[ovs-dev,v4] dpif-netdev: Avoid reordering of packets in a batch with same megaflow

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
buffer.

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@ericsson.com>
Co-authored-by: Venkatesan Pradeep <venkatesan.pradeep@ericsson.com>
Signed-off-by: Venkatesan Pradeep <venkatesan.pradeep@ericsson.com>
---
 lib/dpif-netdev.c | 103 +++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 87 insertions(+), 16 deletions(-)

Tuesday, July 10, 2018 12:14 AM, Vishal Deep Ajmera:
> Subject: [ovs-dev] [PATCH v4] dpif-netdev: Avoid reordering of packets in a
> batch with same megaflow
> 
> 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
> buffer.
> 
> 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@ericsson.com>
> Co-authored-by: Venkatesan Pradeep
> <venkatesan.pradeep@ericsson.com>
> Signed-off-by: Venkatesan Pradeep <venkatesan.pradeep@ericsson.com>

Reviewed-by: Shahaf Shuler <shahafs@mellanox.com> 

> ---
>  lib/dpif-netdev.c | 103
> +++++++++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 87 insertions(+), 16 deletions(-)
> 
> diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c index 8b3556d..d4b8f99
> 100644
> --- a/lib/dpif-netdev.c
> +++ b/lib/dpif-netdev.c
> @@ -208,6 +208,13 @@ 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 *); @@ -5602,6 +5609,19
> @@ dp_netdev_queue_batches(struct dp_packet *pkt,
>      packet_batch_per_flow_update(batch, pkt, tcp_flags);  }
> 
> +static inline void
> +packet_enqueue_to_flow_map(struct dp_packet_flow_map *flow_map,
> +                           struct dp_netdev_flow *flow,
> +                           struct dp_packet *packet,
> +                           uint16_t tcp_flags,
> +                           size_t *map_cnt) {
> +    struct dp_packet_flow_map *map = &flow_map[(*map_cnt)++];
> +    map->flow = flow;
> +    map->packet = packet;
> +    map->tcp_flags = tcp_flags;
> +}
> +
>  /* Try to process all ('cnt') the 'packets' using only the exact match cache
>   * 'pmd->flow_cache'. If a flow is not found for a packet 'packets[i]', the
>   * miniflow is copied into 'keys' and the packet pointer is moved at the @@ -
> 5621,6 +5641,9 @@ 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; @@ -5631,6 +5654,8
> @@ emc_processing(struct dp_netdev_pmd_thread *pmd,
>      uint32_t cur_min;
>      int i;
>      uint16_t tcp_flags;
> +    size_t map_cnt = 0;
> +    bool batch_enable = true;
> 
>      atomic_read_relaxed(&pmd->dp->emc_insert_min, &cur_min);
>      pmd_perf_update_counter(&pmd->perf_stats,
> @@ -5661,11 +5686,20 @@ emc_processing(struct dp_netdev_pmd_thread
> *pmd,
>          if ((*recirc_depth_get() == 0) &&
>              dp_packet_has_flow_mark(packet, &mark)) {
>              flow = mark_to_flow_find(pmd, mark);
> -            if (flow) {
> +            if (OVS_LIKELY(flow)) {
>                  tcp_flags = parse_tcp_flags(packet);
> -                dp_netdev_queue_batches(packet, flow, tcp_flags, batches,
> -                                        n_batches);
> -                continue;
> +                if (OVS_LIKELY(batch_enable)) {
> +                    dp_netdev_queue_batches(packet, flow, tcp_flags, batches,
> +                                            n_batches);
> +                    continue;
> +                } 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. */
> +                    packet_enqueue_to_flow_map(flow_map, flow, packet,
> +                                               tcp_flags, &map_cnt);
> +                }
>              }
>          }
> 
> @@ -5685,8 +5719,18 @@ emc_processing(struct dp_netdev_pmd_thread
> *pmd,
>          }
>          if (OVS_LIKELY(flow)) {
>              tcp_flags = miniflow_get_tcp_flags(&key->mf);
> -            dp_netdev_queue_batches(packet, flow, tcp_flags, batches,
> -                                    n_batches);
> +            if (OVS_LIKELY(batch_enable)) {
> +                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. */
> +                packet_enqueue_to_flow_map(flow_map, flow, packet, tcp_flags,
> +                                           &map_cnt);
> +            }
> +
>          } else {
>              /* Exact match cache missed. Group missed packets together at
>               * the beginning of the 'packets' array. */ @@ -5695,9 +5739,16 @@
> 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);
> 
> @@ -5784,8 +5835,8 @@ 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_); @@ -5864,6 +5915,8
> @@ 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])) {
>              continue;
> @@ -5872,9 +5925,13 @@ 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,
> -                                miniflow_get_tcp_flags(&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, @@ -5905,19 +5962,34 @@
> dp_netdev_input__(struct dp_netdev_pmd_thread *pmd,
>      OVS_ALIGNED_VAR(CACHE_LINE_SIZE)
>          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;
> +    size_t i;
> 
>      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);
>      }
> 
> +    /* 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
>       * packet_batch_per_flow_execute() as it could potentially trigger
>       * recirculation. When a packet matching flow ‘j’ happens to be @@ -
> 5927,7 +5999,6 @@ 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;
>      }
> --
> 1.9.1
> 
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Not a full review.
One comment inline.

