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ring/c11: synchronize load and store of the tail

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Synchronize the load-acquire of the tail and the store-release
within update_tail, the store release ensures all the ring operations,
enqueue or dequeue, are seen by the observers on the other side as soon
as they see the updated tail. The load-acquire is needed here as the
data dependency is not a reliable way for ordering as the compiler might
break it by saving to temporary values to boost performance.
When computing the free_entries and avail_entries, use atomic semantics
to load the heads and tails instead.

The patch was benchmarked with test/ring_perf_autotest and it decreases
the enqueue/dequeue latency by 5% ~ 27.6% with two lcores, the real gains
are dependent on the number of lcores, depth of the ring, SPSC or MPMC.
For 1 lcore, it also improves a little, about 3 ~ 4%.
It is a big improvement, in case of MPMC, with two lcores and ring size
of 32, it saves latency up to (3.26-2.36)/3.26 = 27.6%.

This patch is a bug fix, while the improvement is a bonus. In our analysis
the improvement comes from the cacheline pre-filling after hoisting load-
acquire from _atomic_compare_exchange_n up above.

The test command:
$sudo ./test/test/test -l 16-19,44-47,72-75,100-103 -n 4 --socket-mem=\
1024 -- -i

Test result with this patch(two cores):
 SP/SC bulk enq/dequeue (size: 8): 5.86
 MP/MC bulk enq/dequeue (size: 8): 10.15
 SP/SC bulk enq/dequeue (size: 32): 1.94
 MP/MC bulk enq/dequeue (size: 32): 2.36

In comparison of the test result without this patch:
 SP/SC bulk enq/dequeue (size: 8): 6.67
 MP/MC bulk enq/dequeue (size: 8): 13.12
 SP/SC bulk enq/dequeue (size: 32): 2.04
 MP/MC bulk enq/dequeue (size: 32): 3.26

Fixes: 39368ebfc6 ("ring: introduce C11 memory model barrier option")
Cc: stable@dpdk.org

Signed-off-by: Gavin Hu <gavin.hu@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Ola Liljedahl <ola.liljedahl@arm.com>
Reviewed-by: Jia He <justin.he@arm.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Tested-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
This commit is contained in:
Gavin Hu 2018-11-02 19:21:27 +08:00 committed by Thomas Monjalon
parent 305921f450
commit 9ed8770628
1 changed files with 18 additions and 4 deletions
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22
lib/librte_ring/rte_ring_c11_mem.h
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@ -57,6 +57,7 @@ __rte_ring_move_prod_head(struct rte_ring *r, unsigned int is_sp,
uint32_t *free_entries)
{
const uint32_t capacity = r->capacity;
uint32_t cons_tail;
unsigned int max = n;
int success;
@ -67,13 +68,18 @@ __rte_ring_move_prod_head(struct rte_ring *r, unsigned int is_sp,
*old_head = __atomic_load_n(&r->prod.head,
__ATOMIC_ACQUIRE);
/*
* The subtraction is done between two unsigned 32bits value
/* load-acquire synchronize with store-release of ht->tail
* in update_tail.
*/
cons_tail = __atomic_load_n(&r->cons.tail,
__ATOMIC_ACQUIRE);
/* The subtraction is done between two unsigned 32bits value
* (the result is always modulo 32 bits even if we have
* *old_head > cons_tail). So 'free_entries' is always between 0
* and capacity (which is < size).
*/
*free_entries = (capacity + r->cons.tail - *old_head);
*free_entries = (capacity + cons_tail - *old_head);
/* check that we have enough room in ring */
if (unlikely(n > *free_entries))
@ -125,21 +131,29 @@ __rte_ring_move_cons_head(struct rte_ring *r, int is_sc,
uint32_t *entries)
{
unsigned int max = n;
uint32_t prod_tail;
int success;
/* move cons.head atomically */
do {
/* Restore n as it may change every loop */
n = max;
*old_head = __atomic_load_n(&r->cons.head,
__ATOMIC_ACQUIRE);
/* this load-acquire synchronize with store-release of ht->tail
* in update_tail.
*/
prod_tail = __atomic_load_n(&r->prod.tail,
__ATOMIC_ACQUIRE);
/* The subtraction is done between two unsigned 32bits value
* (the result is always modulo 32 bits even if we have
* cons_head > prod_tail). So 'entries' is always between 0
* and size(ring)-1.
*/
*entries = (r->prod.tail - *old_head);
*entries = (prod_tail - *old_head);
/* Set the actual entries for dequeue */
if (n > *entries)