[dpdk-dev] [PATCH v4] mbuf: fix mbuf free performance with non atomic refcnt

[Olivier Matz <olivier.matz@6wind.com>]

When RTE_MBUF_REFCNT_ATOMIC=n, the decrement of the mbuf reference
counter uses an atomic operation. This is not necessary and impacts
the performance (seen with TRex traffic generator).

We cannot replace rte_atomic16_add_return() by rte_mbuf_refcnt_update()
because it would add an additional check.

Solves this by introducing __rte_mbuf_refcnt_update(), which
updates the reference counter without doing anything else.

Fixes: 8f094a9ac5d7 ("mbuf: set mbuf fields while in pool")
Suggested-by: Hanoch Haim <hhaim@cisco.com>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
---

Hanoh,

The following patch implements what was discussed in the thread.
Are you ok with it?

Thanks,
Olivier


 lib/librte_mbuf/rte_mbuf.h | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index ce8a05ddf..dd08cb72b 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -764,6 +764,13 @@ rte_mbuf_refcnt_set(struct rte_mbuf *m, uint16_t new_value)
 	rte_atomic16_set(&m->refcnt_atomic, new_value);
 }
 
+/* internal */
+static inline uint16_t
+__rte_mbuf_refcnt_update(struct rte_mbuf *m, int16_t value)
+{
+	return (uint16_t)(rte_atomic16_add_return(&m->refcnt_atomic, value));
+}
+
 /**
  * Adds given value to an mbuf's refcnt and returns its new value.
  * @param m
@@ -788,19 +795,26 @@ rte_mbuf_refcnt_update(struct rte_mbuf *m, int16_t value)
 		return 1 + value;
 	}
 
-	return (uint16_t)(rte_atomic16_add_return(&m->refcnt_atomic, value));
+	return __rte_mbuf_refcnt_update(m, value);
 }
 
 #else /* ! RTE_MBUF_REFCNT_ATOMIC */
 
+/* internal */
+static inline uint16_t
+__rte_mbuf_refcnt_update(struct rte_mbuf *m, int16_t value)
+{
+	m->refcnt = (uint16_t)(m->refcnt + value);
+	return m->refcnt;
+}
+
 /**
  * Adds given value to an mbuf's refcnt and returns its new value.
  */
 static inline uint16_t
 rte_mbuf_refcnt_update(struct rte_mbuf *m, int16_t value)
 {
-	m->refcnt = (uint16_t)(m->refcnt + value);
-	return m->refcnt;
+	return __rte_mbuf_refcnt_update(m, value);
 }
 
 /**
@@ -1364,8 +1378,7 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
 
 		return m;
 
-       } else if (rte_atomic16_add_return(&m->refcnt_atomic, -1) == 0) {
-
+	} else if (__rte_mbuf_refcnt_update(m, -1) == 0) {
 
 		if (RTE_MBUF_INDIRECT(m))
 			rte_pktmbuf_detach(m);
-- 
2.11.0