[ovs-dev] [PATCH] dpif-netdev: Avoid reordering of packets in a batch with same megaflow

Vishal Deep Ajmera vishal.deep.ajmera at ericsson.com
Sun Jun 17 02:21:51 UTC 2018

OVS reads packets in batches from a given port and packets in the
batch are subjected to potentially 3 levels of lookups to identify
the datapath megaflow entry (or flow) associated with the packet.
Each megaflow entry has a dedicated buffer in which packets that match
the flow classification criteria are collected. This buffer helps OVS
perform batch processing for all packets associated with a given flow.

Each packet in the received batch is first subjected to lookup in the
Exact Match Cache (EMC). Each EMC entry will point to a flow. If the
EMC lookup is successful, the packet is moved from the rx batch to the
per-flow buffer.

Packets that did not match any EMC entry are rearranged in the rx batch
at the beginning and are now subjected to a lookup in the megaflow cache.
Packets that match a megaflow cache entry are *appended* to the per-flow

Packets that do not match any megaflow entry are subjected to slow-path
processing through the upcall mechanism. This cannot change the order of
packets as by definition upcall processing is only done for packets
without matching megaflow entry.

The EMC entry match fields encompass all potentially significant header
fields, typically more than specified in the associated flow's match
criteria. Hence, multiple EMC entries can point to the same flow. Given
that per-flow batching happens at each lookup stage, packets belonging
to the same megaflow can get re-ordered because some packets match EMC
entries while others do not.

The following example can illustrate the issue better. Consider
following batch of packets (labelled P1 to P8) associated with a single
TCP connection and associated with a single flow. Let us assume that
packets with just the ACK bit set in TCP flags have been received in a
prior batch also and a corresponding EMC entry exists.

1. P1 (TCP Flag: ACK)
2. P2 (TCP Flag: ACK)
3. P3 (TCP Flag: ACK)
4. P4 (TCP Flag: ACK, PSH)
5. P5 (TCP Flag: ACK)
6. P6 (TCP Flag: ACK)
7. P7 (TCP Flag: ACK)
8. P8 (TCP Flag: ACK)

The megaflow classification criteria does not include TCP flags while
the EMC match criteria does. Thus, all packets other than P4 match
the existing EMC entry and are moved to the per-flow packet batch.
Subsequently, packet P4 is moved to the same per-flow packet batch as
a result of the megaflow lookup. Though the packets have all been
correctly classified as being associated with the same flow, the
packet order has not been preserved because of the per-flow batching
performed during the EMC lookup stage. This packet re-ordering has
performance implications for TCP applications.

This patch preserves the packet ordering by performing the per-flow
batching after both the EMC and megaflow lookups are complete. As an
optimization, packets are flow-batched in emc processing till any
packet in the batch has an EMC miss.

A new flow map is maintained to keep the original order of packet
along with flow information. Post fastpath processing, packets from
flow map are *appended* to per-flow buffer.

Signed-off-by: Vishal Deep Ajmera <vishal.deep.ajmera at ericsson.com>
Co-authored-by: Venkatesan Pradeep <venkatesan.pradeep at ericsson.com>
Signed-off-by: Venkatesan Pradeep <venkatesan.pradeep at ericsson.com>
 lib/dpif-netdev.c | 79 ++++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 66 insertions(+), 13 deletions(-)

diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
index 9390fff..2c423ed 100644
--- a/lib/dpif-netdev.c
+++ b/lib/dpif-netdev.c
@@ -207,6 +207,13 @@ struct dpcls_rule {
     /* 'flow' must be the last field, additional space is allocated here. */
+/* data structure to keep packet order till fastpath processing */
+struct dp_packet_flow_map {
+    struct dp_packet *packet;
+    struct dp_netdev_flow *flow;
+    uint16_t tcp_flags;
 static void dpcls_init(struct dpcls *);
 static void dpcls_destroy(struct dpcls *);
 static void dpcls_sort_subtable_vector(struct dpcls *);
@@ -5081,10 +5088,10 @@ struct packet_batch_per_flow {
 static inline void
 packet_batch_per_flow_update(struct packet_batch_per_flow *batch,
                              struct dp_packet *packet,
-                             const struct miniflow *mf)
+                             uint16_t tcp_flags)
     batch->byte_count += dp_packet_size(packet);
-    batch->tcp_flags |= miniflow_get_tcp_flags(mf);
+    batch->tcp_flags |= tcp_flags;
     batch->array.packets[batch->array.count++] = packet;
@@ -5118,7 +5125,7 @@ packet_batch_per_flow_execute(struct packet_batch_per_flow *batch,
 static inline void
 dp_netdev_queue_batches(struct dp_packet *pkt,
-                        struct dp_netdev_flow *flow, const struct miniflow *mf,
+                        struct dp_netdev_flow *flow, uint16_t tcp_flags,
                         struct packet_batch_per_flow *batches,
                         size_t *n_batches)
@@ -5129,7 +5136,7 @@ dp_netdev_queue_batches(struct dp_packet *pkt,
         packet_batch_per_flow_init(batch, flow);
-    packet_batch_per_flow_update(batch, pkt, mf);
+    packet_batch_per_flow_update(batch, pkt, tcp_flags);
 /* Try to process all ('cnt') the 'packets' using only the exact match cache
@@ -5151,6 +5158,9 @@ emc_processing(struct dp_netdev_pmd_thread *pmd,
                struct dp_packet_batch *packets_,
                struct netdev_flow_key *keys,
                struct packet_batch_per_flow batches[], size_t *n_batches,
+               struct dp_packet_flow_map *flow_map,
+               size_t *n_flows,
+               uint8_t *index_map,
                bool md_is_valid, odp_port_t port_no)
     struct emc_cache *flow_cache = &pmd->flow_cache;
@@ -5160,6 +5170,9 @@ emc_processing(struct dp_netdev_pmd_thread *pmd,
     const size_t cnt = dp_packet_batch_size(packets_);
     uint32_t cur_min;
     int i;
+    size_t map_cnt = 0;
+    uint16_t tcp_flags;
+    bool batch_enable = true;
     atomic_read_relaxed(&pmd->dp->emc_insert_min, &cur_min);
@@ -5168,6 +5181,7 @@ emc_processing(struct dp_netdev_pmd_thread *pmd,
     DP_PACKET_BATCH_REFILL_FOR_EACH (i, cnt, packet, packets_) {
         struct dp_netdev_flow *flow;
+        struct dp_packet_flow_map *map;
         if (OVS_UNLIKELY(dp_packet_size(packet) < ETH_HEADER_LEN)) {
@@ -5200,8 +5214,20 @@ emc_processing(struct dp_netdev_pmd_thread *pmd,
             flow = NULL;
         if (OVS_LIKELY(flow)) {
-            dp_netdev_queue_batches(packet, flow, &key->mf, batches,
-                                    n_batches);
+            if (OVS_LIKELY(batch_enable)) {
+                tcp_flags = miniflow_get_tcp_flags(&key->mf);
+                dp_netdev_queue_batches(packet, flow, tcp_flags, batches,
+                                        n_batches);
+            } else {
+                /* Flow batching should be performed only after fast-path processing
+                 * is also completed for packets with emc miss or else it will
+                 * result in reordering of packets with same datapath flows.
+                 */
+                map = &flow_map[map_cnt++];
+                map->flow = flow;
+                map->packet = packet;
+                map->tcp_flags = miniflow_get_tcp_flags(&key->mf);
+            }
         } else {
             /* Exact match cache missed. Group missed packets together at
