From f5bfdc8e3947a7ae489cf8ae9cfd6b3fb357b952 Mon Sep 17 00:00:00 2001 From: Waiman Long Date: Mon, 13 Jan 2020 10:07:35 -0500 Subject: 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 Signed-off-by: Peter Zijlstra (Intel) Acked-by: Will Deacon Link: https://lkml.kernel.org/r/20200113150735.21956-1-longman@redhat.com --- arch/arm64/include/asm/spinlock.h | 9 +++++++++ 1 file changed, 9 insertions(+) (limited to 'arch') 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 */ -- cgit v1.2.3