[dpdk-dev] [PATCH RFCv3 08/19] ring: remove the yield when waiting for tail update

[Bruce Richardson <bruce.richardson@intel.com>]

There was a compile time setting to enable a ring to yield when
it entered a loop in mp or mc rings waiting for the tail pointer update.
Build time settings are not recommended for enabling/disabling features,
and since this was off by default, remove it completely. If needed, a
runtime enabled equivalent can be used.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
---
 config/common_base                              |  1 -
 doc/guides/prog_guide/env_abstraction_layer.rst |  5 ----
 lib/librte_ring/rte_ring.h                      | 35 +++++--------------------
 3 files changed, 6 insertions(+), 35 deletions(-)

diff --git a/config/common_base b/config/common_base
index 3bbe3aa..125ee45 100644
--- a/config/common_base
+++ b/config/common_base
@@ -447,7 +447,6 @@ CONFIG_RTE_LIBRTE_PMD_NULL_CRYPTO=y
 # Compile librte_ring
 #
 CONFIG_RTE_LIBRTE_RING=y
-CONFIG_RTE_RING_PAUSE_REP_COUNT=0
 
 #
 # Compile librte_mempool
diff --git a/doc/guides/prog_guide/env_abstraction_layer.rst b/doc/guides/prog_guide/env_abstraction_layer.rst
index 10a10a8..7c39cd2 100644
--- a/doc/guides/prog_guide/env_abstraction_layer.rst
+++ b/doc/guides/prog_guide/env_abstraction_layer.rst
@@ -352,11 +352,6 @@ Known Issues
 
   3. It MUST not be used by multi-producer/consumer pthreads, whose scheduling policies are SCHED_FIFO or SCHED_RR.
 
-  ``RTE_RING_PAUSE_REP_COUNT`` is defined for rte_ring to reduce contention. It's mainly for case 2, a yield is issued after number of times pause repeat.
-
-  It adds a sched_yield() syscall if the thread spins for too long while waiting on the other thread to finish its operations on the ring.
-  This gives the preempted thread a chance to proceed and finish with the ring enqueue/dequeue operation.
-
 + rte_timer
 
   Running  ``rte_timer_manager()`` on a non-EAL pthread is not allowed. However, resetting/stopping the timer from a non-EAL pthread is allowed.
diff --git a/lib/librte_ring/rte_ring.h b/lib/librte_ring/rte_ring.h
index c059daa..8f940c6 100644
--- a/lib/librte_ring/rte_ring.h
+++ b/lib/librte_ring/rte_ring.h
@@ -114,11 +114,6 @@ enum rte_ring_queue_behavior {
 #define RTE_RING_NAMESIZE (RTE_MEMZONE_NAMESIZE - \
 			   sizeof(RTE_RING_MZ_PREFIX) + 1)
 
-#ifndef RTE_RING_PAUSE_REP_COUNT
-#define RTE_RING_PAUSE_REP_COUNT 0 /**< Yield after pause num of times, no yield
-                                    *   if RTE_RING_PAUSE_REP not defined. */
-#endif
-
 struct rte_memzone; /* forward declaration, so as not to require memzone.h */
 
 /* structure to hold a pair of head/tail values and other metadata */
@@ -388,7 +383,7 @@ __rte_ring_mp_do_enqueue(struct rte_ring *r, void * const *obj_table,
 	uint32_t cons_tail, free_entries;
 	const unsigned max = n;
 	int success;
-	unsigned i, rep = 0;
+	unsigned int i;
 	uint32_t mask = r->mask;
 	int ret;
 
@@ -442,18 +437,9 @@ __rte_ring_mp_do_enqueue(struct rte_ring *r, void * const *obj_table,
 	 * If there are other enqueues in progress that preceded us,
 	 * we need to wait for them to complete
 	 */
-	while (unlikely(r->prod.tail != prod_head)) {
+	while (unlikely(r->prod.tail != prod_head))
 		rte_pause();
 
-		/* Set RTE_RING_PAUSE_REP_COUNT to avoid spin too long waiting
-		 * for other thread finish. It gives pre-empted thread a chance
-		 * to proceed and finish with ring dequeue operation. */
-		if (RTE_RING_PAUSE_REP_COUNT &&
-		    ++rep == RTE_RING_PAUSE_REP_COUNT) {
-			rep = 0;
-			sched_yield();
-		}
-	}
 	r->prod.tail = prod_next;
 	return ret;
 }
@@ -486,7 +472,7 @@ __rte_ring_sp_do_enqueue(struct rte_ring *r, void * const *obj_table,
 {
 	uint32_t prod_head, cons_tail;
 	uint32_t prod_next, free_entries;
-	unsigned i;
+	unsigned int i;
 	uint32_t mask = r->mask;
 	int ret;
 
@@ -563,7 +549,7 @@ __rte_ring_mc_do_dequeue(struct rte_ring *r, void **obj_table,
 	uint32_t cons_next, entries;
 	const unsigned max = n;
 	int success;
-	unsigned i, rep = 0;
+	unsigned int i;
 	uint32_t mask = r->mask;
 
 	/* Avoid the unnecessary cmpset operation below, which is also
@@ -608,18 +594,9 @@ __rte_ring_mc_do_dequeue(struct rte_ring *r, void **obj_table,
 	 * If there are other dequeues in progress that preceded us,
 	 * we need to wait for them to complete
 	 */
-	while (unlikely(r->cons.tail != cons_head)) {
+	while (unlikely(r->cons.tail != cons_head))
 		rte_pause();
 
-		/* Set RTE_RING_PAUSE_REP_COUNT to avoid spin too long waiting
-		 * for other thread finish. It gives pre-empted thread a chance
-		 * to proceed and finish with ring dequeue operation. */
-		if (RTE_RING_PAUSE_REP_COUNT &&
-		    ++rep == RTE_RING_PAUSE_REP_COUNT) {
-			rep = 0;
-			sched_yield();
-		}
-	}
 	r->cons.tail = cons_next;
 
 	return behavior == RTE_RING_QUEUE_FIXED ? 0 : n;
@@ -654,7 +631,7 @@ __rte_ring_sc_do_dequeue(struct rte_ring *r, void **obj_table,
 {
 	uint32_t cons_head, prod_tail;
 	uint32_t cons_next, entries;
-	unsigned i;
+	unsigned int i;
 	uint32_t mask = r->mask;
 
 	cons_head = r->cons.head;
-- 
2.9.3