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

[Stanislav Fomichev <stfomichev@gmail.com>]

On 06/20, Jason Xing wrote:
> On Fri, Jun 20, 2025 at 10:10 PM Stanislav Fomichev
> <stfomichev@gmail.com> wrote:
> >
> > On 06/19, Jason Xing wrote:
> > > From: Jason Xing <kernelxing@tencent.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@tencent.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.

My suspicion is that the same issue exists for the zc case. So would
be nice to test it and fix it as well :-p