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

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Message-ID: <CAL+tcoAm-HitfFS+N+QRzECp5X0-X0FuGQEef5=e6cB1c_9UoA@mail.gmail.com>
Date: Sat, 21 Jun 2025 00:30:43 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Stanislav Fomichev <stfomichev@...il.com>
Cc: Jakub Kicinski <kuba@...nel.org>, 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 Fri, Jun 20, 2025 at 10:25 PM Stanislav Fomichev
<stfomichev@...il.com> 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.

Allow me to ask the similar question that you asked me before: even though I
didn't see the necessity to set the max budget for zc mode (just
because I didn't spot it happening), would it be better if we separate
both of them because it's an uAPI interface. IIUC, if the setsockopt
is set, we will not separate it any more in the future?

We can keep using the hardcoded value (32) in the zc mode like
before and __only__ touch the copy mode? Later if someone or I found
the significance of making it tunable, then another parameter of
setsockopt can be added? Does it make sense?

Thanks,
Jason

>
> 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.