[ovs-dev] [PATCH v4 02/15] netdev-dpdk: fix mbuf sizing

Tiago Lam tiago.lam at intel.com
Tue Jul 10 11:06:01 UTC 2018

From: Mark Kavanagh <mark.b.kavanagh at intel.com>

There are numerous factors that must be considered when calculating
the size of an mbuf:
- the data portion of the mbuf must be sized in accordance With Rx
  buffer alignment (typically 1024B). So, for example, in order to
  successfully receive and capture a 1500B packet, mbufs with a
  data portion of size 2048B must be used.
- in OvS, the elements that comprise an mbuf are:
  * the dp packet, which includes a struct rte mbuf (704B)
  * packet data (aligned to 1k, as previously described)
  * RTE_PKTMBUF_TAILROOM (typically 0)

Some PMDs require that the total mbuf size (i.e. the total sum of all
of the above-listed components' lengths) is cache-aligned. To satisfy
this requirement, it may be necessary to round up the total mbuf size
with respect to cacheline size. In doing so, it's possible that the
dp_packet's data portion is inadvertently increased in size, such that
it no longer adheres to Rx buffer alignment. Consequently, the
following property of the mbuf no longer holds true:

    mbuf.data_len == mbuf.buf_len - mbuf.data_off

This creates a problem in the case of multi-segment mbufs, where that
assumption is assumed to be true for all but the final segment in an
mbuf chain. Resolve this issue by adjusting the size of the mbuf's
private data portion, as opposed to the packet data portion when
aligning mbuf size to cachelines.

Fixes: 4be4d22 ("netdev-dpdk: clean up mbuf initialization")
Fixes: 31b88c9 ("netdev-dpdk: round up mbuf_size to cache_line_size")
CC: Santosh Shukla <santosh.shukla at caviumnetworks.com>
Signed-off-by: Mark Kavanagh <mark.b.kavanagh at intel.com>
Acked-by: Santosh Shukla <santosh.shukla at caviumnetworks.com>
Acked-by: Eelco Chaudron <echaudro at redhat.com>
 lib/netdev-dpdk.c | 44 +++++++++++++++++++++++++++++---------------
 1 file changed, 29 insertions(+), 15 deletions(-)

diff --git a/lib/netdev-dpdk.c b/lib/netdev-dpdk.c
index be8e8b9..0b964c8 100644
--- a/lib/netdev-dpdk.c
+++ b/lib/netdev-dpdk.c
@@ -88,10 +88,6 @@ static struct vlog_rate_limit rl = VLOG_RATE_LIMIT_INIT(5, 20);
 #define MTU_TO_MAX_FRAME_LEN(mtu)   ((mtu) + ETHER_HDR_MAX_LEN)
 #define FRAME_LEN_TO_MTU(frame_len) ((frame_len)                    \
                                      - ETHER_HDR_LEN - ETHER_CRC_LEN)
-#define MBUF_SIZE(mtu)              ROUND_UP((MTU_TO_MAX_FRAME_LEN(mtu) \
-                                             + sizeof(struct dp_packet) \
-                                             + RTE_PKTMBUF_HEADROOM),   \
-                                             RTE_CACHE_LINE_SIZE)
 #define NETDEV_DPDK_MBUF_ALIGN      1024
 #define NETDEV_DPDK_MAX_PKT_LEN     9728
@@ -641,6 +637,7 @@ dpdk_mp_create(struct netdev_dpdk *dev, int mtu, uint32_t mbuf_size,
     uint32_t n_mbufs;
     uint32_t hash = hash_string(netdev_name, 0);
     struct dpdk_mp *dmp = NULL;
+    uint16_t pkt_size, aligned_mbuf_size, mbuf_priv_data_len;
     int ret;
     dmp = dpdk_rte_mzalloc(sizeof *dmp);
@@ -662,8 +659,8 @@ dpdk_mp_create(struct netdev_dpdk *dev, int mtu, uint32_t mbuf_size,
          * so this is not an issue for tasks such as debugging.
         ret = snprintf(mp_name, RTE_MEMPOOL_NAMESIZE,
-                           "ovs%08x%02d%05d%07u",
-                           hash, socket_id, mtu, n_mbufs);
+                       "ovs%08x%02d%05d%07u",
+                        hash, socket_id, mtu, n_mbufs);
         if (ret < 0 || ret >= RTE_MEMPOOL_NAMESIZE) {
             VLOG_DBG("snprintf returned %d. "
                      "Failed to generate a mempool name for \"%s\". "
@@ -675,16 +672,33 @@ dpdk_mp_create(struct netdev_dpdk *dev, int mtu, uint32_t mbuf_size,
         VLOG_DBG("Port %s: Requesting a mempool of %u mbufs of size %u "
-                  "on socket %d for %d Rx and %d Tx queues.",
+                  "on socket %d for %d Rx and %d Tx queues, "
+                  "cache line size of %u",
                   netdev_name, n_mbufs, mbuf_size, socket_id,
-                  dev->requested_n_rxq, dev->requested_n_txq);
-        dmp->mp = rte_pktmbuf_pool_create(mp_name, n_mbufs,
-                                          MP_CACHE_SZ,
-                                          sizeof (struct dp_packet)
-                                          - sizeof (struct rte_mbuf),
-                                          MBUF_SIZE(mbuf_size)
-                                          - sizeof(struct dp_packet),
+                  dev->requested_n_rxq, dev->requested_n_txq,
+                  RTE_CACHE_LINE_SIZE);
+        mbuf_priv_data_len = sizeof(struct dp_packet) -
+                                 sizeof(struct rte_mbuf);
+        /* The size of the entire dp_packet. */
+        pkt_size = sizeof (struct dp_packet) +
+                                mbuf_size + RTE_PKTMBUF_HEADROOM;
+        /* mbuf size, rounded up to cacheline size. */
+        aligned_mbuf_size = ROUND_UP(pkt_size, RTE_CACHE_LINE_SIZE);
+        /* If there is a size discrepancy, add padding to mbuf_priv_data_len.
+         * This maintains mbuf size cache alignment, while also honoring RX
+         * buffer alignment in the data portion of the mbuf. If this adjustment
+         * is not made, there is a possiblity later on that for an element of
+         * the mempool, buf, buf->data_len < (buf->buf_len - buf->data_off).
+         * This is problematic in the case of multi-segment mbufs, particularly
+         * when an mbuf segment needs to be resized (when [push|popp]ing a VLAN
+         * header, for example.
+         */
+        mbuf_priv_data_len += (aligned_mbuf_size - pkt_size);
+        dmp->mp = rte_pktmbuf_pool_create(mp_name, n_mbufs, MP_CACHE_SZ,
+                                          mbuf_priv_data_len,
+                                          mbuf_size + RTE_PKTMBUF_HEADROOM,
         if (dmp->mp) {

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