Re: [dpdk-stable] [PATCH v2] vhost: fix virtio_net false sharing

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 03/23/2017 04:44 PM, Kevin Traynor wrote:
> The broadcast_rarp field in the virtio_net struct is checked in the
> dequeue datapath regardless of whether descriptors are available or not.
>
> As it is checked with cmpset leading to a write, false sharing on the
> virtio_net struct can happen between enqueue and dequeue datapaths
> regardless of whether a RARP is requested. In OVS, the issue can cause
> a uni-directional performance drop of up to 15%.
>
> Fix that by only performing the cmpset if a read of broadcast_rarp
> indicates that the cmpset is likely to succeed.
>
> Fixes: a66bcad32240 ("vhost: arrange struct fields for better cache sharing")
> Cc: stable@dpdk.org
>
> Signed-off-by: Kevin Traynor <ktraynor@redhat.com>
> ---
>
> V2:
> Change from fixing by moving broadcast_rarp location in virtio_net struct
> to performing a read before cmpset.
>
>  lib/librte_vhost/virtio_net.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)

Nice!
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>

I'll try to benchmark it with testpmd only to see if we measure the
same gain without OVS.

Thanks,
Maxime