[ovs-dev] [PATCH v8] netdev-dpdk:Detailed packet drop statistics

Sriram Vatala sriram.v at altencalsoftlabs.com
Tue Sep 10 17:31:53 UTC 2019


-----Original Message-----
From: Ilya Maximets <i.maximets at samsung.com>
Sent: 10 September 2019 19:29
To: Sriram Vatala <sriram.v at altencalsoftlabs.com>; ovs-dev at openvswitch.org
Cc: ktraynor at redhat.com; ian.stokes at intel.com
Subject: Re: [PATCH v8] netdev-dpdk:Detailed packet drop statistics

On 08.09.2019 19:01, Sriram Vatala wrote:
> OVS may be unable to transmit packets for multiple reasons and today
> there is a single counter to track packets dropped due to any of those
> reasons. The most common reason is that a VM is unable to read packets
> fast enough causing the vhostuser port transmit queue on the OVS side
> to become full. This manifests as a problem with VNFs not receiving
> all packets. Having a separate drop counter to track packets dropped
> because the transmit queue is full will clearly indicate that the
> problem is on the VM side and not in OVS. Similarly maintaining
> separate counters for all possible drops helps in indicating sensible
> cause for packet drops.
>
> This patch adds custom software stats counters to track packets
> dropped at port level and these counters are displayed along with
> other stats in "ovs-vsctl get interface <iface> statistics"
> command. The detailed stats will be available for both dpdk and
> vhostuser ports.
>
> Signed-off-by: Sriram Vatala <sriram.v at altencalsoftlabs.com>
> ---
> Changes since v7 :
> Defined structure netdev_dpdk_sw_stats and moved all custom stats into
> it.
> Placed a pointer to netdev_dpdk_sw_stats structure in netdev_dpdk
> strucure.
> stats are reported with prefix with netdev_dpdk
> ---
>  include/openvswitch/netdev.h                  |  14 +++
>  lib/netdev-dpdk.c                             | 109 +++++++++++++++---
>  utilities/bugtool/automake.mk                 |   3 +-
>  utilities/bugtool/ovs-bugtool-get-port-stats  |  25 ++++
>  .../plugins/network-status/openvswitch.xml    |   1 +
>  vswitchd/vswitch.xml                          |  24 ++++
>  6 files changed, 156 insertions(+), 20 deletions(-)  create mode
> 100755 utilities/bugtool/ovs-bugtool-get-port-stats
>
> diff --git a/include/openvswitch/netdev.h
> b/include/openvswitch/netdev.h index 0c10f7b48..c17e6a97d 100644
> --- a/include/openvswitch/netdev.h
> +++ b/include/openvswitch/netdev.h
> @@ -89,6 +89,20 @@ struct netdev_stats {
>      uint64_t rx_jabber_errors;
>  };
>
> +/* Custom software stats for dpdk ports */ struct
> +netdev_dpdk_sw_stats {
> +    /* No. of retries when unable to transmit. */
> +    uint64_t tx_retries;
> +    /* Pkt drops when unable to transmit; Probably Tx queue is full */
> +    uint64_t tx_failure_drops;
> +    /* Pkt len greater than max dev MTU */
> +    uint64_t tx_mtu_exceeded_drops;
> +    /* Pkt drops in egress policier processing */
> +    uint64_t tx_qos_drops;
> +    /* Pkt drops in ingress policier processing */
> +    uint64_t rx_qos_drops;
> +};

This should not be in a common header since the only user is netdev-dpdk.c.
Regarding this code itself:
1. s/policier/policer/
2. s/Pkt/Packet/, s/len/length/
3. s/max dev MTU/device MTU/ (MTU already has 'maximum' word).
4. All comments should end with a period.

Sorry I missed to check spell. I will fix this in my next patch v9
I will move this structure definition to netdev-dpdk.c  and make it static.

