[PATCH v6 10/10] cpuidle/poll_state: limit POLL_IDLE_RELAX_COUNT on arm64

[Ankur Arora]

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 store to a region.

Limit the POLL_IDLE_RELAX_COUNT to 1 for that case.

Suggested-by: Haris Okanovic <harisokn at amazon.com>
Signed-off-by: Ankur Arora <ankur.a.arora at oracle.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 532e4ed19e0f..b69fe7b67cb4 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