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 Fri, Jun 20, 2025 at 9:50 PM Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
> >
> > 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.
> 
> Actually I mentioned it in the commit message. One of advantages is to
> contribute to less frequencies of sendto() and overall higher
> transmission speed.

Understood. It is just informative to have more quantitative data.
What value worked for you.
 
> >
> > 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?
> 
> One naive question from me is why the number of packets to be sent is
> definitely required to be limited within a small number by default?
> Let me set tcp as an example, a simple sendmsg call will not be
> stopped because of the hardcoded limitation.
> 
> For one application I saw, I suggested using 128 because I saw two
> limitations without changing any default configuration: 1)
> XDP_MAX_TX_BUDGET, 2) socket sndbuf which is 212992 decided by
> net.core.wmem_default. As to XDP_MAX_TX_BUDGET, the scenario behind
> this was I counted how many desc are transmitted to the driver at one
> time of sendto() based on [1] patch and then I calculated the
> possibility of hitting the upper bound. Finally I chose 128 as a
> suitable value because 1) it covers most of the cases, 2) a higher
> number would not bring evident results.
> 
> [1]: https://lore.kernel.org/all/20250619093641.70700-1-kerneljasonxing@gmail.com/

This is indeed helpful context.

Another limiting factor is the XSK TX queue length?

So even if a user passes UINT_MAX, nothing terrible will happen.

Still, it is better to not accept obviously bad input to begin with.

Normal packet processing loops give up control after tens or maybe
a few hundred packets at a time.