[dpdk-dev] [PATCH v3] lib/timer: relax barrier for status update

[Phil Yang]

Volatile has no ordering semantics. The rte_timer structure defines
timer status as a volatile variable and uses the rte_r/wmb barrier
to guarantee inter-thread visibility.

This patch optimized the volatile operation with c11 atomic operations
and one-way barrier to save the performance penalty. According to the
timer_perf_autotest benchmarking results, this patch can uplift 10%~16%
timer appending performance, 3%~20% timer resetting performance and 45%
timer callbacks scheduling performance on aarch64 and no loss in
performance for x86.

Suggested-by: Honnappa Nagarahalli <honnappa.nagarahalli at arm.com>
Signed-off-by: Phil Yang <phil.yang at arm.com>
Reviewed-by: Gavin Hu <gavin.hu at arm.com>
Acked-by: Erik Gabriel Carrillo <erik.g.carrillo at intel.com>

---
This patch depends on patch:
http://patchwork.dpdk.org/patch/65997/

It is still using built-ins as the wrapper functions for C11 built-ins
are not defined yet

v3: fix typo.

v2:
1. Changed the memory ordering comment in timer_set_config_state.

 lib/librte_timer/rte_timer.c | 87 ++++++++++++++++++++++++++++++--------------
 lib/librte_timer/rte_timer.h |  2 +-
 2 files changed, 61 insertions(+), 28 deletions(-)

diff --git a/lib/librte_timer/rte_timer.c b/lib/librte_timer/rte_timer.c
index 269e921..6d19ce4 100644
--- a/lib/librte_timer/rte_timer.c
+++ b/lib/librte_timer/rte_timer.c
@@ -10,7 +10,6 @@
 #include <assert.h>
 #include <sys/queue.h>
 
-#include <rte_atomic.h>
 #include <rte_common.h>
 #include <rte_cycles.h>
 #include <rte_eal_memconfig.h>
@@ -218,7 +217,7 @@ rte_timer_init(struct rte_timer *tim)
 
 	status.state = RTE_TIMER_STOP;
 	status.owner = RTE_TIMER_NO_OWNER;
-	tim->status.u32 = status.u32;
+	__atomic_store_n(&tim->status.u32, status.u32, __ATOMIC_RELAXED);
 }
 
 /*
@@ -239,9 +238,9 @@ timer_set_config_state(struct rte_timer *tim,
 
 	/* wait that the timer is in correct status before update,
 	 * and mark it as being configured */
-	while (success == 0) {
-		prev_status.u32 = tim->status.u32;
+	prev_status.u32 = __atomic_load_n(&tim->status.u32, __ATOMIC_RELAXED);
 
+	while (success == 0) {
 		/* timer is running on another core
 		 * or ready to run on local core, exit
 		 */
@@ -258,9 +257,15 @@ timer_set_config_state(struct rte_timer *tim,
 		 * mark it atomically as being configured */
 		status.state = RTE_TIMER_CONFIG;
 		status.owner = (int16_t)lcore_id;
-		success = rte_atomic32_cmpset(&tim->status.u32,
-					      prev_status.u32,
-					      status.u32);
+		/* CONFIG states are acting as locked states. If the
+		 * timer is in CONFIG state, the state cannot be changed
+		 * by other threads. So, we should use ACQUIRE here.
+		 */
+		success = __atomic_compare_exchange_n(&tim->status.u32,
+					      &prev_status.u32,
+					      status.u32, 0,
+					      __ATOMIC_ACQUIRE,
+					      __ATOMIC_RELAXED);
 	}
 
 	ret_prev_status->u32 = prev_status.u32;
@@ -279,20 +284,27 @@ timer_set_running_state(struct rte_timer *tim)
 
 	/* wait that the timer is in correct status before update,
 	 * and mark it as running */
-	while (success == 0) {
-		prev_status.u32 = tim->status.u32;
+	prev_status.u32 = __atomic_load_n(&tim->status.u32, __ATOMIC_RELAXED);
 
+	while (success == 0) {
 		/* timer is not pending anymore */
 		if (prev_status.state != RTE_TIMER_PENDING)
 			return -1;
 
-		/* here, we know that timer is stopped or pending,
-		 * mark it atomically as being configured */
+		/* we know that the timer will be pending at this point,
+		 * mark it atomically as being running
+		 */
 		status.state = RTE_TIMER_RUNNING;
 		status.owner = (int16_t)lcore_id;
-		success = rte_atomic32_cmpset(&tim->status.u32,
-					      prev_status.u32,
-					      status.u32);
+		/* RUNNING states are acting as locked states. If the
+		 * timer is in RUNNING state, the state cannot be changed
+		 * by other threads. So, we should use ACQUIRE here.
+		 */
+		success = __atomic_compare_exchange_n(&tim->status.u32,
+					      &prev_status.u32,
+					      status.u32, 0,
+					      __ATOMIC_ACQUIRE,
+					      __ATOMIC_RELAXED);
 	}
 
 	return 0;
@@ -520,10 +532,12 @@ __rte_timer_reset(struct rte_timer *tim, uint64_t expire,
 
