| Message ID | 1464120374-8950-4-git-send-email-cota@braap.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 24/05/2016 22:06, Emilio G. Cota wrote: > Currently we emit a consume-load in atomic_rcu_read. This is > overkill for non-Sparc hosts, and is only useful to make > things easier for Thread Sanitizer, which as far as I understand > works best without explicit fences. Likewise: 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. and +/* See above: most compilers currently treat consume and acquire the + * same, but this slows down atomic_rcu_read unnecessarily. + */ Thanks, Paolo
diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h index 4a4f2fb..c5b6c8d 100644 --- a/include/qemu/atomic.h +++ b/include/qemu/atomic.h @@ -63,13 +63,20 @@ __atomic_store(ptr, &_val, __ATOMIC_RELAXED); \ } while(0) -/* Atomic RCU operations imply weak memory barriers */ +#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; \ })
Currently we emit a consume-load in atomic_rcu_read. This is overkill for non-Sparc hosts, and is only useful to make things easier for Thread Sanitizer, which as far as I understand works best without explicit fences. The appended 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> --- include/qemu/atomic.h | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)