RE: [PATCH V3 net-next] net: fec: add XDP_TX feature support

[Wei Fang]

> 
> 
> On 02/08/2023 14.33, Wei Fang wrote:
> >>> +	struct xdp_frame *xdpf = xdp_convert_buff_to_frame(xdp);
> >> XDP_TX can avoid this conversion to xdp_frame.
> >> It would requires some refactor of fec_enet_txq_xmit_frame().
> >>
> > Yes, but I'm not intend to change it, using the existing interface is enough.
> >
> >>> +	struct fec_enet_private *fep = netdev_priv(ndev);
> >>> +	struct fec_enet_priv_tx_q *txq;
> >>> +	int cpu = smp_processor_id();
> >>> +	struct netdev_queue *nq;
> >>> +	int queue, ret;
> >>> +
> >>> +	queue = fec_enet_xdp_get_tx_queue(fep, cpu);
> >>> +	txq = fep->tx_queue[queue];
> 
> Notice how TXQ gets selected based on CPU.
> Thus it will be the same for all the frames.
> 
Yes, I'll optimize it, thanks!

> >>> +	nq = netdev_get_tx_queue(fep->netdev, queue);
> >>> +
> >>> +	__netif_tx_lock(nq, cpu);
> >>
> >> It is sad that XDP_TX takes a lock for each frame.
> >>
> > Yes, but the XDP path share the queue with the kernel network stack,
> > so we need a lock here, unless there is a dedicated queue for XDP
> > path. Do you have a better solution?
> >
> 
> Yes, the solution would be to keep a stack local (or per-CPU) queue for all the
> XDP_TX frames, and send them at the xdp_do_flush_map() call site. This is
> basically what happens with xdp_do_redirect() in cpumap.c and devmap.c
> code, that have a per-CPU bulk queue and sends a bulk of packets into
> fec_enet_xdp_xmit / ndo_xdp_xmit.
> 
> I understand if you don't want to add the complexity to the driver.
> And I guess, it should be a followup patch to make sure this actually
> improves performance.
> 
Thanks, I got it. I'll optimize in a followup patch if it really improves the performance.