cpuidle: menu: Avoid selecting shallow states with stopped tick

If the scheduler tick has been stopped already and the governor
selects a shallow idle state, the CPU can spend a long time in that
state if the selection is based on an inaccurate prediction of idle
time.  That effect turns out to be relevant, so it needs to be
mitigated.

To that end, modify the menu governor to discard the result of the
idle time prediction if the tick is stopped and the predicted idle
time is less than the tick period length, unless the tick timer is
going to expire soon.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
index 267982e471e026e791bc8ac42b7839d8ebfbdeb5..1bfe03ceb236c3cee8df46fc284f1cc17e30c9ec 100644 (file)
--- a/drivers/cpuidle/governors/menu.c
+++ b/drivers/cpuidle/governors/menu.c
@@ -352,13 +352,28 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
         */
        data->predicted_us = min(data->predicted_us, expected_interval);
 
-       /*
-        * Use the performance multiplier and the user-configurable
-        * latency_req to determine the maximum exit latency.
-        */
-       interactivity_req = data->predicted_us / performance_multiplier(nr_iowaiters, cpu_load);
-       if (latency_req > interactivity_req)
-               latency_req = interactivity_req;
+       if (tick_nohz_tick_stopped()) {
+               /*
+                * If the tick is already stopped, the cost of possible short
+                * idle duration misprediction is much higher, because the CPU
+                * may be stuck in a shallow idle state for a long time as a
+                * result of it.  In that case say we might mispredict and try
+                * to force the CPU into a state for which we would have stopped
+                * the tick, unless a timer is going to expire really soon
+                * anyway.
+                */
+               if (data->predicted_us < TICK_USEC)
+                       data->predicted_us = min_t(unsigned int, TICK_USEC,
+                                                  ktime_to_us(delta_next));
+       } else {
+               /*
+                * Use the performance multiplier and the user-configurable
+                * latency_req to determine the maximum exit latency.
+                */
+               interactivity_req = data->predicted_us / performance_multiplier(nr_iowaiters, cpu_load);
+               if (latency_req > interactivity_req)
+                       latency_req = interactivity_req;
+       }
 
        expected_interval = data->predicted_us;
        /*