Commit 4dc2375c authored Mar 27, 2018 by Rafael J. Wysocki

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>

parent 64bdff69

drivers/cpuidle/poll_state.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -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;
		}