[PATCH v13 12/15] cpuidle/poll_state: Wait for need-resched via tif_need_resched_relaxed_wait()

[Ankur Arora <ankur.a.arora@oracle.com>]

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, since we want to minimize power consumption in idle, building
of cpuidle/poll_state.c continues 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@kernel.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: linux-pm@vger.kernel.org
Suggested-by: Rafael J. Wysocki <rafael@kernel.org>
Acked-by: Rafael J. Wysocki (Intel) <rafael@kernel.org>
Tested-by: Haris Okanovic <harisokn@amazon.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
Notes: does not address comment about breaking the timeout contract [1]. 
as the interface isn't meant to be precise.

[1] https://lore.kernel.org/all/20260608083136.20A521F00898@smtp.kernel.org/
---
 drivers/cpuidle/poll_state.c | 21 +--------------------
 1 file changed, 1 insertion(+), 20 deletions(-)

diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
index c7524e4c522a..7443b3e971ba 100644
--- a/drivers/cpuidle/poll_state.c
+++ b/drivers/cpuidle/poll_state.c
@@ -6,41 +6,22 @@
 #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;
 
 		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;
-			}
-		}
+		dev->poll_time_limit = !tif_need_resched_relaxed_wait(limit);
 	}
 	raw_local_irq_disable();
 
-- 
2.43.7