 	/* update state: as we are in CONFIG state, only us can modify
 	 * the state so we don't need to use cmpset() here */
-	rte_wmb();
 	status.state = RTE_TIMER_PENDING;
 	status.owner = (int16_t)tim_lcore;
-	tim->status.u32 = status.u32;
+	/* The "RELEASE" ordering guarantees the memory operations above
+	 * the status update are observed before the update by all threads
+	 */
+	__atomic_store_n(&tim->status.u32, status.u32, __ATOMIC_RELEASE);
 
 	if (tim_lcore != lcore_id || !local_is_locked)
 		rte_spinlock_unlock(&priv_timer[tim_lcore].list_lock);
@@ -600,10 +614,12 @@ __rte_timer_stop(struct rte_timer *tim, int local_is_locked,
 	}
 
 	/* mark timer as stopped */
-	rte_wmb();
 	status.state = RTE_TIMER_STOP;
 	status.owner = RTE_TIMER_NO_OWNER;
-	tim->status.u32 = status.u32;
+	/* The "RELEASE" ordering guarantees the memory operations above
+	 * the status update are observed before the update by all threads
+	 */
+	__atomic_store_n(&tim->status.u32, status.u32, __ATOMIC_RELEASE);
 
 	return 0;
 }
@@ -637,7 +653,8 @@ rte_timer_stop_sync(struct rte_timer *tim)
 int
 rte_timer_pending(struct rte_timer *tim)
 {
-	return tim->status.state == RTE_TIMER_PENDING;
+	return __atomic_load_n(&tim->status.state,
+				__ATOMIC_RELAXED) == RTE_TIMER_PENDING;
 }
 
 /* must be called periodically, run all timer that expired */
@@ -739,8 +756,12 @@ __rte_timer_manage(struct rte_timer_data *timer_data)
 			/* remove from done list and mark timer as stopped */
 			status.state = RTE_TIMER_STOP;
 			status.owner = RTE_TIMER_NO_OWNER;
-			rte_wmb();
-			tim->status.u32 = status.u32;
+			/* The "RELEASE" ordering guarantees the memory
+			 * operations above the status update are observed
+			 * before the update by all threads
+			 */
+			__atomic_store_n(&tim->status.u32, status.u32,
+				__ATOMIC_RELEASE);
 		}
 		else {
 			/* keep it in list and mark timer as pending */
@@ -748,8 +769,12 @@ __rte_timer_manage(struct rte_timer_data *timer_data)
 			status.state = RTE_TIMER_PENDING;
 			__TIMER_STAT_ADD(priv_timer, pending, 1);
 			status.owner = (int16_t)lcore_id;
-			rte_wmb();
-			tim->status.u32 = status.u32;
+			/* The "RELEASE" ordering guarantees the memory
+			 * operations above the status update are observed
+			 * before the update by all threads
+			 */
+			__atomic_store_n(&tim->status.u32, status.u32,
+				__ATOMIC_RELEASE);
 			__rte_timer_reset(tim, tim->expire + tim->period,
 				tim->period, lcore_id, tim->f, tim->arg, 1,
 				timer_data);
@@ -919,8 +944,12 @@ rte_timer_alt_manage(uint32_t timer_data_id,
 			/* remove from done list and mark timer as stopped */
 			status.state = RTE_TIMER_STOP;
 			status.owner = RTE_TIMER_NO_OWNER;
-			rte_wmb();
-			tim->status.u32 = status.u32;
+			/* The "RELEASE" ordering guarantees the memory
+			 * operations above the status update are observed
+			 * before the update by all threads
+			 */
+			__atomic_store_n(&tim->status.u32, status.u32,
+				__ATOMIC_RELEASE);
 		} else {
 			/* keep it in list and mark timer as pending */
 			rte_spinlock_lock(
@@ -928,8 +957,12 @@ rte_timer_alt_manage(uint32_t timer_data_id,
 			status.state = RTE_TIMER_PENDING;
 			__TIMER_STAT_ADD(data->priv_timer, pending, 1);
 			status.owner = (int16_t)this_lcore;
-			rte_wmb();
-			tim->status.u32 = status.u32;
+			/* The "RELEASE" ordering guarantees the memory
+			 * operations above the status update are observed
+			 * before the update by all threads
+			 */
+			__atomic_store_n(&tim->status.u32, status.u32,
+				__ATOMIC_RELEASE);
 			__rte_timer_reset(tim, tim->expire + tim->period,
 				tim->period, this_lcore, tim->f, tim->arg, 1,
 				data);
diff --git a/lib/librte_timer/rte_timer.h b/lib/librte_timer/rte_timer.h
index c6b3d45..df533fa 100644
--- a/lib/librte_timer/rte_timer.h
+++ b/lib/librte_timer/rte_timer.h
@@ -101,7 +101,7 @@ struct rte_timer
 {
 	uint64_t expire;       /**< Time when timer expire. */
 	struct rte_timer *sl_next[MAX_SKIPLIST_DEPTH];
-	volatile union rte_timer_status status; /**< Status of timer. */
+	union rte_timer_status status; /**< Status of timer. */
 	uint64_t period;       /**< Period of timer (0 if not periodic). */
 	rte_timer_cb_t f;      /**< Callback function. */
 	void *arg;             /**< Argument to callback function. */
-- 
2.7.4