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

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Date: Wed, 27 Mar 2024 17:32:58 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: Heng Qi <hengqi@...ux.alibaba.com>, <netdev@...r.kernel.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>, "Michael S. Tsirkin" <mst@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, vadim.fedorenko@...ux.dev
Subject: Re: [PATCH net-next v2 2/2] virtio-net: support dim profile
 fine-tuning

On Wed, 27 Mar 2024 15:45:50 +0100 Alexander Lobakin wrote:
> > +/* 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.

I've been wondering lately if adaptive IRQ moderation isn't exactly
the kind of heuristic we would be best off deferring to BPF.
I have done 0 experiments -- are the thresholds enough
or do more interesting algos come to mind for anyone?