> +
>  /* Structure representation of custom statistics counter */  struct
> netdev_custom_counter {
>      uint64_t value;
> diff --git a/lib/netdev-dpdk.c b/lib/netdev-dpdk.c index
> bc20d6843..39aab4672 100644
> --- a/lib/netdev-dpdk.c
> +++ b/lib/netdev-dpdk.c
> @@ -413,11 +413,10 @@ struct netdev_dpdk {
>
>      PADDED_MEMBERS(CACHE_LINE_SIZE,
>          struct netdev_stats stats;
> -        /* Custom stat for retries when unable to transmit. */
> -        uint64_t tx_retries;
> +        struct netdev_dpdk_sw_stats *sw_stats;
>          /* Protects stats */
>          rte_spinlock_t stats_lock;
> -        /* 4 pad bytes here. */
> +        /* 36 pad bytes here. */
>      );
>
>      PADDED_MEMBERS(CACHE_LINE_SIZE,
> @@ -1171,7 +1170,8 @@ common_construct(struct netdev *netdev, dpdk_port_t 
> port_no,
>      dev->rte_xstats_ids = NULL;
>      dev->rte_xstats_ids_size = 0;
>
> -    dev->tx_retries = 0;
> +    dev->sw_stats = (struct netdev_dpdk_sw_stats *)
> +                        xcalloc(1,sizeof(struct
> + netdev_dpdk_sw_stats));

This should be:
       dev->sw_stats = xzalloc(sizeof *dev->sw_stats); Or
       dev->sw_stats = dpdk_rte_mzalloc(sizeof *dev->sw_stats);

The later variant will require proper error handling as it could return NULL.
I will change this in next patch v9.

See the Documentation/internals/contributing/coding-style.rst for details.

>
>      return 0;
>  }
> @@ -1357,6 +1357,7 @@ common_destruct(struct netdev_dpdk *dev)
>      ovs_list_remove(&dev->list_node);
>      free(ovsrcu_get_protected(struct ingress_policer *,
>                                &dev->ingress_policer));
> +    free(dev->sw_stats);
>      ovs_mutex_destroy(&dev->mutex);
>  }
>
> @@ -2171,6 +2172,7 @@ netdev_dpdk_vhost_rxq_recv(struct netdev_rxq *rxq,
>      struct ingress_policer *policer = netdev_dpdk_get_ingress_policer(dev);
>      uint16_t nb_rx = 0;
>      uint16_t dropped = 0;
> +    uint16_t qos_drops = 0;
>      int qid = rxq->queue_id * VIRTIO_QNUM + VIRTIO_TXQ;
>      int vid = netdev_dpdk_get_vid(dev);
>
> @@ -2202,11 +2204,13 @@ netdev_dpdk_vhost_rxq_recv(struct netdev_rxq *rxq,
>                                      (struct rte_mbuf **) batch->packets,
>                                      nb_rx, true);
>          dropped -= nb_rx;
> +        qos_drops = dropped;

'dropped' here already counts 'qos_drops' only.  I don't think we need an 
extra variable here.
I used separate variable for readability purpose. I can remove this.

>      }
>
>      rte_spinlock_lock(&dev->stats_lock);
>      netdev_dpdk_vhost_update_rx_counters(&dev->stats, batch->packets,
>                                           nb_rx, dropped);
> +    dev->sw_stats->rx_qos_drops += qos_drops;
>      rte_spinlock_unlock(&dev->stats_lock);
>
>      batch->count = nb_rx;
> @@ -2232,6 +2236,7 @@ netdev_dpdk_rxq_recv(struct netdev_rxq *rxq, struct 
> dp_packet_batch *batch,
>      struct ingress_policer *policer = netdev_dpdk_get_ingress_policer(dev);
>      int nb_rx;
>      int dropped = 0;
> +    int qos_drops = 0;
>
>      if (OVS_UNLIKELY(!(dev->flags & NETDEV_UP))) {
>          return EAGAIN;
> @@ -2250,12 +2255,14 @@ netdev_dpdk_rxq_recv(struct netdev_rxq *rxq, struct 
> dp_packet_batch *batch,
>                                      (struct rte_mbuf **) batch->packets,
>                                      nb_rx, true);
>          dropped -= nb_rx;
> +        qos_drops = dropped;

Same here.

