Re: [PATCH net-next v3 1/2] net: xsk: update tx queue consumer immediately after transmission

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Wed, Jun 25, 2025 at 06:10:13PM +0800, Jason Xing wrote:
> From: Jason Xing <kernelxing@tencent.com>
> 
> For afxdp, the return value of sendto() syscall doesn't reflect how many
> descs handled in the kernel. One of use cases is that when user-space
> application tries to know the number of transmitted skbs and then decides
> if it continues to send, say, is it stopped due to max tx budget?
> 
> The following formular can be used after sending to learn how many
> skbs/descs the kernel takes care of:
> 
>   tx_queue.consumers_before - tx_queue.consumers_after
> 
> Prior to the current patch, in non-zc mode, the consumer of tx queue is
> not immediately updated at the end of each sendto syscall when error
> occurs, which leads to the consumer value out-of-dated from the perspective
> of user space. So this patch requires store operation to pass the cached
> value to the shared value to handle the problem.
> 
> More than those explicit errors appearing in the while() loop in
> __xsk_generic_xmit(), there are a few possible error cases that might
> be neglected in the following call trace:
> __xsk_generic_xmit()
>     xskq_cons_peek_desc()
>         xskq_cons_read_desc()
> 	    xskq_cons_is_valid_desc()
> It will also cause the premature exit in the while() loop even if not
> all the descs are consumed.
> 
> Based on the above analysis, using 'cached_prod != cached_cons' could
> cover all the possible cases because it represents there are remaining
> descs that are not handled and cached_cons are not updated to the global
> state of consumer at this time.
> 
> Signed-off-by: Jason Xing <kernelxing@tencent.com>
> ---
> v3
> Link: https://lore.kernel.org/all/20250623073129.23290-1-kerneljasonxing@gmail.com/
> 1. use xskq_has_descs helper.
> 2. add selftest
> 
> V2
> Link: https://lore.kernel.org/all/20250619093641.70700-1-kerneljasonxing@gmail.com/
> 1. filter out those good cases because only those that return error need
> updates.
> Side note:
> 1. in non-batched zero copy mode, at the end of every caller of
> xsk_tx_peek_desc(), there is always a xsk_tx_release() function that used
> to update the local consumer to the global state of consumer. So for the
> zero copy mode, no need to change at all.
> 2. Actually I have no strong preference between v1 (see the above link)
> and v2 because smp_store_release() shouldn't cause side effect.
> Considering the exactitude of writing code, v2 is a more preferable
> one.
> ---
>  net/xdp/xsk.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 5542675dffa9..ab6351b24ac8 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -856,6 +856,9 @@ static int __xsk_generic_xmit(struct sock *sk)
>  	}
>  
>  out:
> +	if (xskq_has_descs(xs->tx))
> +		__xskq_cons_release(xs->tx);
> +
>  	if (sent_frame)
>  		if (xsk_tx_writeable(xs))
>  			sk->sk_write_space(sk);

Hi Jason,
IMHO below should be enough to address the issue:

	if (sent_frame) {
		__xskq_cons_release(xs->tx);
		if (xsk_tx_writeable(xs))
			sk->sk_write_space(sk);
	}

which basically is what xsk_tx_release() does for each tx socket in list.
zc drivers call it whenever there was a single descriptor produced to HW
ring. So should we on generic xmit side, based on @sent_frame.

We could even wrap these 3 lines onto internal function, say
__xsk_tx_release() and use it in xsk_tx_release() as well.

> -- 
> 2.41.3
>