linux-kernel - [PATCH v8 10/11] cpuidle/poll_state: limit POLL_IDLE_RELAX

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Message-Id: <20240925232425.2763385-11-ankur.a.arora@oracle.com>
Date: Wed, 25 Sep 2024 16:24:24 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: linux-pm@...r.kernel.org, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc: catalin.marinas@....com, will@...nel.org, tglx@...utronix.de,
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Subject: [PATCH v8 10/11] cpuidle/poll_state: limit POLL_IDLE_RELAX_COUNT on arm64

smp_cond_load_relaxed(), in its generic polling variant, polls on
the loop condition waiting for it to change, eventually exiting
the loop if the time limit has been exceeded.

To limit the frequency of the relatively expensive time check it
is limited to once every POLL_IDLE_RELAX_COUNT iterations.

arm64, however, uses an event based mechanism where instead of
polling, we wait for a store to a region.

Limit the POLL_IDLE_RELAX_COUNT to 1 for that case.

Suggested-by: Haris Okanovic <harisokn@...zon.com>
Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
---
 drivers/cpuidle/poll_state.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
index fc1204426158..61df2395585e 100644
--- a/drivers/cpuidle/poll_state.c
+++ b/drivers/cpuidle/poll_state.c
@@ -8,7 +8,18 @@
 #include <linux/sched/clock.h>
 #include <linux/sched/idle.h>
 
+#ifdef CONFIG_ARM64
+/*
+ * POLL_IDLE_RELAX_COUNT determines how often we check for timeout
+ * while polling for TIF_NEED_RESCHED in thread_info->flags.
+ *
+ * Set this to a low value since arm64, instead of polling, uses a
+ * event based mechanism.
+ */
+#define POLL_IDLE_RELAX_COUNT	1
+#else
 #define POLL_IDLE_RELAX_COUNT	200
+#endif
 
 static int __cpuidle poll_idle(struct cpuidle_device *dev,
 			       struct cpuidle_driver *drv, int index)
-- 
2.43.5