netdev - Re: [PATCH net-next v4 1/2] net: xsk: update tx queue consumer immediately after transmission

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Message-ID: <aGJ5t2hvN9wX+vxh@boxer>
Date: Mon, 30 Jun 2025 13:49:11 +0200
From: Maciej Fijalkowski <maciej.fijalkowski@...el.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>,
	<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 v4 1/2] net: xsk: update tx queue consumer
 immediately after transmission

On Fri, Jun 27, 2025 at 04:57:44PM +0800, 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, 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 @sent_frame could cover all the possible
> cases where it might lead to out-of-dated global state of consumer after
> finishing __xsk_generic_xmit().
> 
> The patch also adds a common helper __xsk_tx_release() to keep align
> with the zc mode usage in xsk_tx_release().
> 
> Signed-off-by: Jason Xing <kernelxing@...cent.com>

Acked-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>

> ---
> v4
> Link: https://lore.kernel.org/all/20250625101014.45066-1-kerneljasonxing@gmail.com/
> 1. use the common helper
> 2. keep align with the zc mode usage in xsk_tx_release()
> 
> 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 | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 72c000c0ae5f..bd61b0bc9c24 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -300,6 +300,13 @@ static bool xsk_tx_writeable(struct xdp_sock *xs)
>  	return true;
>  }
>  
> +static void __xsk_tx_release(struct xdp_sock *xs)
> +{
> +	__xskq_cons_release(xs->tx);
> +	if (xsk_tx_writeable(xs))
> +		xs->sk.sk_write_space(&xs->sk);
> +}
> +
>  static bool xsk_is_bound(struct xdp_sock *xs)
>  {
>  	if (READ_ONCE(xs->state) == XSK_BOUND) {
> @@ -407,11 +414,8 @@ void xsk_tx_release(struct xsk_buff_pool *pool)
>  	struct xdp_sock *xs;
>  
>  	rcu_read_lock();
> -	list_for_each_entry_rcu(xs, &pool->xsk_tx_list, tx_list) {
> -		__xskq_cons_release(xs->tx);
> -		if (xsk_tx_writeable(xs))
> -			xs->sk.sk_write_space(&xs->sk);
> -	}
> +	list_for_each_entry_rcu(xs, &pool->xsk_tx_list, tx_list)
> +		__xsk_tx_release(xs);
>  	rcu_read_unlock();
>  }
>  EXPORT_SYMBOL(xsk_tx_release);
> @@ -858,8 +862,7 @@ static int __xsk_generic_xmit(struct sock *sk)
>  
>  out:
>  	if (sent_frame)
> -		if (xsk_tx_writeable(xs))
> -			sk->sk_write_space(sk);
> +		__xsk_tx_release(xs);
>  
>  	mutex_unlock(&xs->mutex);
>  	return err;
> -- 
> 2.41.3
>