netdev - Re: [PATCH net-next v2] net: xsk: update tx queue consumer immediately after transmission

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Message-ID: <aFlmQ94TeHb9v-OC@mini-arch>
Date: Mon, 23 Jun 2025 07:35:47 -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 v2] net: xsk: update tx queue consumer
 immediately after transmission

On 06/23, 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 '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@...cent.com>
> ---
> 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..b9223a2a6ada 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 (xs->tx->cached_prod != xs->tx->cached_cons)

Can we use xskq_has_descs() here instead?

And still would be nice to verify this with a test. Should not be much work,
most of the things (setup/etc) are already there, so you won't have
to write everything from scratch...