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

[Jesper Dangaard Brouer <hawk@kernel.org>]

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.

>>> +	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.

--Jesper