[PATCH v8 12/12] cpuidle/poll_state: Wait for need-resched via tif_need_resched_relaxed_wait()

[Ankur Arora]

The inner loop in poll_idle() polls over the thread_info flags,
waiting to see if the thread has TIF_NEED_RESCHED set. The loop
exits once the condition is met, or if the poll time limit has
been exceeded.

To minimize the number of instructions executed in each iteration,
the time check is rate-limited. In addition, each loop iteration
executes cpu_relax() which on certain platforms provides a hint to
the pipeline that the loop busy-waits, allowing the processor to
reduce power consumption.

Switch over to tif_need_resched_relaxed_wait() instead, since that
provides exactly that.

However, given that when running in idle we want to minimize our power
consumption, continue to depend on CONFIG_ARCH_HAS_CPU_RELAX as that
serves as an indicator that the platform supports an optimized version
of tif_need_resched_relaxed_wait() (via
smp_cond_load_acquire_timeout()).

Cc: "Rafael J. Wysocki" <rafael at kernel.org>
Cc: Daniel Lezcano <daniel.lezcano at linaro.org>
Cc: linux-pm at vger.kernel.org
Suggested-by: "Rafael J. Wysocki" <rafael at kernel.org>
Signed-off-by: Ankur Arora <ankur.a.arora at oracle.com>
---

Notes:
  - use tif_need_resched_relaxed_wait() instead of
    smp_cond_load_relaxed_timeout()

 drivers/cpuidle/poll_state.c | 27 +++++----------------------
 1 file changed, 5 insertions(+), 22 deletions(-)

diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
index c7524e4c522a..20136b3a08c2 100644
--- a/drivers/cpuidle/poll_state.c
+++ b/drivers/cpuidle/poll_state.c
@@ -6,41 +6,24 @@
 #include <linux/cpuidle.h>
 #include <linux/export.h>
 #include <linux/irqflags.h>
-#include <linux/sched.h>
-#include <linux/sched/clock.h>
 #include <linux/sched/idle.h>
 #include <linux/sprintf.h>
 #include <linux/types.h>
 
-#define POLL_IDLE_RELAX_COUNT	200
-
 static int __cpuidle poll_idle(struct cpuidle_device *dev,
 			       struct cpuidle_driver *drv, int index)
 {
-	u64 time_start;
-
-	time_start = local_clock_noinstr();
-
 	dev->poll_time_limit = false;
 
 	raw_local_irq_enable();
 	if (!current_set_polling_and_test()) {
-		unsigned int loop_count = 0;
-		u64 limit;
+		s64 limit;
+		bool nr_set;
 
-		limit = cpuidle_poll_time(drv, dev);
+		limit = (s64)cpuidle_poll_time(drv, dev);
 
-		while (!need_resched()) {
-			cpu_relax();
-			if (loop_count++ < POLL_IDLE_RELAX_COUNT)
-				continue;
-
-			loop_count = 0;
-			if (local_clock_noinstr() - time_start > limit) {
-				dev->poll_time_limit = true;
-				break;
-			}
-		}
+		nr_set = tif_need_resched_relaxed_wait(limit);
+		dev->poll_time_limit = !nr_set;
 	}
 	raw_local_irq_disable();
 
-- 
2.31.1