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| author | Emilio G. Cota | 2016-05-24 22:06:14 +0200 |
|---|---|---|
| committer | Paolo Bonzini | 2016-05-29 09:11:11 +0200 |
| commit | 15487aa132109891482f79d78a30d6cfd465a391 (patch) | |
| tree | c4f272a7dce759a217247f5e9a25b166a5977dd1 /include | |
| parent | atomics: emit an smp_read_barrier_depends() barrier only for Alpha and Thread... (diff) | |
| download | qemu-15487aa132109891482f79d78a30d6cfd465a391.tar.gz qemu-15487aa132109891482f79d78a30d6cfd465a391.tar.xz qemu-15487aa132109891482f79d78a30d6cfd465a391.zip | |
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 <cota@braap.org>
Message-Id: <1464120374-8950-4-git-send-email-cota@braap.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'include')
| -rw-r--r-- | include/qemu/atomic.h | 14 |
1 files changed, 12 insertions, 2 deletions
diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h index 96db6e9db5..7a590969b5 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; \ }) |