netdev - Re: [PATCH net-next v3] net: xsk: introduce XDP_MAX_TX

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Message-ID: <CAL+tcoA=KQCLdthH3VXPhd-z=sieKQu_xOPgQEzxdy0Mtnycag@mail.gmail.com>
Date: Fri, 20 Jun 2025 08:17:48 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: davem@...emloft.net, edumazet@...gle.com, 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 v3] net: xsk: introduce XDP_MAX_TX_BUDGET set/getsockopt

On Thu, Jun 19, 2025 at 11:09 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Thu, 19 Jun 2025 17:04:40 +0800 Jason Xing wrote:
> > @@ -424,7 +421,9 @@ bool xsk_tx_peek_desc(struct xsk_buff_pool *pool, struct xdp_desc *desc)
> >       rcu_read_lock();
> >  again:
> >       list_for_each_entry_rcu(xs, &pool->xsk_tx_list, tx_list) {
> > -             if (xs->tx_budget_spent >= MAX_PER_SOCKET_BUDGET) {
> > +             int max_budget = READ_ONCE(xs->max_tx_budget);
> > +
> > +             if (xs->tx_budget_spent >= max_budget) {
> >                       budget_exhausted = true;
> >                       continue;
> >               }
> > @@ -779,7 +778,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> >  static int __xsk_generic_xmit(struct sock *sk)
> >  {
> >       struct xdp_sock *xs = xdp_sk(sk);
> > -     u32 max_batch = TX_BATCH_SIZE;
> > +     u32 max_budget = READ_ONCE(xs->max_tx_budget);
>
> Hm, maybe a question to Stan / Willem & other XSK experts but are these
> two max values / code paths really related? Question 2 -- is generic
> XSK a legit optimization target, legit enough to add uAPI?

I'm not an expert but my take is:
#1, I don't see the correlation actually while I don't see any reason
to use the different values for both of them.
#2, These two definitions are improvement points because whether to do
the real send is driven by calling sendto(). Enlarging a little bit of
this value could save many times of calling sendto(). As for the uAPI,
I don't know if it's worth it, sorry. If not, the previous version 2
patch (regarding per-netns policy) will be revived.

So I will leave those two questions to XSK experts as well.

>
> Jason, I think some additions to Documentation/ and quantification of
> the benefits would be needed as well.

Got it.

#1 Documentation. I would add one small section 'XDP_MAX_TX_BUDGET
setsockopt' in Documentation/networking/af_xdp.rst.

#2 quantification
It's really hard to do so mainly because of various stacks implemented
in the user-space. AF_XDP is providing a fundamental mechanism only
and its upper layer is prosperous.

Thanks,
Jason

> --
> pw-bot: cr