Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed

La 11.12.2023 13:46, Will Deacon a scris:

On Mon, Nov 20, 2023 at 04:01:38PM +0200, Mihai Carabas wrote:

quoted

cpu_relax on ARM64 does a simple "yield". Thus we replace it with
smp_cond_load_relaxed which basically does a "wfe".

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Mihai Carabas <redacted>
---
  drivers/cpuidle/poll_state.c | 14 +++++++++-----
  1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
index 9b6d90a72601..440cd713e39a 100644
--- a/drivers/cpuidle/poll_state.c
+++ b/drivers/cpuidle/poll_state.c

@@ -26,12 +26,16 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
  
  		limit = cpuidle_poll_time(drv, dev);
  
-		while (!need_resched()) {
-			cpu_relax();
-			if (loop_count++ < POLL_IDLE_RELAX_COUNT)
-				continue;
-
+		for (;;) {
  			loop_count = 0;
+
+			smp_cond_load_relaxed(&current_thread_info()->flags,
+					      (VAL & _TIF_NEED_RESCHED) ||
+					      (loop_count++ >= POLL_IDLE_RELAX_COUNT));
+
+			if (loop_count < POLL_IDLE_RELAX_COUNT)
+				break;
+
  			if (local_clock_noinstr() - time_start > limit) {
  				dev->poll_time_limit = true;
  				break;

Doesn't this make ARCH_HAS_CPU_RELAX a complete misnomer?

This controls the build of poll_state.c and the generic definition of 
smp_cond_load_relaxed (used by x86) is using cpu_relax(). Do you propose 
other approach here?

Will

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