linux-pm/CAJZ5v0jJxHj65r2HXBTd3wfbZtsg=_StzwO1kA5STDnaPe_dWA@xxxxxxxxxxxxxx
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
---

v2 -> v3:
   * Make the handling of the "2 idle state and utilized CPU" case more
     straightforward.

v1 -> v2:
   * Rework the code handling the special case when the CPU is utilized and
     there are only 2 idle states (drop the loop, avoid using state 0 when
     the tick has been stopped already and it is too shallow, check if
     state 1 is not disabled when about to use it, set low idle duration
     estimate).
   * Changelog edits.

---
 drivers/cpuidle/governors/teo.c |   40 ++++++++++++++++++++++++++++++----------
 1 file changed, 30 insertions(+), 10 deletions(-)

Index: linux-pm/drivers/cpuidle/governors/teo.c
===================================================================
--- linux-pm.orig/drivers/cpuidle/governors/teo.c
+++ linux-pm/drivers/cpuidle/governors/teo.c
@@ -397,13 +397,23 @@ static int teo_select(struct cpuidle_dri
 	 * the shallowest non-polling state and exit.
 	 */
 	if (drv->state_count < 3 && cpu_data->utilized) {
-		for (i = 0; i < drv->state_count; ++i) {
-			if (!dev->states_usage[i].disable &&
-			    !(drv->states[i].flags & CPUIDLE_FLAG_POLLING)) {
-				idx = i;
-				goto end;
-			}
-		}
+		/* The CPU is utilized, so assume a short idle duration. */
+		duration_ns = teo_middle_of_bin(0, drv);
+		/*
+		 * If state 0 is enabled and it is not a polling one, select it
+		 * right away unless the scheduler tick has been stopped, in
+		 * which case care needs to be taken to leave the CPU in a deep
+		 * enough state in case it is not woken up any time soon after
+		 * all.  If state 1 is disabled, though, state 0 must be used
+		 * anyway.
+		 */
+		if ((!idx && !(drv->states[0].flags & CPUIDLE_FLAG_POLLING) &&
+		    teo_time_ok(duration_ns)) || dev->states_usage[1].disable)
+			idx = 0;
+		else /* Assume that state 1 is not a polling one and use it. */
+			idx = 1;
+
+		goto end;
 	}
 
 	/*
@@ -539,10 +549,20 @@ static int teo_select(struct cpuidle_dri
 
 	/*
 	 * If the CPU is being utilized over the threshold, choose a shallower
-	 * non-polling state to improve latency
+	 * non-polling state to improve latency, unless the scheduler tick has
+	 * been stopped already and the shallower state's target residency is
+	 * not sufficiently large.
 	 */
-	if (cpu_data->utilized)
-		idx = teo_find_shallower_state(drv, dev, idx, duration_ns, true);
+	if (cpu_data->utilized) {
+		s64 span_ns;
+
+		i = teo_find_shallower_state(drv, dev, idx, duration_ns, true);
+		span_ns = teo_middle_of_bin(i, drv);
+		if (teo_time_ok(span_ns)) {
+			idx = i;
+			duration_ns = span_ns;
+		}
+	}
 
 end:
 	/*