On Fri, Jun 20, 2025 at 11:35 PM Maciej Fijalkowski
<maciej.fijalkowski@xxxxxxxxx> wrote:
>
> On Fri, Jun 20, 2025 at 07:10:51AM -0700, Stanislav Fomichev wrote:
> > On 06/19, Jason Xing wrote:
> > > From: Jason Xing <kernelxing@xxxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxxx>
> > > ---
> > > 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?