>      }
>
>      /* Update stats to reflect dropped packets */
>      if (OVS_UNLIKELY(dropped)) {
>          rte_spinlock_lock(&dev->stats_lock);
>          dev->stats.rx_dropped += dropped;
> +        dev->sw_stats->rx_qos_drops += qos_drops;
>          rte_spinlock_unlock(&dev->stats_lock);
>      }
>
> @@ -2339,6 +2346,10 @@ __netdev_dpdk_vhost_send(struct netdev *netdev, int 
> qid,
>      struct rte_mbuf **cur_pkts = (struct rte_mbuf **) pkts;
>      unsigned int total_pkts = cnt;
>      unsigned int dropped = 0;
> +    unsigned int tx_failure;
> +    unsigned int mtu_drops;
> +    unsigned int qos_drops;
> +    struct netdev_dpdk_sw_stats *sw_stats = dev->sw_stats;
>      int i, retries = 0;
>      int max_retries = VHOST_ENQ_RETRY_MIN;
>      int vid = netdev_dpdk_get_vid(dev); @@ -2356,9 +2367,12 @@
> __netdev_dpdk_vhost_send(struct netdev *netdev, int qid,
>      rte_spinlock_lock(&dev->tx_q[qid].tx_lock);
>
>      cnt = netdev_dpdk_filter_packet_len(dev, cur_pkts, cnt);
> +    mtu_drops = total_pkts - cnt;
> +    qos_drops = cnt;
>      /* Check has QoS has been configured for the netdev */
>      cnt = netdev_dpdk_qos_run(dev, cur_pkts, cnt, true);
> -    dropped = total_pkts - cnt;
> +    qos_drops -= cnt;
> +    dropped = qos_drops + mtu_drops;
>
>      do {
>          int vhost_qid = qid * VIRTIO_QNUM + VIRTIO_RXQ; @@ -2383,12
> +2397,16 @@ __netdev_dpdk_vhost_send(struct netdev *netdev, int qid,
>          }
>      } while (cnt && (retries++ < max_retries));
>
> +    tx_failure = cnt;
>      rte_spinlock_unlock(&dev->tx_q[qid].tx_lock);
>
>      rte_spinlock_lock(&dev->stats_lock);
>      netdev_dpdk_vhost_update_tx_counters(&dev->stats, pkts, total_pkts,
>                                           cnt + dropped);
> -    dev->tx_retries += MIN(retries, max_retries);
> +    sw_stats->tx_retries += MIN(retries, max_retries);
> +    sw_stats->tx_failure_drops += tx_failure;
> +    sw_stats->tx_mtu_exceeded_drops += mtu_drops;
> +    sw_stats->tx_qos_drops += qos_drops;
>      rte_spinlock_unlock(&dev->stats_lock);
>
>  out:
> @@ -2413,12 +2431,16 @@ dpdk_do_tx_copy(struct netdev *netdev, int qid, 
> struct dp_packet_batch *batch)
>      struct rte_mbuf *pkts[PKT_ARRAY_SIZE];
>      uint32_t cnt = batch_cnt;
>      uint32_t dropped = 0;
> +    uint32_t tx_failure = 0;
> +    uint32_t mtu_drops = 0;
> +    uint32_t qos_drops = 0;
> +    struct netdev_dpdk_sw_stats *sw_stats = dev->sw_stats;
>
>      if (dev->type != DPDK_DEV_VHOST) {
>          /* Check if QoS has been configured for this netdev. */
>          cnt = netdev_dpdk_qos_run(dev, (struct rte_mbuf **) batch->packets,
>                                    batch_cnt, false);
> -        dropped += batch_cnt - cnt;
> +        qos_drops = batch_cnt - cnt;
>      }
>
>      uint32_t txcnt = 0;
> @@ -2431,7 +2453,7 @@ dpdk_do_tx_copy(struct netdev *netdev, int qid, struct 
> dp_packet_batch *batch)
>              VLOG_WARN_RL(&rl, "Too big size %u max_packet_len %d",
>                           size, dev->max_packet_len);
>
> -            dropped++;
> +            mtu_drops++;
>              continue;
>          }
>
> @@ -2454,13 +2476,17 @@ dpdk_do_tx_copy(struct netdev *netdev, int qid, 
> struct dp_packet_batch *batch)
>              __netdev_dpdk_vhost_send(netdev, qid, (struct dp_packet **) 
> pkts,
>                                       txcnt);
>          } else {
> -            dropped += netdev_dpdk_eth_tx_burst(dev, qid, pkts, txcnt);
> +            tx_failure = netdev_dpdk_eth_tx_burst(dev, qid, pkts,
> + txcnt);
>          }
>      }
>
> +    dropped += qos_drops + mtu_drops + tx_failure;
>      if (OVS_UNLIKELY(dropped)) {
>          rte_spinlock_lock(&dev->stats_lock);
>          dev->stats.tx_dropped += dropped;
> +        sw_stats->tx_failure_drops += tx_failure;
> +        sw_stats->tx_mtu_exceeded_drops += mtu_drops;
> +        sw_stats->tx_qos_drops += qos_drops;
>          rte_spinlock_unlock(&dev->stats_lock);
>      }
>  }
> @@ -2485,6 +2511,8 @@ netdev_dpdk_send__(struct netdev_dpdk *dev, int qid,
>                     struct dp_packet_batch *batch,
>                     bool concurrent_txq)  {
> +    struct netdev_dpdk_sw_stats *sw_stats = dev->sw_stats;

