[PATCH v9 02/15] cpuidle/poll_state: poll via smp_cond_load_relaxed_timeout()

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

The inner loop in poll_idle() polls to see if the thread's
TIF_NEED_RESCHED bit is 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 is busy-waiting, which allows the processor to reduce
power consumption.

However, cpu_relax() is defined optimally only on x86. On arm64, for
instance, it is implemented as a YIELD which only serves as a hint
to the CPU that it prioritize a different hardware thread if one is
available. arm64, does expose a more optimal polling mechanism via
smp_cond_load_relaxed_timeout() which uses LDXR, WFE to wait until a
store to a specified region, or until a timeout.

These semantics are essentially identical to what we want
from poll_idle(). So, restructure the loop to use
smp_cond_load_relaxed_timeout() instead.

The generated code remains close to the original version.

Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 drivers/cpuidle/poll_state.c | 27 ++++++++-------------------
 1 file changed, 8 insertions(+), 19 deletions(-)

diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
index 9b6d90a72601..0b42971393c9 100644
--- a/drivers/cpuidle/poll_state.c
+++ b/drivers/cpuidle/poll_state.c
@@ -8,35 +8,24 @@
 #include <linux/sched/clock.h>
 #include <linux/sched/idle.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;
+		unsigned long flags;
+		u64 time_start = local_clock_noinstr();
+		u64 limit = cpuidle_poll_time(drv, dev);
 
-		limit = cpuidle_poll_time(drv, dev);
+		flags = smp_cond_load_relaxed_timeout(&current_thread_info()->flags,
+						      VAL & _TIF_NEED_RESCHED,
+						      local_clock_noinstr(),
+						      time_start + limit);
 
-		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 = !(flags & _TIF_NEED_RESCHED);
 	}
 	raw_local_irq_disable();
 
-- 
2.43.5