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

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Message-ID: <CAL+tcoDHe=bMESuJe-zVXyU6r7QHmZ3w6CK0g=N6Dqvf8ONh3g@mail.gmail.com>
Date: Mon, 23 Jun 2025 13:31:45 +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 11:58 PM Stanislav Fomichev
<stfomichev@...il.com> wrote:
>
> On 06/20, Jason Xing wrote:
> > 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.
>
> 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

After digging into the logic around xsk_tx_peek_desc(), I can say that
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 :)

I will soon send the v2 with the 'if (error)' statement in the
__xsk_generic_xmit().

Thanks,
Jason