[PATCH v2 5/7] arm64: barrier: add fine wait for smp_cond_load_relaxed_timewait()

[Ankur Arora]

Define __smp_cond_timewait_fine for callers which need fine grained
timeout.

To do this, use a narrowing timeout slack, equal to the remaining
duration. This allows us to optimistically wait in WFE until
the remaining duration drops below ARCH_TIMER_EVT_STREAM_PERIOD_US/2.
Once we reach that point, we go into the spin-wait state.

Cc: Will Deacon <will at kernel.org>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Kumar Kartikeya Dwivedi <memxor at gmail.com>
Cc: Alexei Starovoitov <ast at kernel.org>
Signed-off-by: Ankur Arora <ankur.a.arora at oracle.com>
---
 arch/arm64/include/asm/barrier.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index f4a184a96933..e4abb8f5dd97 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -247,6 +247,9 @@ static inline u64 ___cond_timewait(u64 now, u64 prev, u64 end,
 	if (now >= end)
 		return 0;
 
+	if (slack == 0)
+		slack = max(remaining, SMP_TIMEWAIT_CHECK_US);
+
 	/*
 	 * Use WFE if there's enough slack to get an event-stream wakeup even
 	 * if we don't come out of the WFE due to natural causes.
@@ -273,6 +276,16 @@ static inline u64 ___cond_timewait(u64 now, u64 prev, u64 end,
 	return now;
 }
 
+/*
+ * Fine wait_policy: minimize the timeout delay while balancing against the
+ * time spent in the WFE wait state.
+ *
+ * The worst case timeout delay is ARCH_TIMER_EVT_STREAM_PERIOD_US/2, which
+ * would also be the worst case spin period.
+ */
+#define __smp_cond_timewait_fine(now, prev, end, spin, wait)		\
+	__smp_cond_timewait(now, prev, end, spin, wait,			\
+			    0)
 /*
  * Coarse wait_policy: minimizes the duration spent spinning at the cost of
  * potentially spending the available slack in a WFE wait state.
-- 
2.43.5