[ovs-dev] [PATCH] dpif-netdev: Check for PKT_RX_RSS_HASH flag.

[Pravin Shelar]

On Tue, Jun 16, 2015 at 11:39 AM, Daniele Di Proietto
<diproiettod at vmware.com> wrote:
> DPDK mbufs contain a valid RSS hash only if PKT_RX_RSS_HASH is
> set in 'ol_flags'.  Otherwise the hash is garbage and doesn't
> relate to the packet.
>
> This fixes an issue with vhost, which, being a virtual NIC, doesn't
> compute the hash.
>
> Unfortunately the ixgbe vPMD doesn't set the PKT_RX_RSS_HASH, forcing
> OVS to compute an hash is software.  This has a significant impact on
> performance (-30% throughput in a single flow setup) which can be
> mitigated in the CPU supports crc32c instructions.
>
> Reported-by: Dongjun <dongj at dtdream.com>
> Suggested-by: Flavio Leitner <fbl at sysclose.org>
> Signed-off-by: Daniele Di Proietto <diproiettod at vmware.com>
> ---
>  lib/dp-packet.h   | 11 +++++++++++
>  lib/dpif-netdev.c |  2 +-
>  2 files changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/lib/dp-packet.h b/lib/dp-packet.h
> index e4c2593..6840750 100644
> --- a/lib/dp-packet.h
> +++ b/lib/dp-packet.h
> @@ -529,11 +529,22 @@ dp_packet_set_rss_hash(struct dp_packet *p, uint32_t hash)
>  {
>  #ifdef DPDK_NETDEV
>      p->mbuf.hash.rss = hash;
> +    p->mbuf.ol_flags |= PKT_RX_RSS_HASH;
>  #else
>      p->rss_hash = hash;
>  #endif
>  }
>
> +static inline bool
> +dp_packet_rss_valid(struct dp_packet *p)
> +{
> +#ifdef DPDK_NETDEV
> +    return p->mbuf.ol_flags & PKT_RX_RSS_HASH;
> +#else
> +    return true;
> +#endif
> +}
> +
>  #ifdef  __cplusplus
>  }
>  #endif
> diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
> index f13169c..c4a4b3a 100644
> --- a/lib/dpif-netdev.c
> +++ b/lib/dpif-netdev.c
> @@ -3036,7 +3036,7 @@ dpif_netdev_packet_get_rss_hash(struct dp_packet *packet,
>  {
>      uint32_t hash, recirc_depth;
>
> -    hash = dp_packet_get_rss_hash(packet);
> +    hash = dp_packet_rss_valid(packet) ? dp_packet_get_rss_hash(packet) : 0;
>      if (OVS_UNLIKELY(!hash)) {
We can just check for PKT_RX_RSS_HASH, it saves one check for zero
hash. Is zero hash not valid hash value?

>          hash = miniflow_hash_5tuple(mf, 0);
>          dp_packet_set_rss_hash(packet, hash);
> --
> 2.1.4
>
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