Best regards, Ilya Maximets.

On 10.07.2018 00:13, Vishal Deep Ajmera wrote:
> 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
> buffer.
> 
> 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@ericsson.com>
> Co-authored-by: Venkatesan Pradeep <venkatesan.pradeep@ericsson.com>
> Signed-off-by: Venkatesan Pradeep <venkatesan.pradeep@ericsson.com>
> ---
>  lib/dpif-netdev.c | 103 +++++++++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 87 insertions(+), 16 deletions(-)
> 
> diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
> index 8b3556d..d4b8f99 100644
> --- a/lib/dpif-netdev.c
> +++ b/lib/dpif-netdev.c
> @@ -208,6 +208,13 @@ 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 *);
> @@ -5602,6 +5609,19 @@ dp_netdev_queue_batches(struct dp_packet *pkt,
>      packet_batch_per_flow_update(batch, pkt, tcp_flags);
>  }
>  
> +static inline void
> +packet_enqueue_to_flow_map(struct dp_packet_flow_map *flow_map,
> +                           struct dp_netdev_flow *flow,
> +                           struct dp_packet *packet,
> +                           uint16_t tcp_flags,
> +                           size_t *map_cnt)
> +{
> +    struct dp_packet_flow_map *map = &flow_map[(*map_cnt)++];
> +    map->flow = flow;
> +    map->packet = packet;
> +    map->tcp_flags = tcp_flags;
> +}
> +
>  /* Try to process all ('cnt') the 'packets' using only the exact match cache
>   * 'pmd->flow_cache'. If a flow is not found for a packet 'packets[i]', the
>   * miniflow is copied into 'keys' and the packet pointer is moved at the
> @@ -5621,6 +5641,9 @@ 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;
> @@ -5631,6 +5654,8 @@ emc_processing(struct dp_netdev_pmd_thread *pmd,
>      uint32_t cur_min;
>      int i;
>      uint16_t tcp_flags;
> +    size_t map_cnt = 0;
> +    bool batch_enable = true;
>  
>      atomic_read_relaxed(&pmd->dp->emc_insert_min, &cur_min);
>      pmd_perf_update_counter(&pmd->perf_stats,
> @@ -5661,11 +5686,20 @@ emc_processing(struct dp_netdev_pmd_thread *pmd,
>          if ((*recirc_depth_get() == 0) &&
>              dp_packet_has_flow_mark(packet, &mark)) {
>              flow = mark_to_flow_find(pmd, mark);
> -            if (flow) {
> +            if (OVS_LIKELY(flow)) {
>                  tcp_flags = parse_tcp_flags(packet);
> -                dp_netdev_queue_batches(packet, flow, tcp_flags, batches,
> -                                        n_batches);
> -                continue;
> +                if (OVS_LIKELY(batch_enable)) {
> +                    dp_netdev_queue_batches(packet, flow, tcp_flags, batches,
> +                                            n_batches);
> +                    continue;
> +                } 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. */
> +                    packet_enqueue_to_flow_map(flow_map, flow, packet,
> +                                               tcp_flags, &map_cnt);
> +                }
>              }
>          }
>  
> @@ -5685,8 +5719,18 @@ emc_processing(struct dp_netdev_pmd_thread *pmd,
>          }
>          if (OVS_LIKELY(flow)) {
>              tcp_flags = miniflow_get_tcp_flags(&key->mf);
> -            dp_netdev_queue_batches(packet, flow, tcp_flags, batches,
> -                                    n_batches);
> +            if (OVS_LIKELY(batch_enable)) {
> +                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. */
> +                packet_enqueue_to_flow_map(flow_map, flow, packet, tcp_flags,
> +                                           &map_cnt);
> +            }
> +
>          } else {
>              /* Exact match cache missed. Group missed packets together at
>               * the beginning of the 'packets' array. */
> @@ -5695,9 +5739,16 @@ 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;

Using the "packets_->count" without accessor + inside a refill loop is
highly inaccurate/potentially dangerous. Isn't it equal to "n_missed"?

