Re: [dpdk-dev] [PATCH 2/3] eventdev: use c11 atomics for lcore timer armed flag

-----Original Message-----
From: Phil Yang <redacted>
Sent: Friday, June 12, 2020 6:20 AM
To: dev@dpdk.org; Carrillo, Erik G <redacted>
Cc: drc@linux.vnet.ibm.com; honnappa.nagarahalli@arm.com;
ruifeng.wang@arm.com; dharmik.thakkar@arm.com; nd@arm.com
Subject: [PATCH 2/3] eventdev: use c11 atomics for lcore timer armed flag

The in_use flag is a per core variable which is not shared between lcores in
the normal case and the access of this variable should be ordered on the
same core. However, if non-EAL thread pick the highest lcore to insert timers
into, there is the possibility of conflicts on this flag between threads. Then
the atomic CAS operation is needed.

Use the c11 atomic CAS instead of the generic rte_atomic operations to avoid
the unnecessary barrier on aarch64.

Signed-off-by: Phil Yang <redacted>
Reviewed-by: Dharmik Thakkar <redacted>
Reviewed-by: Ruifeng Wang <redacted>
---
 lib/librte_eventdev/rte_event_timer_adapter.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/lib/librte_eventdev/rte_event_timer_adapter.c

b/lib/librte_eventdev/rte_event_timer_adapter.c
index 6a0e283..6947efb 100644

--- a/lib/librte_eventdev/rte_event_timer_adapter.c
+++ b/lib/librte_eventdev/rte_event_timer_adapter.c

@@ -554,7 +554,7 @@ struct swtim {
 	uint32_t timer_data_id;
 	/* Track which cores have actually armed a timer */
 	struct {
-		rte_atomic16_t v;
+		int16_t v;
 	} __rte_cache_aligned in_use[RTE_MAX_LCORE];
 	/* Track which cores' timer lists should be polled */
 	unsigned int poll_lcores[RTE_MAX_LCORE]; @@ -606,7 +606,8 @@

swtim_callback(struct rte_timer *tim)
 				      "with immediate expiry value");
 		}

-		if (unlikely(rte_atomic16_test_and_set(&sw-

quoted

in_use[lcore].v))) {

+		if (unlikely(sw->in_use[lcore].v == 0)) {
+			sw->in_use[lcore].v = 1;
 			n_lcores = __atomic_fetch_add(&sw->n_poll_lcores,
1,
 						__ATOMIC_RELAXED);
 			__atomic_store_n(&sw->poll_lcores[n_lcores],
lcore, @@ -834,7 +835,7 @@ swtim_init(struct rte_event_timer_adapter
*adapter)

 	/* Initialize the variables that track in-use timer lists */
 	for (i = 0; i < RTE_MAX_LCORE; i++)
-		rte_atomic16_init(&sw->in_use[i].v);
+		sw->in_use[i].v = 0;

 	/* Initialize the timer subsystem and allocate timer data instance */
 	ret = rte_timer_subsystem_init();

@@ -1017,6 +1018,8 @@ __swtim_arm_burst(const struct

rte_event_timer_adapter *adapter,
 	struct rte_timer *tim, *tims[nb_evtims];
 	uint64_t cycles;
 	int n_lcores;
+	/* Timer is not armed state */