[dpdk-dev] [RFC PATCH 06/14] mbuf: reorder fields by time-of-use

[Bruce Richardson]

*  Reorder the fields in the mbuf so that we have fields that are used
together side-by-side in the structure. This means that we have a
contiguous block of 8-bytes in the mbuf which are used to reset an mbuf
of descriptor rearm.
*  Where needed add in a dummy fields to overwrite values 8 or 16 bytes
at a time, when doing RX or RX descriptor rearm. This avoids compiler
warnings when using uint64_t values to overwrite a set of smaller
values.
* At the end, place fields that are only used for TX or for the slower
RX path, and mark them as down to be moved to a second cache line.

Signed-off-by: Bruce Richardson <bruce.richardson at intel.com>
---
 lib/librte_mbuf/rte_mbuf.c |  2 +-
 lib/librte_mbuf/rte_mbuf.h | 37 +++++++++++++++++++++----------------
 2 files changed, 22 insertions(+), 17 deletions(-)

diff --git a/lib/librte_mbuf/rte_mbuf.c b/lib/librte_mbuf/rte_mbuf.c
index 64f1587..594b910 100644
--- a/lib/librte_mbuf/rte_mbuf.c
+++ b/lib/librte_mbuf/rte_mbuf.c
@@ -161,7 +161,7 @@ rte_pktmbuf_dump(FILE *f, const struct rte_mbuf *m, unsigned dump_len)
 
 	fprintf(f, "dump mbuf at 0x%p, phys=%"PRIx64", buf_len=%u\n",
 	       m, (uint64_t)m->buf_physaddr, (unsigned)m->buf_len);
-	fprintf(f, "  pkt_len=%"PRIu32", ol_flags=%"PRIx16", nb_segs=%u, "
+	fprintf(f, "  pkt_len=%"PRIu32", ol_flags=%"PRIx64", nb_segs=%u, "
 	       "in_port=%u\n", m->pkt_len, m->ol_flags,
 	       (unsigned)m->nb_segs, (unsigned)m->port);
 	nb_segs = m->nb_segs;
diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index e0981c9..566bb7e 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -132,22 +132,20 @@ union rte_vlan_macip {
 /**< MAC+IP  length. */
 #define TX_MACIP_LEN_CMP_MASK   (TX_MAC_LEN_CMP_MASK | TX_IP_LEN_CMP_MASK)
 
+
 /**
  * The generic rte_mbuf, containing a packet mbuf.
  */
 struct rte_mbuf {
-	struct rte_mempool *pool; /**< Pool from which mbuf was allocated. */
 	void *buf_addr;           /**< Virtual address of segment buffer. */
 	phys_addr_t buf_physaddr; /**< Physical address of segment buffer. */
-	uint16_t buf_len;         /**< Length of segment buffer. */
 
-	/* valid for any segment */
-	struct rte_mbuf *next;    /**< Next segment of scattered packet. */
+	/* next 8 bytes are initialised on RX descriptor rearm */
+	uint64_t rearm_data[0]; /**< dummy element so we can get uin64_t ptrs
+	                         * to this part of the mbuf without alias error
+	                         */
+	uint16_t buf_len;       /**< Length of segment buffer. */
 	uint16_t data_off;
-	uint16_t data_len;        /**< Amount of data in segment buffer. */
-	uint32_t pkt_len;         /**< Total pkt len: sum of all segments. */
-
-#ifdef RTE_MBUF_REFCNT
 	/**
 	 * 16-bit Reference counter.
 	 * It should only be accessed using the following functions:
@@ -157,20 +155,23 @@ struct rte_mbuf {
 	 * config option.
 	 */
 	union {
+#ifdef RTE_MBUF_REFCNT
 		rte_atomic16_t refcnt_atomic;   /**< Atomically accessed refcnt */
 		uint16_t refcnt;                /**< Non-atomically accessed refcnt */
-	};
-#else
-	uint16_t refcnt_reserved;     /**< Do not use this field */
 #endif
-
-	/* these fields are valid for first segment only */
+		uint16_t refcnt_reserved;     /**< Do not use this field */
+	};
 	uint8_t nb_segs;        /**< Number of segments. */
 	uint8_t port;        /**< Input port. */
-	uint16_t ol_flags;            /**< Offload features. */
-	uint16_t reserved;             /**< Unused field. Required for padding. */
 
-	/* offload features, valid for first segment only */
+	/* remaining bytes are set on RX when pulling packet from descriptor */
+	uint64_t ol_flags;        /**< Offload features. */
+
+	__m128i rx_descriptor_fields1[0]; /**< dummy field used as marker for
+	                                   * writes in a vector driver */
+	uint16_t packet_type;     /**< Type of packet, e.g. protocols used */
+	uint16_t data_len;        /**< Amount of data in segment buffer. */
+	uint32_t pkt_len;         /**< Total pkt len: sum of all segments. */
 	union rte_vlan_macip vlan_macip;
 	union {
 		uint32_t rss;       /**< RSS hash result if RSS enabled */
@@ -181,6 +182,10 @@ struct rte_mbuf {
 		uint32_t sched;     /**< Hierarchical scheduler */
 	} hash;                 /**< hash information */
 
+	/* second cache line, fields only used in slow path or on TX */
+	struct rte_mempool *pool; /**< Pool from which mbuf was allocated. */
+	struct rte_mbuf *next;    /**< Next segment of scattered packet. */
+
 	union {
 		uint8_t metadata[0];
 		uint16_t metadata16[0];
-- 
1.9.3