> +            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);
>  
> @@ -5784,8 +5835,8 @@ 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_);
> @@ -5864,6 +5915,8 @@ 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])) {
>              continue;
> @@ -5872,9 +5925,13 @@ 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,
> -                                miniflow_get_tcp_flags(&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,
> @@ -5905,19 +5962,34 @@ dp_netdev_input__(struct dp_netdev_pmd_thread *pmd,
>      OVS_ALIGNED_VAR(CACHE_LINE_SIZE)
>          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;
> +    size_t i;
>  
>      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);
>      }
>  
> +    /* 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
>       * packet_batch_per_flow_execute() as it could potentially trigger
>       * recirculation. When a packet matching flow ‘j’ happens to be
> @@ -5927,7 +5999,6 @@ 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;
>      }
>

> > +
> > +            /* preserve the order of packet for flow batching */
> > +            index_map[packets_->count - 1] = map_cnt;
> 
> Using the "packets_->count" without accessor + inside a refill loop is highly
> inaccurate/potentially dangerous. Isn't it equal to "n_missed"?
> 
Hi Ilya,

Thanks for the review. However I could not understand your point about
inaccurate/potentially dangerous code. Can you elaborate on a scenario 
where this is likely to fail/crash ?

On 10.07.2018 14:14, Vishal Deep Ajmera wrote:
>>> +
>>> +            /* preserve the order of packet for flow batching */
>>> +            index_map[packets_->count - 1] = map_cnt;
>>
>> Using the "packets_->count" without accessor + inside a refill loop is highly
>> inaccurate/potentially dangerous. Isn't it equal to "n_missed"?
>>
> Hi Ilya,
> 
> Thanks for the review. However I could not understand your point about
> inaccurate/potentially dangerous code. Can you elaborate on a scenario 
> where this is likely to fail/crash ?

This is potentially dangerous from the future modifications and hard to
read for reviewer/person who tries to understand how it works.

Current implementation will fail if someone will change the logic of
'DP_PACKET_BATCH_REFILL_FOR_EACH', for example.

The key point is that 'DP_PACKET_BATCH_REFILL_FOR_EACH' and
'dp_packet_batch_refill' manipulates with the batch size in a hidden
from the end user manner. Using of the 'packets_->count' directly
requires knowledge of how refilling works internally, but these
functions was intended to hide internals of batch manipulations.

Also, as I understand, you're storing 'map_cnt' for each missed packet.
So, why not just use 'n_missed' for that purpose?

Best regards, Ilya Maximets.

> 
> This is potentially dangerous from the future modifications and hard to read for
> reviewer/person who tries to understand how it works.
> 
> Current implementation will fail if someone will change the logic of
> 'DP_PACKET_BATCH_REFILL_FOR_EACH', for example.
> 
> The key point is that 'DP_PACKET_BATCH_REFILL_FOR_EACH' and
> 'dp_packet_batch_refill' manipulates with the batch size in a hidden from the
> end user manner. Using of the 'packets_->count' directly requires knowledge of
> how refilling works internally, but these functions was intended to hide internals
> of batch manipulations.
> 
> Also, as I understand, you're storing 'map_cnt' for each missed packet.
> So, why not just use 'n_missed' for that purpose?
> 

map_cnt keeps count of not just the emc miss packet but also all subsequent
packets (with emc hit) after one emc miss.

On 10.07.2018 17:48, Vishal Deep Ajmera wrote:
>>
>> This is potentially dangerous from the future modifications and hard to read for
>> reviewer/person who tries to understand how it works.
>>
>> Current implementation will fail if someone will change the logic of
>> 'DP_PACKET_BATCH_REFILL_FOR_EACH', for example.
>>
>> The key point is that 'DP_PACKET_BATCH_REFILL_FOR_EACH' and
>> 'dp_packet_batch_refill' manipulates with the batch size in a hidden from the
>> end user manner. Using of the 'packets_->count' directly requires knowledge of
>> how refilling works internally, but these functions was intended to hide internals
>> of batch manipulations.
>>
>> Also, as I understand, you're storing 'map_cnt' for each missed packet.
>> So, why not just use 'n_missed' for that purpose?
>>
> 
> map_cnt keeps count of not just the emc miss packet but also all subsequent
> packets (with emc hit) after one emc miss.

Yes. But we're talking about replacing 'packets_->count' with 'n_missed'.
Not about 'map_cnt'.

> >>
> >> This is potentially dangerous from the future modifications and hard to read
> for
> >> reviewer/person who tries to understand how it works.
> >>
> >> Current implementation will fail if someone will change the logic of
> >> 'DP_PACKET_BATCH_REFILL_FOR_EACH', for example.
> >>
> >> The key point is that 'DP_PACKET_BATCH_REFILL_FOR_EACH' and
> >> 'dp_packet_batch_refill' manipulates with the batch size in a hidden from the
> >> end user manner. Using of the 'packets_->count' directly requires knowledge
> of
> >> how refilling works internally, but these functions was intended to hide
> internals
> >> of batch manipulations.
> >>
> >> Also, as I understand, you're storing 'map_cnt' for each missed packet.
> >> So, why not just use 'n_missed' for that purpose?
> >>
> >
> > map_cnt keeps count of not just the emc miss packet but also all subsequent
> > packets (with emc hit) after one emc miss.
> 
> Yes. But we're talking about replacing 'packets_->count' with 'n_missed'.
> Not about 'map_cnt'.

Thanks Ilya. I have fixed this in v5 patch-set.