[dpdk-dev] [PATCH v4 5/6] spinlock: use wfe to reduce contention on aarch64

[Gavin Hu <gavin.hu@arm.com>]

In acquiring a spinlock, cores repeatedly poll the lock variable.
This is replaced by rte_wait_until_equal API.

Running the micro benchmarking and the testpmd and l3fwd traffic tests
on ThunderX2, Ampere eMAG80 and Arm N1SDP, everything went well and no
notable performance gain nor degradation was measured.

Signed-off-by: Gavin Hu <gavin.hu@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Ola Liljedahl <ola.liljedahl@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Tested-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
---
 .../common/include/arch/arm/rte_spinlock.h         | 25 ++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/lib/librte_eal/common/include/arch/arm/rte_spinlock.h b/lib/librte_eal/common/include/arch/arm/rte_spinlock.h
index 1a6916b..7b8328e 100644
--- a/lib/librte_eal/common/include/arch/arm/rte_spinlock.h
+++ b/lib/librte_eal/common/include/arch/arm/rte_spinlock.h
@@ -16,6 +16,31 @@ extern "C" {
 #include <rte_common.h>
 #include "generic/rte_spinlock.h"
 
+/* armv7a does support WFE, but an explicit wake-up signal using SEV is
+ * required (must be preceded by DSB to drain the store buffer) and
+ * this is less performant, so keep armv7a implementation unchanged.
+ */
+#ifndef RTE_FORCE_INTRINSICS
+static inline void
+rte_spinlock_lock(rte_spinlock_t *sl)
+{
+	unsigned int tmp;
+	/* http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.
+	 * faqs/ka16809.html
+	 */
+	asm volatile(
+		"sevl\n"
+		"1:	wfe\n"
+		"2:	ldaxr %w[tmp], %w[locked]\n"
+		"cbnz   %w[tmp], 1b\n"
+		"stxr   %w[tmp], %w[one], %w[locked]\n"
+		"cbnz   %w[tmp], 2b\n"
+		: [tmp] "=&r" (tmp), [locked] "+Q"(sl->locked)
+		: [one] "r" (1)
+		: "cc", "memory");
+}
+#endif
+
 static inline int rte_tm_supported(void)
 {
 	return 0;
-- 
2.7.4