netdev - Re: [PATCH net-next] net: xsk: update tx queue consumer immdiately after transmission

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Message-ID: <aFVr60tw3QJopcOo@mini-arch>
Date: Fri, 20 Jun 2025 07:10:51 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, bjorn@...nel.org, magnus.karlsson@...el.com,
	maciej.fijalkowski@...el.com, jonathan.lemon@...il.com,
	sdf@...ichev.me, ast@...nel.org, daniel@...earbox.net,
	hawk@...nel.org, john.fastabend@...il.com, joe@...a.to,
	willemdebruijn.kernel@...il.com, bpf@...r.kernel.org,
	netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next] net: xsk: update tx queue consumer immdiately
 after transmission

On 06/19, Jason Xing wrote:
> From: Jason Xing <kernelxing@...cent.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, the consumer of tx queue is not immdiately
> updated at the end of each sendto syscall, which leads 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.
> 
> Signed-off-by: Jason Xing <kernelxing@...cent.com>
> ---
>  net/xdp/xsk.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 7c47f665e9d1..3288ab2d67b4 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -856,6 +856,8 @@ static int __xsk_generic_xmit(struct sock *sk)
>  	}
>  
>  out:
> +	__xskq_cons_release(xs->tx);
> +
>  	if (sent_frame)
>  		if (xsk_tx_writeable(xs))
>  			sk->sk_write_space(sk);

So for the "good" case we are going to write the cons twice? From
xskq_cons_peek_desc and from here? Maybe make this __xskq_cons_release
conditional ('if (err)')?

I also wonder whether we should add a test for that? Should be easy to
verify by sending more than 32 packets. Is there a place in
tools/testing/selftests/bpf/xskxceiver.c to add that?