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

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

Stanislav Fomichev wrote:
> On 06/19, Jakub Kicinski 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?
> 
> 1) xsk_tx_peek_desc is for zc case and xsk_build_skb is copy mode;
> whether we want to affect zc case given the fact that Jason seemingly
> cares about copy mode is a good question.

The two constants seem to be only tangentially created.

If there is fear that one tunable modifies both, it is simple enough
to remove the unnecessary dependency and only tune the first.
 
> 2) I do find it surprising as well. Recent busy polling patches were
> also using/targeting copy mode. But from my pow, if people use it, I see
> no reason to make it more usable.

That's a very fair question.

Jason, have you tried XDP_ZEROCOPY? It's quite plausible that that
would address your issue.

I have had a use for copy mode, but that was rather obscure. A small
packet workload where copy cost is negligible, and with copy mode it
was easy to make to reinstate flow steering in XDP to specific XSKs,
akin to

https://lore.kernel.org/netdev/65c0f032ac71a_7396029419@willemb.c.googlers.com.notmuch/

The main issue with that remained that driver copy mode also implies
the slower generic copy based Tx path, which goes through the full
dev_queue_xmit stack.