From 15487aa132109891482f79d78a30d6cfd465a391 Mon Sep 17 00:00:00 2001 From: "Emilio G. Cota" Date: Tue, 24 May 2016 16:06:14 -0400 Subject: [PATCH] atomics: do not emit consume barrier for atomic_rcu_read Currently we emit a consume-load in atomic_rcu_read. Because of limitations in current compilers, this is overkill for non-Alpha hosts and it is only useful to make Thread Sanitizer work. This patch leaves the consume-load in atomic_rcu_read when compiling with Thread Sanitizer enabled, and resorts to a relaxed load + smp_read_barrier_depends otherwise. On an RMO host architecture, such as aarch64, the performance improvement of this change is easily measurable. For instance, qht-bench performs an atomic_rcu_read on every lookup. Performance before and after applying this patch: $ tests/qht-bench -d 5 -n 1 Before: 9.78 MT/s After: 10.96 MT/s Signed-off-by: Emilio G. Cota Message-Id: <1464120374-8950-4-git-send-email-cota@braap.org> Signed-off-by: Paolo Bonzini --- include/qemu/atomic.h | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h index 96db6e9..7a59096 100644 --- a/include/qemu/atomic.h +++ b/include/qemu/atomic.h @@ -67,13 +67,23 @@ __atomic_store(ptr, &_val, __ATOMIC_RELAXED); \ } while(0) -/* Atomic RCU operations imply weak memory barriers */ +/* See above: most compilers currently treat consume and acquire the + * same, but this slows down atomic_rcu_read unnecessarily. + */ +#ifdef __SANITIZE_THREAD__ +#define atomic_rcu_read__nocheck(ptr, valptr) \ + __atomic_load(ptr, valptr, __ATOMIC_CONSUME); +#else +#define atomic_rcu_read__nocheck(ptr, valptr) \ + __atomic_load(ptr, valptr, __ATOMIC_RELAXED); \ + smp_read_barrier_depends(); +#endif #define atomic_rcu_read(ptr) \ ({ \ QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \ typeof(*ptr) _val; \ - __atomic_load(ptr, &_val, __ATOMIC_CONSUME); \ + atomic_rcu_read__nocheck(ptr, &_val); \ _val; \ }) -- 2.7.4