cpuidle/poll_state: Poll via smp_cond_load_relaxed_timeout()

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 done only intermittently (once every
POLL_IDLE_RELAX_COUNT iterations). 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.

This is close to what smp_cond_load_relaxed_timeout() provides. So,
restructure the loop and fold the loop condition and the timeout check
in smp_cond_load_relaxed_timeout().

Cc: "Rafael J. Wysocki" <[email protected]>
Cc: Daniel Lezcano <[email protected]>
Signed-off-by: Ankur Arora <[email protected]>

Author: terminus

drivers/cpuidle/poll_state.c

@@ -8,35 +8,22 @@
 #include <linux/sched/clock.h>
 #include <linux/sched/idle.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();
+	u64 time_end;
+	u32 flags = 0;
 
 	dev->poll_time_limit = false;
 
+	time_end = local_clock_noinstr() + cpuidle_poll_time(drv, dev);
+
 	raw_local_irq_enable();
 	if (!current_set_polling_and_test()) {
-		unsigned int loop_count = 0;
-		u64 limit;
-
-		limit = 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;
-			}
-		}
+		flags = smp_cond_load_relaxed_timeout(&current_thread_info()->flags,
+						      (VAL & _TIF_NEED_RESCHED),
+						      (local_clock_noinstr() >= time_end));
+		dev->poll_time_limit = !(flags & _TIF_NEED_RESCHED);
 	}
 	raw_local_irq_disable();