haraldwolff/qemu-patch-raspberry4
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atomics: do not emit consume barrier for atomic_rcu_read

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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>
This commit is contained in:
Emilio G. Cota 2016-05-24 16:06:14 -04:00 committed by Paolo Bonzini
parent c983895258
commit 15487aa132
1 changed files with 12 additions and 2 deletions
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14
include/qemu/atomic.h
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@ -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; \
})