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authorjaszha03 <jason.zhang2@arm.com>2019-06-26 17:06:59 -0500
committerDave Barach <openvpp@barachs.net>2019-08-01 11:54:17 +0000
commitfb1ccc7c369d879a20567df609d04cfdcec5f8c1 (patch)
treec2f289132da3c8e0a71c186df990270bf9c26043 /src
parent635acbf4b76038023b0b5a9bdae0dc87e13fd308 (diff)
vppinfra: refactor clib_spinlock_t to use compare and swap
Tested performance of a CAS implementation (using __atomic_compare_exchange) against a TAS implementation (using __atomic_exchange) using test_spinlock.c and found some performance improvement. Generated assembly for CAS and TAS implementations show that TAS always executes with a load-store dependency, but CAS moves a branch condition between the load and store so that only a load occurs when the lock is free. Benchmarking: The results below are the cycle counts from test_spinlock.c, configured so that for 10000 iterations, 12 threads on separate cores are spawned, each of which increments a global counter 10000 times in each iteration. For A72, 8 threads are spawned in each test. x86 Xeon TAS: 7.333e8, 7.605e8, 7.535e8, 7.485e8, 7.321e8 x86 Xeon CAS: 5.842e8, 5.433e8, 5.389e8, 5.983e8, 5.552e8 Aarch64 ThX2* TAS: 9.852e7, 10.209e7, 9.190e7, 9.600e7, 9.224e7 Aarch64 ThX2* CAS: 7.640e7, 7.486e7, 7.425e7, 7.269e7, 7.534e7 A72 TAS: 7.289e6, 6.963e6, 7.208e6, 6.976e6, 7.200e6 A72 CAS: 1.695e6, 1.608e6, 1.600e6, 1.634e6, 1.746e6 *ThunderX2 used additional gcc options "-march=armv8.1-a+crc+crypto+lse" Type: refactor Change-Id: Ic5cd97991804f6b012707fad1a5d1a6edb96cd3d Signed-off-by: Jason Zhang <jason.zhang2@arm.com> Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com> Reviewed-by: Lijian Zhang <Lijian.Zhang@arm.com>
Diffstat (limited to 'src')
-rw-r--r--src/vppinfra/atomics.h3
-rw-r--r--src/vppinfra/lock.h12
2 files changed, 13 insertions, 2 deletions
diff --git a/src/vppinfra/atomics.h b/src/vppinfra/atomics.h
index df8e534b683..ce2177df5c2 100644
--- a/src/vppinfra/atomics.h
+++ b/src/vppinfra/atomics.h
@@ -37,11 +37,14 @@
#define clib_atomic_cmp_and_swap(addr,old,new) __sync_val_compare_and_swap(addr, old, new)
#define clib_atomic_bool_cmp_and_swap(addr,old,new) __sync_bool_compare_and_swap(addr, old, new)
+#define clib_atomic_cmp_and_swap_acq_relax_n(addr,exp,new,weak) __atomic_compare_exchange_n ((addr), (exp), (new), (weak), __ATOMIC_ACQUIRE, __ATOMIC_RELAXED)
+
#define clib_atomic_test_and_set(a) __atomic_exchange_n(a, 1, __ATOMIC_ACQUIRE)
#define clib_atomic_release(a) __atomic_store_n(a, 0, __ATOMIC_RELEASE)
#define clib_atomic_fence_rel() __atomic_thread_fence(__ATOMIC_RELEASE);
+#define clib_atomic_load_relax_n(a) __atomic_load_n((a), __ATOMIC_RELAXED)
#define clib_atomic_load_acq_n(a) __atomic_load_n((a), __ATOMIC_ACQUIRE)
#define clib_atomic_store_rel_n(a, b) __atomic_store_n ((a), (b), __ATOMIC_RELEASE)
diff --git a/src/vppinfra/lock.h b/src/vppinfra/lock.h
index cc6a7f086b1..c1f0b87dfc6 100644
--- a/src/vppinfra/lock.h
+++ b/src/vppinfra/lock.h
@@ -17,6 +17,7 @@
#define included_clib_lock_h
#include <vppinfra/clib.h>
+#include <vppinfra/atomics.h>
#if __x86_64__
#define CLIB_PAUSE() __builtin_ia32_pause ()
@@ -76,8 +77,15 @@ clib_spinlock_free (clib_spinlock_t * p)
static_always_inline void
clib_spinlock_lock (clib_spinlock_t * p)
{
- while (clib_atomic_test_and_set (&(*p)->lock))
- CLIB_PAUSE ();
+ u32 free = 0;
+ while (!clib_atomic_cmp_and_swap_acq_relax_n (&(*p)->lock, &free, 1, 0))
+ {
+ /* atomic load limits number of compare_exchange executions */
+ while (clib_atomic_load_relax_n (&(*p)->lock))
+ CLIB_PAUSE ();
+ /* on failure, compare_exchange writes (*p)->lock into free */
+ free = 0;
+ }
CLIB_LOCK_DBG (p);
}