              * the beginning of the 'packets' array. */
@@ -5210,9 +5236,17 @@ emc_processing(struct dp_netdev_pmd_thread *pmd,
              * must be returned to the caller. The next key should be extracted
              * to 'keys[n_missed + 1]'. */
             key = &keys[++n_missed];
+            /* preserve the order of packet for flow batching */
+            index_map[packets_->count - 1] = map_cnt;
+            flow_map[map_cnt++].flow = NULL;
+            /* skip batching for subsequent packets to avoid reordering */
+            batch_enable = false;
+    *n_flows = map_cnt;
     pmd_perf_update_counter(&pmd->perf_stats, PMD_STAT_EXACT_HIT,
                             cnt - n_dropped - n_missed);
@@ -5299,8 +5333,8 @@ static inline void
 fast_path_processing(struct dp_netdev_pmd_thread *pmd,
                      struct dp_packet_batch *packets_,
                      struct netdev_flow_key *keys,
-                     struct packet_batch_per_flow batches[],
-                     size_t *n_batches,
+                     struct dp_packet_flow_map *flow_map,
+                     uint8_t *index_map,
                      odp_port_t in_port)
     const size_t cnt = dp_packet_batch_size(packets_);
@@ -5379,6 +5413,8 @@ fast_path_processing(struct dp_netdev_pmd_thread *pmd,
     DP_PACKET_BATCH_FOR_EACH (i, packet, packets_) {
         struct dp_netdev_flow *flow;
+        /* get the original order of this packet in received batch */
+        int recv_idx = index_map[i];
         if (OVS_UNLIKELY(!rules[i])) {
@@ -5387,7 +5423,12 @@ fast_path_processing(struct dp_netdev_pmd_thread *pmd,
         flow = dp_netdev_flow_cast(rules[i]);
         emc_probabilistic_insert(pmd, &keys[i], flow);
-        dp_netdev_queue_batches(packet, flow, &keys[i].mf, batches, n_batches);
+        /* add these packets into the flow map in the same order
+         * as received.
+         */
+        flow_map[recv_idx].packet = packet;
+        flow_map[recv_idx].flow = flow;
+        flow_map[recv_idx].tcp_flags = miniflow_get_tcp_flags(&keys[i].mf);
     pmd_perf_update_counter(&pmd->perf_stats, PMD_STAT_MASKED_HIT,
@@ -5418,17 +5459,30 @@ dp_netdev_input__(struct dp_netdev_pmd_thread *pmd,
         struct netdev_flow_key keys[PKT_ARRAY_SIZE];
     struct packet_batch_per_flow batches[PKT_ARRAY_SIZE];
-    size_t n_batches;
+    struct dp_packet_flow_map flow_map[PKT_ARRAY_SIZE];
+    uint8_t index_map[PKT_ARRAY_SIZE];
+    size_t n_batches, n_flows = 0;
     odp_port_t in_port;
     n_batches = 0;
     emc_processing(pmd, packets, keys, batches, &n_batches,
-                            md_is_valid, port_no);
+                   flow_map, &n_flows, index_map, md_is_valid, port_no);
     if (!dp_packet_batch_is_empty(packets)) {
         /* Get ingress port from first packet's metadata. */
         in_port = packets->packets[0]->md.in_port.odp_port;
         fast_path_processing(pmd, packets, keys,
-                             batches, &n_batches, in_port);
+                             flow_map, index_map, in_port);
+    }
+    size_t i;
+    /* batch rest of packets which are in flow map */
+    for (i = 0;i < n_flows; i++) {
+        struct dp_packet_flow_map *map = &flow_map[i];
+        if (OVS_UNLIKELY(!map->flow)) {
+            continue;
+        }
+        dp_netdev_queue_batches(map->packet, map->flow, map->tcp_flags, batches, &n_batches);
     /* All the flow batches need to be reset before any call to
@@ -5440,7 +5494,6 @@ dp_netdev_input__(struct dp_netdev_pmd_thread *pmd,
      * already its own batches[k] still waiting to be served.  So if its
      * ‘batch’ member is not reset, the recirculated packet would be wrongly
      * appended to batches[k] of the 1st call to dp_netdev_input__(). */
-    size_t i;
     for (i = 0; i < n_batches; i++) {
         batches[i].flow->batch = NULL;

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