cpuidle: menu: Retain tick when shallow state is selected

rafaeljw · rafaeljw · commit 757ab15c3f49 · 2018-08-25T13:16:08.000+02:00
The case addressed by commit 5ef499c (cpuidle: menu: Handle stopped tick more aggressively) in the stopped tick case is present when the tick has not been stopped yet too. Namely, if only two CPU idle states, shallow state A with target residency significantly below the tick boundary and deep state B with target residency significantly above it, are available and the predicted idle duration is above the tick boundary, but below the target residency of state B, state A will be selected and the CPU may spend indefinite amount of time in it, which is not quite energy-efficient. However, if the tick has not been stopped yet and the governor is about to select a shallow idle state for the CPU even though the idle duration predicted by it is above the tick boundary, it should be fine to wake up the CPU early, so the tick can be retained then and the governor will have a chance to select a deeper state when it runs next time. [Note that when this really happens, it will make the idle duration predictor believe that the CPU might be idle longer than predicted, which will make it more likely to predict longer idle durations going forward, but that will also cause deeper idle states to be selected going forward, on average, which is what's needed here.] Fixes: 87c9fe6 (cpuidle: menu: Avoid selecting shallow states with stopped tick) Reported-by: Leo Yan <leo.yan@linaro.org> Cc: 4.17+ <stable@vger.kernel.org> # 4.17+: 5ef499c (cpuidle: menu: Handle ...) Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
@@ -379,9 +379,20 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 		if (idx == -1)
 			idx = i; /* first enabled state */
 		if (s->target_residency > data->predicted_us) {
-			if (!tick_nohz_tick_stopped())
+			if (data->predicted_us < TICK_USEC)
 				break;
 
+			if (!tick_nohz_tick_stopped()) {
+				/*
+				 * If the state selected so far is shallow,
+				 * waking up early won't hurt, so retain the
+				 * tick in that case and let the governor run
+				 * again in the next iteration of the loop.
+				 */
+				expected_interval = drv->states[idx].target_residency;
+				break;
+			}
+
 			/*
 			 * If the state selected so far is shallow and this
 			 * state's target residency matches the time till the
