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

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Date: Thu, 28 Mar 2024 03:27:40 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: Heng Qi <hengqi@...ux.alibaba.com>, netdev@...r.kernel.org,
	Jakub Kicinski <kuba@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Vladimir Oltean <vladimir.oltean@....com>,
	"David S. Miller" <davem@...emloft.net>,
	Jason Wang <jasowang@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [PATCH net-next v2 2/2] virtio-net: support dim profile
 fine-tuning

On Wed, Mar 27, 2024 at 03:45:50PM +0100, Alexander Lobakin wrote:
> From: Heng Qi <hengqi@...ux.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@...ux.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.

Yes I'd expect anything global to be const.

> 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