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authorWaiman Long <longman@redhat.com>2020-01-13 10:07:35 -0500
committerPeter Zijlstra <peterz@infradead.org>2020-01-17 10:19:30 +0100
commitf5bfdc8e3947a7ae489cf8ae9cfd6b3fb357b952 (patch)
treee3203e215a968b8f0681ec15d9952c4c05583005
parent57097124cbbd310cc2b5884189e22e60a3c20514 (diff)
locking/osq: Use optimized spinning loop for arm64
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> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Will Deacon <will@kernel.org> Link: https://lkml.kernel.org/r/20200113150735.21956-1-longman@redhat.com
-rw-r--r--arch/arm64/include/asm/spinlock.h9
-rw-r--r--kernel/locking/osq_lock.c23
2 files changed, 19 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..1f7734949ac8 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -134,20 +134,17 @@ 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;
+ /*
+ * Wait to acquire the lock or cancelation. Note that need_resched()
+ * will come with an IPI, which will wake smp_cond_load_relaxed() if it
+ * is implemented with a monitor-wait. vcpu_is_preempted() relies on
+ * polling, be careful.
+ */
+ 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
*