Re: [PATCH net-next v2 2/2] virtio-net: support dim profile fine-tuning

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Heng Qi <hengqi@linux.alibaba.com>
Date: Wed, 27 Mar 2024 17:19:06 +0800

> Virtio-net has different types of back-end device
> implementations. In order to effectively optimize
> the dim library's gains for different device
> implementations, let's use the new interface params
> to fine-tune the profile list.

Nice idea, but

> 
> Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
> ---
>  drivers/net/virtio_net.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 52 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index e709d44..9b6c727 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -57,6 +57,16 @@
>  
>  #define VIRTNET_DRIVER_VERSION "1.0.0"
>  
> +/* This is copied from NET_DIM_RX_EQE_PROFILES in DIM library */
> +#define VIRTNET_DIM_RX_PKTS 256
> +static struct dim_cq_moder rx_eqe_conf[] = {
> +	{.usec = 1,   .pkts = VIRTNET_DIM_RX_PKTS,},
> +	{.usec = 8,   .pkts = VIRTNET_DIM_RX_PKTS,},
> +	{.usec = 64,  .pkts = VIRTNET_DIM_RX_PKTS,},
> +	{.usec = 128, .pkts = VIRTNET_DIM_RX_PKTS,},
> +	{.usec = 256, .pkts = VIRTNET_DIM_RX_PKTS,}
> +};

This is wrong.
This way you will have one global table for ALL the virtio devices in
the system, while Ethtool performs configuration on a per-netdevice basis.
What you need is to have 1 dim_cq_moder per each virtio netdevice,
embedded somewhere into its netdev_priv(). Then
virtio_dim_{rx,tx}_work() will take profiles from there, not the global
struct. The global struct can stay here as const to initialize default
per-netdevice params.

> +
>  static const unsigned long guest_offloads[] = {
>  	VIRTIO_NET_F_GUEST_TSO4,
>  	VIRTIO_NET_F_GUEST_TSO6,

Thanks,
Olek