linux-kernel - [PATCH v3 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load

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Date: Mon, 29 Jan 2024 12:40:34 +0200
From: Mihai Carabas <mihai.carabas@...cle.com>
To: linux-arm-kernel@...ts.infradead.org
Cc: kvm@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, catalin.marinas@....com, will@...nel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, pbonzini@...hat.com, wanpengli@...cent.com,
        vkuznets@...hat.com, rafael@...nel.org, daniel.lezcano@...aro.org,
        akpm@...ux-foundation.org, pmladek@...e.com, peterz@...radead.org,
        dianders@...omium.org, npiggin@...il.com, rick.p.edgecombe@...el.com,
        joao.m.martins@...cle.com, juerg.haefliger@...onical.com,
        mic@...ikod.net, mihai.carabas@...cle.com, arnd@...db.de,
        ankur.a.arora@...cle.com
Subject: [PATCH v3 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed

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@...radead.org>
Signed-off-by: Mihai Carabas <mihai.carabas@...cle.com>
---
 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;
-- 
1.8.3.1