cpuidle: poll_state: Avoid invoking local_clock() too often

Rik reports that he sees an increase in CPU use in one benchmark
due to commit 612f1a22f067 "cpuidle: poll_state: Add time limit to
poll_idle()" that caused poll_idle() to call local_clock() in every
iteration of the loop.  Utilization increase generally means more
non-idle time with respect to total CPU time (on the average) which
implies reduced CPU frequency.

Doug reports that limiting the rate of local_clock() invocations
in there causes much less power to be drawn during a CPU-intensive
parallel workload (with idle states 1 and 2 disabled to enforce more
state 0 residency).

These two reports together suggest that executing local_clock() on
multiple CPUs in parallel at a high rate may cause chips to get hot
and trigger thermal/power limits on them to kick in, so reduce the
rate of local_clock() invocations in poll_idle() to avoid that issue.

Fixes: 612f1a22f067 "cpuidle: poll_state: Add time limit to poll_idle()"
Reported-by: Rik van Riel <riel@surriel.com>
Reported-by: Doug Smythies <dsmythies@telus.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Rik van Riel <riel@surriel.com>
Reviewed-by: Rik van Riel <riel@surriel.com>

diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
index 68cdc506524b62df30ec4e84d2a51c342b8f5e6d..3f86d23c592ec0cdce4b6e8019a02256f4146d78 100644 (file)
--- a/drivers/cpuidle/poll_state.c
+++ b/drivers/cpuidle/poll_state.c
@@ -10,6 +10,7 @@
 #include <linux/sched/idle.h>
 
 #define POLL_IDLE_TIME_LIMIT   (TICK_NSEC / 16)
+#define POLL_IDLE_RELAX_COUNT  200
 
 static int __cpuidle poll_idle(struct cpuidle_device *dev,
                               struct cpuidle_driver *drv, int index)
@@ -18,9 +19,14 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
 
        local_irq_enable();
        if (!current_set_polling_and_test()) {
+               unsigned int loop_count = 0;
+
                while (!need_resched()) {
                        cpu_relax();
+                       if (loop_count++ < POLL_IDLE_RELAX_COUNT)
+                               continue;
 
+                       loop_count = 0;
                        if (local_clock() - time_start > POLL_IDLE_TIME_LIMIT)
                                break;
                }