[dpdk-dev] [PATCH v7] mbuf: provide rte_pktmbuf_alloc_bulk API

[Huawei Xie]

v7 changes:
 rte_pktmbuf_alloc_bulk isn't exported as API, so shouldn't be listed in
version map

v6 changes:
 reflect the changes in release notes and library version map file
 revise our duff's code style a bit to make it more readable

v5 changes:
 add comment about duff's device and our variant implementation

v3 changes:
 move while after case 0
 add context about duff's device and why we use while loop in the commit
message

v2 changes:
 unroll the loop a bit to help the performance

rte_pktmbuf_alloc_bulk allocates a bulk of packet mbufs.

There is related thread about this bulk API.
http://dpdk.org/dev/patchwork/patch/4718/
Thanks to Konstantin's loop unrolling.

Attached the wiki page about duff's device. It explains the performance
optimization through loop unwinding, and also the most dramatic use of
case label fall-through.
https://en.wikipedia.org/wiki/Duff%27s_device

In this implementation, while() loop is used because we could not assume
count is strictly positive. Using while() loop saves one line of check.

Signed-off-by: Gerald Rogers <gerald.rogers at intel.com>
Signed-off-by: Huawei Xie <huawei.xie at intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev at intel.com>
Acked-by: Olivier Matz <olivier.matz at 6wind.com>
---
 doc/guides/rel_notes/release_16_04.rst |  3 ++
 lib/librte_mbuf/rte_mbuf.h             | 55 ++++++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+)

diff --git a/doc/guides/rel_notes/release_16_04.rst b/doc/guides/rel_notes/release_16_04.rst
index e2219d0..b10a11b 100644
--- a/doc/guides/rel_notes/release_16_04.rst
+++ b/doc/guides/rel_notes/release_16_04.rst
@@ -46,6 +46,9 @@ This section should contain new features added in this release. Sample format:
 
 * **Added vhost-user live migration support.**
 
+* **Enable bulk allocation of mbufs.**
+  A new function ``rte_pktmbuf_alloc_bulk()`` has been added to allow the user
+  to allocate a bulk of mbufs.
 
 Resolved Issues
 ---------------
diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index c973e9b..c1f6bc4 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -1336,6 +1336,61 @@ static inline struct rte_mbuf *rte_pktmbuf_alloc(struct rte_mempool *mp)
 }
 
 /**
+ * Allocate a bulk of mbufs, initialize refcnt and reset the fields to default
+ * values.
+ *
+ *  @param pool
+ *    The mempool from which mbufs are allocated.
+ *  @param mbufs
+ *    Array of pointers to mbufs
+ *  @param count
+ *    Array size
+ *  @return
+ *   - 0: Success
+ */
+static inline int rte_pktmbuf_alloc_bulk(struct rte_mempool *pool,
+	 struct rte_mbuf **mbufs, unsigned count)
+{
+	unsigned idx = 0;
+	int rc;
+
+	rc = rte_mempool_get_bulk(pool, (void **)mbufs, count);
+	if (unlikely(rc))
+		return rc;
+
+	/* To understand duff's device on loop unwinding optimization, see
+	 * https://en.wikipedia.org/wiki/Duff's_device.
+	 * Here while() loop is used rather than do() while{} to avoid extra
+	 * check if count is zero.
+	 */
+	switch (count % 4) {
+	case 0:
+		while (idx != count) {
+			RTE_MBUF_ASSERT(rte_mbuf_refcnt_read(mbufs[idx]) == 0);
+			rte_mbuf_refcnt_set(mbufs[idx], 1);
+			rte_pktmbuf_reset(mbufs[idx]);
+			idx++;
+	case 3:
+			RTE_MBUF_ASSERT(rte_mbuf_refcnt_read(mbufs[idx]) == 0);
+			rte_mbuf_refcnt_set(mbufs[idx], 1);
+			rte_pktmbuf_reset(mbufs[idx]);
+			idx++;
+	case 2:
+			RTE_MBUF_ASSERT(rte_mbuf_refcnt_read(mbufs[idx]) == 0);
+			rte_mbuf_refcnt_set(mbufs[idx], 1);
+			rte_pktmbuf_reset(mbufs[idx]);
+			idx++;
+	case 1:
+			RTE_MBUF_ASSERT(rte_mbuf_refcnt_read(mbufs[idx]) == 0);
+			rte_mbuf_refcnt_set(mbufs[idx], 1);
+			rte_pktmbuf_reset(mbufs[idx]);
+			idx++;
+		}
+	}
+	return 0;
+}
+
+/**
  * Attach packet mbuf to another packet mbuf.
  *
  * After attachment we refer the mbuf we attached as 'indirect',
-- 
1.8.1.4