[PATCH v2] locking/osq: Use optimized spinning loop for arm64

[Waiman Long <longman@redhat.com>]

Arm64 has a more optimized spinning loop (atomic_cond_read_acquire)
for spinlock that can boost performance of sibling threads by putting
the current cpu to a shallow sleep state that is woken up only when
the monitored variable changes or an external event happens.

OSQ has a more complicated spinning loop. Besides the lock value, it
also checks for need_resched() and vcpu_is_preempted(). The check for
need_resched() is not a problem as it is only set by the tick interrupt
handler. That will be detected by the spinning cpu right after iret.

The vcpu_is_preempted() check, however, is a problem as changes to the
preempt state of of previous node will not affect the sleep state. For
ARM64, vcpu_is_preempted is not defined and so is a no-op. To guard
against future addition of vcpu_is_preempted() to arm64, code is added
to cause build error when vcpu_is_preempted becomes defined in arm64
without the corresponding changes in the OSQ spinning code.

On a 2-socket 56-core 224-thread ARM64 system, a kernel mutex locking
microbenchmark was run for 10s with and without the patch. The
performance numbers before patch were:

Running locktest with mutex [runtime = 10s, load = 1]
Threads = 224, Min/Mean/Max = 316/123,143/2,121,269
Threads = 224, Total Rate = 2,757 kop/s; Percpu Rate = 12 kop/s

After patch, the numbers were:

Running locktest with mutex [runtime = 10s, load = 1]
Threads = 224, Min/Mean/Max = 334/147,836/1,304,787
Threads = 224, Total Rate = 3,311 kop/s; Percpu Rate = 15 kop/s

So there was about 20% performance improvement.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 arch/arm64/include/asm/barrier.h | 10 ++++++++++
 kernel/locking/osq_lock.c        | 25 ++++++++++++-------------
 2 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index 7d9cc5ec4971..8eb5f1239885 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -152,6 +152,16 @@ do {									\
 	VAL;								\
 })
 
+/*
+ * In osq_lock(), smp_cond_load_relaxed() is called with a condition
+ * that includes vcpu_is_preempted(). For arm64, vcpu_is_preempted is not
+ * currently defined. So it is a no-op. If vcpu_is_preempted is defined in
+ * the future, smp_cond_load_relaxed() will not response to changes in the
+ * preempt state in a timely manner. So code changes will have to be made
+ * to address this deficiency.
+ */
+#define vcpu_is_preempted_not_used
+
 #define smp_cond_load_acquire(ptr, cond_expr)				\
 ({									\
 	typeof(ptr) __PTR = (ptr);					\
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 6ef600aa0f47..69ec5161c3cc 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -13,6 +13,14 @@
  */
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
 
+/*
+ * The optimized smp_cond_load_relaxed() spin loop should not be used with
+ * vcpu_is_preempted defined.
+ */
+#if defined(vcpu_is_preempted) && defined(vcpu_is_preempted_not_used)
+#error "vcpu_is_preempted() inside smp_cond_load_relaxed() may not work!"
+#endif
+
 /*
  * We use the value 0 to represent "no CPU", thus the encoded value
  * will be the CPU number incremented by 1.
@@ -134,20 +142,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 	 * cmpxchg in an attempt to undo our queueing.
 	 */
 
-	while (!READ_ONCE(node->locked)) {
-		/*
-		 * If we need to reschedule bail... so we can block.
-		 * Use vcpu_is_preempted() to avoid waiting for a preempted
-		 * lock holder:
-		 */
-		if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))
-			goto unqueue;
-
-		cpu_relax();
-	}
-	return true;
+	if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() ||
+				  vcpu_is_preempted(node_cpu(node->prev))))
+		return true;
 
-unqueue:
+	/* unqueue */
 	/*
 	 * Step - A  -- stabilize @prev
 	 *
-- 
2.18.1