This should be defined under the 'else' branch.
Sorry, I missed this. I will move this.
> +
>      if (OVS_UNLIKELY(!(dev->flags & NETDEV_UP))) {
>          dp_packet_delete_batch(batch, true);
>          return;
> @@ -2502,18 +2530,25 @@ netdev_dpdk_send__(struct netdev_dpdk *dev, int qid,
>          dp_packet_delete_batch(batch, true);
>      } else {
>          int tx_cnt, dropped;
> +        int tx_failure, mtu_drops, qos_drops;
>          int batch_cnt = dp_packet_batch_size(batch);
>          struct rte_mbuf **pkts = (struct rte_mbuf **) batch->packets;
>
>          tx_cnt = netdev_dpdk_filter_packet_len(dev, pkts, batch_cnt);
> +        mtu_drops = batch_cnt - tx_cnt;
> +        qos_drops = tx_cnt;
>          tx_cnt = netdev_dpdk_qos_run(dev, pkts, tx_cnt, true);
> -        dropped = batch_cnt - tx_cnt;
> +        qos_drops -= tx_cnt;
>
> -        dropped += netdev_dpdk_eth_tx_burst(dev, qid, pkts, tx_cnt);
> +        tx_failure = netdev_dpdk_eth_tx_burst(dev, qid, pkts,
> + tx_cnt);
>
> +        dropped = tx_failure + mtu_drops + qos_drops;
>          if (OVS_UNLIKELY(dropped)) {
>              rte_spinlock_lock(&dev->stats_lock);
>              dev->stats.tx_dropped += dropped;
> +            sw_stats->tx_failure_drops += tx_failure;
> +            sw_stats->tx_mtu_exceeded_drops += mtu_drops;
> +            sw_stats->tx_qos_drops += qos_drops;
>              rte_spinlock_unlock(&dev->stats_lock);
>          }
>      }
> @@ -2772,6 +2807,17 @@ netdev_dpdk_get_custom_stats(const struct netdev 
> *netdev,
>      uint32_t i;
>      struct netdev_dpdk *dev = netdev_dpdk_cast(netdev);
>      int rte_xstats_ret;
> +    uint16_t index = 0;
> +
> +#define DPDK_CSTATS                         \
> +    DPDK_CSTAT(tx_failure_drops)            \
> +    DPDK_CSTAT(tx_mtu_exceeded_drops)       \
> +    DPDK_CSTAT(tx_qos_drops)                \
> +    DPDK_CSTAT(rx_qos_drops)
> +
> +#define DPDK_CSTAT(NAME) +1
> +    custom_stats->size = DPDK_CSTATS; #undef DPDK_CSTAT
>
>      ovs_mutex_lock(&dev->mutex);
>
> @@ -2786,9 +2832,10 @@ netdev_dpdk_get_custom_stats(const struct netdev 
> *netdev,
>          if (rte_xstats_ret > 0 &&
>              rte_xstats_ret <= dev->rte_xstats_ids_size) {
>
> -            custom_stats->size = rte_xstats_ret;
> +            index = rte_xstats_ret;
> +            custom_stats->size += rte_xstats_ret;
>              custom_stats->counters =
> -                    (struct netdev_custom_counter *) 
> xcalloc(rte_xstats_ret,
> +                   (struct netdev_custom_counter *)
> + xcalloc(custom_stats->size,
>                              sizeof(struct netdev_custom_counter));
>
>              for (i = 0; i < rte_xstats_ret; i++) { @@ -2802,7 +2849,6
> @@ netdev_dpdk_get_custom_stats(const struct netdev *netdev,
>              VLOG_WARN("Cannot get XSTATS values for port: 
> "DPDK_PORT_ID_FMT,
>                        dev->port_id);
>              custom_stats->counters = NULL;
> -            custom_stats->size = 0;
>              /* Let's clear statistics cache, so it will be
>               * reconfigured */
>              netdev_dpdk_clear_xstats(dev); @@ -2811,6 +2857,27 @@
> netdev_dpdk_get_custom_stats(const struct netdev *netdev,
>          free(values);
>      }
>
> +    if (custom_stats->counters == NULL) {
> +        custom_stats->counters =
> +            (struct netdev_custom_counter *) xcalloc(custom_stats->size,
> +                                  sizeof(struct netdev_custom_counter));
> +    }
> +
> +    rte_spinlock_lock(&dev->stats_lock);
> +    i = index;
> +#define DPDK_CSTAT(NAME)                                                \
> +    ovs_strlcpy(custom_stats->counters[i++].name, "netdev_dpdk_"#NAME,  \
> +                NETDEV_CUSTOM_STATS_NAME_SIZE);
> +    DPDK_CSTATS;
> +#undef DPDK_CSTAT
> +
> +    i = index;
> +#define DPDK_CSTAT(NAME)                                     \
> +    custom_stats->counters[i++].value = dev->sw_stats->NAME;
> +    DPDK_CSTATS;
> +#undef DPDK_CSTAT
> +    rte_spinlock_unlock(&dev->stats_lock);
> +


