[dpdk-dev] [PATCH v8 3/3] spinlock: reimplement with atomic one-way barrier builtins

[Gavin Hu]

The __sync builtin based implementation generates full memory barriers
('dmb ish') on Arm platforms. Using C11 atomic builtins to generate one way
barriers.

Here is the assembly code of __sync_compare_and_swap builtin.
__sync_bool_compare_and_swap(dst, exp, src);
   0x000000000090f1b0 <+16>:    e0 07 40 f9 ldr x0, [sp, #8]
   0x000000000090f1b4 <+20>:    e1 0f 40 79 ldrh    w1, [sp, #6]
   0x000000000090f1b8 <+24>:    e2 0b 40 79 ldrh    w2, [sp, #4]
   0x000000000090f1bc <+28>:    21 3c 00 12 and w1, w1, #0xffff
   0x000000000090f1c0 <+32>:    03 7c 5f 48 ldxrh   w3, [x0]
   0x000000000090f1c4 <+36>:    7f 00 01 6b cmp w3, w1
   0x000000000090f1c8 <+40>:    61 00 00 54 b.ne    0x90f1d4
<rte_atomic16_cmpset+52>  // b.any
   0x000000000090f1cc <+44>:    02 fc 04 48 stlxrh  w4, w2, [x0]
   0x000000000090f1d0 <+48>:    84 ff ff 35 cbnz    w4, 0x90f1c0
<rte_atomic16_cmpset+32>
   0x000000000090f1d4 <+52>:    bf 3b 03 d5 dmb ish
   0x000000000090f1d8 <+56>:    e0 17 9f 1a cset    w0, eq  // eq = none

The benchmarking results showed constant improvements on all available
platforms:
1. Cavium ThunderX2: 126% performance;
2. Hisilicon 1616: 30%;
3. Qualcomm Falkor: 13%;
4. Marvell ARMADA 8040 with A72 cores on macchiatobin: 3.7%

Here is the example test result on TX2:
$sudo ./build/app/test -l 16-27 -- i
RTE>>spinlock_autotest

*** spinlock_autotest without this patch ***
Test with lock on 12 cores...
Core [16] Cost Time = 53886 us
Core [17] Cost Time = 53605 us
Core [18] Cost Time = 53163 us
Core [19] Cost Time = 49419 us
Core [20] Cost Time = 34317 us
Core [21] Cost Time = 53408 us
Core [22] Cost Time = 53970 us
Core [23] Cost Time = 53930 us
Core [24] Cost Time = 53283 us
Core [25] Cost Time = 51504 us
Core [26] Cost Time = 50718 us
Core [27] Cost Time = 51730 us
Total Cost Time = 612933 us

*** spinlock_autotest with this patch ***
Test with lock on 12 cores...
Core [16] Cost Time = 18808 us
Core [17] Cost Time = 29497 us
Core [18] Cost Time = 29132 us
Core [19] Cost Time = 26150 us
Core [20] Cost Time = 21892 us
Core [21] Cost Time = 24377 us
Core [22] Cost Time = 27211 us
Core [23] Cost Time = 11070 us
Core [24] Cost Time = 29802 us
Core [25] Cost Time = 15793 us
Core [26] Cost Time = 7474 us
Core [27] Cost Time = 29550 us
Total Cost Time = 270756 us

In the tests on ThunderX2, with more cores contending, the performance gain
was even higher, indicating the __atomic implementation scales up better
than __sync.

Fixes: af75078fece3 ("first public release")
Cc: stable at dpdk.org

Signed-off-by: Gavin Hu <gavin.hu at arm.com>
Reviewed-by: Phil Yang <phil.yang at arm.com>
Reviewed-by: Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>
Reviewed-by: Ola Liljedahl <Ola.Liljedahl at arm.com>
Reviewed-by: Steve Capper <Steve.Capper at arm.com>
---
 lib/librte_eal/common/include/generic/rte_spinlock.h | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/lib/librte_eal/common/include/generic/rte_spinlock.h b/lib/librte_eal/common/include/generic/rte_spinlock.h
index c4c3fc3..87ae7a4 100644
--- a/lib/librte_eal/common/include/generic/rte_spinlock.h
+++ b/lib/librte_eal/common/include/generic/rte_spinlock.h
@@ -61,9 +61,14 @@ rte_spinlock_lock(rte_spinlock_t *sl);
 static inline void
 rte_spinlock_lock(rte_spinlock_t *sl)
 {
-	while (__sync_lock_test_and_set(&sl->locked, 1))
-		while(sl->locked)
+	int exp = 0;
+
+	while (!__atomic_compare_exchange_n(&sl->locked, &exp, 1, 0,
+				__ATOMIC_ACQUIRE, __ATOMIC_RELAXED)) {
+		while (__atomic_load_n(&sl->locked, __ATOMIC_RELAXED))
 			rte_pause();
+		exp = 0;
+	}
 }
 #endif
 
@@ -80,7 +85,7 @@ rte_spinlock_unlock (rte_spinlock_t *sl);
 static inline void
 rte_spinlock_unlock (rte_spinlock_t *sl)
 {
-	__sync_lock_release(&sl->locked);
+	__atomic_store_n(&sl->locked, 0, __ATOMIC_RELEASE);
 }
 #endif
 
@@ -99,7 +104,10 @@ rte_spinlock_trylock (rte_spinlock_t *sl);
 static inline int
 rte_spinlock_trylock (rte_spinlock_t *sl)
 {
-	return __sync_lock_test_and_set(&sl->locked,1) == 0;
+	int exp = 0;
+	return __atomic_compare_exchange_n(&sl->locked, &exp, 1,
+				0, /* disallow spurious failure */
+				__ATOMIC_ACQUIRE, __ATOMIC_RELAXED);
 }
 #endif
 
@@ -113,7 +121,7 @@ rte_spinlock_trylock (rte_spinlock_t *sl)
  */
 static inline int rte_spinlock_is_locked (rte_spinlock_t *sl)
 {
-	return sl->locked;
+	return __atomic_load_n(&sl->locked, __ATOMIC_ACQUIRE);
 }
 
 /**
-- 
2.7.4