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

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Message-ID: <CAL+tcoBLAMWXjBz9BYb84MmJxGztHFOLbqZL-YX0s7ykBjNT7g@mail.gmail.com>
Date: Fri, 20 Jun 2025 23:25:15 +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] net: xsk: update tx queue consumer immdiately
 after transmission

On Fri, Jun 20, 2025 at 10:10 PM Stanislav Fomichev
<stfomichev@...il.com> wrote:
>
> 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)')?

One unlikely exception:
xskq_cons_peek_desc()->xskq_cons_read_desc()->xskq_cons_is_valid_desc()->return
false;
?

There are still two possible 'return false' in xskq_cons_peek_desc()
while so far I didn't spot a single one happening.

Admittedly, your suggestion covers the majority of normal good ones. I
can adjust it as you said.

>
> 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?

Well, sorry, if it's not required, please don't force me to do so :S
The patch is only one simple update of the consumer that is shared
between user-space and kernel.

Thanks,
Jason