Above is still a big code duplication. I think, it's better to rename
netdev_dpdk_vhost_get_custom_stats() to something neutral like
netdev_dpdk_get_sw_custom_stats() and re-use it here. DPDK xstats could be 
added by calling xrealloc() on resulted sw custom_stats.
For tracking unsupported statistics (tx_retries), we can use same trick as 
bridge.c, i.e. setting UINT64_MAX for unsupported and skip them while 
reporting.

What do you think?
I could draft another refactoring patch for this if needed.

Yes. This can be done.  I can do this change in my next patch v9. You can 
draft patch if you want this particular change to go as a separate patch. Let 
me know your decision.

>      ovs_mutex_unlock(&dev->mutex);
>
>      return 0;
> @@ -2823,8 +2890,12 @@ netdev_dpdk_vhost_get_custom_stats(const struct 
> netdev *netdev,
>      struct netdev_dpdk *dev = netdev_dpdk_cast(netdev);
>      int i;
>
> -#define VHOST_CSTATS \
> -    VHOST_CSTAT(tx_retries)
> +#define VHOST_CSTATS                        \
> +    VHOST_CSTAT(tx_retries)                 \
> +    VHOST_CSTAT(tx_failure_drops)           \
> +    VHOST_CSTAT(tx_mtu_exceeded_drops)      \
> +    VHOST_CSTAT(tx_qos_drops)               \
> +    VHOST_CSTAT(rx_qos_drops)
>
>  #define VHOST_CSTAT(NAME) + 1
>      custom_stats->size = VHOST_CSTATS; @@ -2832,8 +2903,8 @@
> netdev_dpdk_vhost_get_custom_stats(const struct netdev *netdev,
>      custom_stats->counters = xcalloc(custom_stats->size,
>                                       sizeof *custom_stats->counters);
>      i = 0;
> -#define VHOST_CSTAT(NAME) \
> -    ovs_strlcpy(custom_stats->counters[i++].name, #NAME, \
> +#define VHOST_CSTAT(NAME)                                                \
> +    ovs_strlcpy(custom_stats->counters[i++].name, "netdev_dpdk_"#NAME,   \
>                  NETDEV_CUSTOM_STATS_NAME_SIZE);
>      VHOST_CSTATS;
>  #undef VHOST_CSTAT
> @@ -2843,7 +2914,7 @@ netdev_dpdk_vhost_get_custom_stats(const struct netdev 
> *netdev,
>      rte_spinlock_lock(&dev->stats_lock);
>      i = 0;
>  #define VHOST_CSTAT(NAME) \
> -    custom_stats->counters[i++].value = dev->NAME;
> +    custom_stats->counters[i++].value = dev->sw_stats->NAME;
>      VHOST_CSTATS;
>  #undef VHOST_CSTAT
>      rte_spinlock_unlock(&dev->stats_lock);
> diff --git a/utilities/bugtool/automake.mk
> b/utilities/bugtool/automake.mk index 40980b367..dda58e0a1 100644
> --- a/utilities/bugtool/automake.mk
> +++ b/utilities/bugtool/automake.mk
> @@ -22,7 +22,8 @@ bugtool_scripts = \
>  	utilities/bugtool/ovs-bugtool-ovs-bridge-datapath-type \
>  	utilities/bugtool/ovs-bugtool-ovs-vswitchd-threads-affinity \
>  	utilities/bugtool/ovs-bugtool-qos-configs \
> -	utilities/bugtool/ovs-bugtool-get-dpdk-nic-numa
> +	utilities/bugtool/ovs-bugtool-get-dpdk-nic-numa \
> +	utilities/bugtool/ovs-bugtool-get-port-stats
>
>  scripts_SCRIPTS += $(bugtool_scripts)
>
> diff --git a/utilities/bugtool/ovs-bugtool-get-port-stats
> b/utilities/bugtool/ovs-bugtool-get-port-stats
> new file mode 100755
> index 000000000..0fe175e6b
> --- /dev/null
> +++ b/utilities/bugtool/ovs-bugtool-get-port-stats
> @@ -0,0 +1,25 @@
> +#! /bin/bash
> +
> +# This library is free software; you can redistribute it and/or #
> +modify it under the terms of version 2.1 of the GNU Lesser General #
> +Public License as published by the Free Software Foundation.
> +#
> +# This library is distributed in the hope that it will be useful, #
> +but WITHOUT ANY WARRANTY; without even the implied warranty of #
