netdev - Re: [PATCH net-next 2/5] ixgbe: xsk: resolve the underflow of budget in ixgbe_xmit

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Message-ID: <CAL+tcoBTejWSmv6XTpFqvgy4Qk4c39-OJm8Vqcwraa0cAST=sw@mail.gmail.com>
Date: Fri, 25 Jul 2025 07:18:11 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Tony Nguyen <anthony.l.nguyen@...el.com>
Cc: przemyslaw.kitszel@...el.com, andrew+netdev@...n.ch, davem@...emloft.net, 
	edumazet@...gle.com, kuba@...nel.org, 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, 
	bpf@...r.kernel.org, intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org, 
	Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next 2/5] ixgbe: xsk: resolve the underflow of budget
 in ixgbe_xmit_zc

Hi Tony,

On Fri, Jul 25, 2025 at 4:21 AM Tony Nguyen <anthony.l.nguyen@...el.com> wrote:
>
>
>
> On 7/20/2025 2:11 AM, Jason Xing wrote:
> > From: Jason Xing <kernelxing@...cent.com>
> >
> > Resolve the budget underflow which leads to returning true in ixgbe_xmit_zc
> > even when the budget of descs are thoroughly consumed.
> >
> > Before this patch, when the budget is decreased to zero and finishes
> > sending the last allowed desc in ixgbe_xmit_zc, it will always turn back
> > and enter into the while() statement to see if it should keep processing
> > packets, but in the meantime it unexpectedly decreases the value again to
> > 'unsigned int (0--)', namely, UINT_MAX. Finally, the ixgbe_xmit_zc returns
> > true, showing 'we complete cleaning the budget'. That also means
> > 'clean_complete = true' in ixgbe_poll.
> >
> > The true theory behind this is if that budget number of descs are consumed,
> > it implies that we might have more descs to be done. So we should return
> > false in ixgbe_xmit_zc to tell napi poll to find another chance to start
> > polling to handle the rest of descs. On the contrary, returning true here
> > means job done and we know we finish all the possible descs this time and
> > we don't intend to start a new napi poll.
> >
> > It is apparently against our expectations. Please also see how
> > ixgbe_clean_tx_irq() handles the problem: it uses do..while() statement
> > to make sure the budget can be decreased to zero at most and the underflow
> > never happens.
> >
> > Fixes: 8221c5eba8c1 ("ixgbe: add AF_XDP zero-copy Tx support")
>
> Hi Jason,
>
> Seems like this one should be split off and go to iwl-net/net like the
> other similar ones [1]? Also, some of the updates made to the other
> series apply here as well?

The other three patches are built on top of this one. If without the
patch, the whole series will be warned because of build failure. I was
thinking we could backport this patch that will be backported to the
net branch after the whole series goes into the net-next branch.

Or you expect me to cook four patches without this one first so that
you could easily cherry pick this one then without building conflict?

>
> Thanks,
> Tony
>
> [1]
> https://lore.kernel.org/netdev/20250723142327.85187-1-kerneljasonxing@gmail.com/

Regarding this one, should I send a v4 version with the current patch
included? And target [iwl-net/net] explicitly as well?

I'm not sure if I follow you. Could you instruct me on how to proceed
next in detail?

Thanks,
Jason