[ovs-dev] [PATCH 1/3] netdev-dpdk: Fix and document vhost tx retries.

[Flavio Leitner]

On Fri, Jun 21, 2019 at 02:41:56PM +0100, Kevin Traynor wrote:
> Fix minor issue of one additional retry and add
> documentation about vhost tx retries and ways to
> reduce/remove them.
> 
> Signed-off-by: Kevin Traynor <ktraynor at redhat.com>
> 
> ---
> There is a checkpatch warning that one of the libvirt
> lines in docs is a few characters too long. It is similar
> for some others in the file and I think it makes sense to
> leave it as is but can wrap if required.
> ---
>  Documentation/topics/dpdk/vhost-user.rst | 31 ++++++++++++++++++++++++
>  lib/netdev-dpdk.c                        |  2 +-
>  2 files changed, 32 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/topics/dpdk/vhost-user.rst b/Documentation/topics/dpdk/vhost-user.rst
> index f7b4b338e..d1682fd05 100644
> --- a/Documentation/topics/dpdk/vhost-user.rst
> +++ b/Documentation/topics/dpdk/vhost-user.rst
> @@ -76,4 +76,35 @@ mode ports require QEMU version 2.7.  Ports of type vhost-user are currently
>  deprecated and will be removed in a future release.
>  
> +vhost tx retries
> +~~~~~~~~~~~~~~~~
> +
> +When sending a batch of packets (max is 32) to a vhost-user or

Maybe use the actual define name (NETDEV_MAX_BURST) since it might change?

> +vhost-user-client interface, it may happen that some but not all of the packets
> +in the batch are able to be sent to the guest. This is often because there is
> +not enough free descriptors in the virtqueue for all the packets to be sent.
> +In this case there will be a retry, with a default maximum of 8 occurring. If
> +at any time no packets can be sent, it may mean the guest is not accepting
> +packets, so there are no (more) retries.
> +
> +Tx Retries may be reduced or removed by some external configuration, such

s/removed/even avoided/ ?

> +as increasing the virtqueue size through the ``rx_queue_size`` parameter in
> +libvirt::
> +
> +  <interface type='vhostuser'>
> +      <mac address='56:48:4f:53:54:01'/>
> +      <source type='unix' path='/tmp/dpdkvhostclient0' mode='server'/>
> +      <model type='virtio'/>
> +      <driver name='vhost' rx_queue_size='1024' tx_queue_size='1024'/>
> +      <address type='pci' domain='0x0000' bus='0x00' slot='0x10' function='0x0'/>
> +  </interface>

Do we need to worry about the qemu version?

> +
> +The guest application will also need need to provide enough descriptors. For
> +example with ``testpmd`` the command line argument can be used::
> +
> + --rxd=1024 --txd=1024
> +
> +The guest should also have sufficient cores dedicated for consuming and
> +processing packets at the required rate.
> +
>  .. _dpdk-vhost-user:
>  
> diff --git a/lib/netdev-dpdk.c b/lib/netdev-dpdk.c
> index 4856c56aa..0f3b9c6f4 100644
> --- a/lib/netdev-dpdk.c
> +++ b/lib/netdev-dpdk.c
> @@ -2353,5 +2353,5 @@ __netdev_dpdk_vhost_send(struct netdev *netdev, int qid,
>              break;
>          }
> -    } while (cnt && (retries++ <= VHOST_ENQ_RETRY_NUM));
> +    } while (cnt && (retries++ < VHOST_ENQ_RETRY_NUM));

You're removing the packet's last hope to reach home, are you sure? :-)
fbl

>  
>      rte_spinlock_unlock(&dev->tx_q[qid].tx_lock);
> -- 
> 2.20.1
> 
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