Re: [PATCH net-next v3] net: xsk: introduce XDP_MAX_TX_BUDGET set/getsockopt

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Jason Xing wrote:
> On Thu, Jun 19, 2025 at 11:09 PM Jakub Kicinski <kuba@kernel.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.

You're proposing the code change, so I think it's on you to make
this argument?
 
> #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.

I think it's a hard sell to argue adding a tunable, if no plausible
recommendation can be given on how the tunable is to be used.

It's not necessary, and most cases infeasible, to give a heuristic
that fits all possible users. But at a minimum the one workload that
prompted the patch. What value do you set it to and how did you
arrive at that number?