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Subject[PATCH v3] locking/osq: Use optimized spinning loop for arm64
Date
Arm64 has a more optimized spinning loop (atomic_cond_read_acquire)
using wfe for spinlock that can boost performance of sibling threads
by putting the current cpu to a wait state that is broken 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 wait state. For
ARM64, vcpu_is_preempted is not currently defined and so is a no-op.
Will has indicated that he is planning to para-virtualize wfe instead
of defining vcpu_is_preempted for PV support. So just add a comment in
arch/arm64/include/asm/spinlock.h to indicate that vcpu_is_preempted()
should not be defined as suggested.

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/spinlock.h | 9 +++++++++
kernel/locking/osq_lock.c | 17 ++++-------------
2 files changed, 13 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/include/asm/spinlock.h b/arch/arm64/include/asm/spinlock.h
index b093b287babf..102404dc1e13 100644
--- a/arch/arm64/include/asm/spinlock.h
+++ b/arch/arm64/include/asm/spinlock.h
@@ -11,4 +11,13 @@
/* See include/linux/spinlock.h */
#define smp_mb__after_spinlock() smp_mb()

+/*
+ * Changing this will break osq_lock() thanks to the call inside
+ * smp_cond_load_relaxed().
+ *
+ * See:
+ * https://lore.kernel.org/lkml/20200110100612.GC2827@hirez.programming.kicks-ass.net
+ */
+#define vcpu_is_preempted(cpu) false
+
#endif /* __ASM_SPINLOCK_H */
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 6ef600aa0f47..e42893f2fcbc 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -134,20 +134,11 @@ 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
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