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

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Message-ID: <CAL+tcoAEQgefeBWeXQmkvLTnbojDfSWbPtJKX3G_=OYNDNdfLg@mail.gmail.com>
Date: Tue, 24 Jun 2025 20:53:47 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Stanislav Fomichev <stfomichev@...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 Mon, Jun 23, 2025 at 10:35 PM Stanislav Fomichev
<stfomichev@...il.com> wrote:
>
> 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?

Sure, thanks.

>
> 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...

Ah, finally I managed to make it work but a bit ugly... Let me try if
I can find a
better way to write the test patch.

Thanks,
Jason