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

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Message-ID: <CAL+tcoCw=N0+btRMv9R=-n6XeJGhupyOd7eOyjNwiS7+Nny9wg@mail.gmail.com>
Date: Fri, 20 Jun 2025 23:42:01 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc: Stanislav Fomichev <stfomichev@...il.com>, 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] net: xsk: update tx queue consumer immdiately
 after transmission

On Fri, Jun 20, 2025 at 11:35 PM Maciej Fijalkowski
<maciej.fijalkowski@...el.com> wrote:
>
> On Fri, Jun 20, 2025 at 07:10:51AM -0700, Stanislav Fomichev 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)')?
>
> this patch updates a global state of producer whereas generic xmit loop
> updates local value. this global state is also updated within peeking
> function.

Stanislav also pointed out the normal/majority of good cases. I will
filter out the good case then.

>
> from quick look patch seems to be correct however my mind is in vacation
> mode so i'll take a second look on monday.

Thanks. I'm very sure that the line this patch introduces can be
helpful because I manually printk the delta to verify before/after
__xskq_cons_release(xs->tx); and then spot a few numbers larger than
zero during a simple test.

Thanks,
Jason

>
> >
> > 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?