> +MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU #
> +Lesser General Public License for more details.
> +#
> +# Copyright (C) 2019 Ericsson AB
> +
> +for bridge in `ovs-vsctl -- --real list-br` do
> +    echo -e "\nBridge : ${bridge}\n"
> +    for iface in `ovs-vsctl list-ifaces ${bridge}`
> +    do
> +        echo -e "iface : ${iface}"
> +        ovs-vsctl get interface ${iface} statistics
> +        echo -e "\n"
> +    done
> +    echo -e "iface : ${bridge}"
> +    ovs-vsctl get interface ${bridge} statistics done
> diff --git a/utilities/bugtool/plugins/network-status/openvswitch.xml
> b/utilities/bugtool/plugins/network-status/openvswitch.xml
> index d39867c6e..8c1ec643e 100644
> --- a/utilities/bugtool/plugins/network-status/openvswitch.xml
> +++ b/utilities/bugtool/plugins/network-status/openvswitch.xml
> @@ -40,4 +40,5 @@
>      <command label="ovs-ofctl-dump-groups" 
> filters="ovs">/usr/share/openvswitch/scripts/ovs-bugtool-ovs-ofctl-loop-over-bridges 
> "dump-groups"</command>
>      <command label="ovs-ofctl-dump-group-stats" filters="ovs" 
> repeat="2">/usr/share/openvswitch/scripts/ovs-bugtool-ovs-ofctl-loop-over-bridges 
> "dump-group-stats"</command>
>      <command label="get_dpdk_nic_numa"
> filters="ovs">/usr/share/openvswitch/scripts/ovs-bugtool-get-dpdk-nic-
> numa</command>
> +    <command label="get_port_statistics"
> + filters="ovs">/usr/share/openvswitch/scripts/ovs-bugtool-get-port-st
> + ats</command>
>  </collect>
> diff --git a/vswitchd/vswitch.xml b/vswitchd/vswitch.xml index
> 9a743c05b..402f3c8ec 100644
> --- a/vswitchd/vswitch.xml
> +++ b/vswitchd/vswitch.xml
> @@ -3486,6 +3486,30 @@ ovs-vsctl add-port br0 p0 -- set Interface p0 
> type=patch options:peer=p1 \
>            the above.
>          </column>
>        </group>
> +      <group title="Statistics: Custom Software stats for dpdk ports">
> +        <column name="statistics" key="tx_retries">
> +          Total number of transmit retries on a vhost-user or 
> vhost-user-client
> +          interface.
> +        </column>
> +        <column name="statistics" key="tx_failure_drops">
> +          Total number of packets dropped because DPDP transmit API for
> +          physical/vhost ports fails to transmit the packets. This happens
> +          most likely because the transmit queue is full or has been filled
> +          up. There are other reasons as well which are unlikely to happen.
> +        </column>
> +        <column name="statistics" key="tx_mtu_exceeded_drops">
> +          Number of packets dropped due to packet length exceeding the max
> +          device MTU.
> +        </column>
> +        <column name="statistics" key="tx_qos_drops">
> +          Total number of packets dropped due to transmission rate 
> exceeding
> +          the configured egress policer rate.
> +        </column>
> +        <column name="statistics" key="rx_qos_drops">
> +          Total number of packets dropped due to reception rate exceeding
> +          the configured ingress policer rate.
> +        </column>
> +      </group>

I still think that we should not document these stats here.

I am bit unclear on where to document the stats. Initially I documented this 
stats under Documentation/topics/dpdk/bridge.rst.  Later on Ben's suggestion I 
moved this to vswitchd.xml

Thanks for your comments. I will fix the comments in my next patch v9.

Thanks & Regards,
Sriram.

>      </group>
>
>      <group title="Ingress Policing">
>


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