linux-kernel - Re: [PATCH] net: Revert tx queue length on partial failure in dev_qdisc_change_tx_queue

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Message-ID: <aIPDLjCUHCf+iI1O@pop-os.localdomain>
Date: Fri, 25 Jul 2025 10:47:26 -0700
From: Cong Wang <xiyou.wangcong@...il.com>
To: Suchit K <suchitkarunakaran@...il.com>
Cc: Eric Dumazet <edumazet@...gle.com>, davem@...emloft.net,
	kuba@...nel.org, pabeni@...hat.com, horms@...nel.org,
	jhs@...atatu.com, jiri@...nulli.us, sdf@...ichev.me,
	kuniyu@...gle.com, aleksander.lobakin@...el.com,
	netdev@...r.kernel.org, skhan@...uxfoundation.org,
	linux-kernel-mentees@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: Revert tx queue length on partial failure in
 dev_qdisc_change_tx_queue_len()

Hi Suchit,

On Wed, Jul 23, 2025 at 11:47:09PM +0530, Suchit K wrote:
> >
> > WRITE_ONCE() is missing.
> >
> > > +               while (i >= 0) {
> > > +                       qdisc_change_tx_queue_len(dev, &dev->_tx[i]);
> >
> > What happens if one of these calls fails ?
> >
> > I think a fix will be more complicated...
> 
> Hi Eric,
> Given that pfifo_fast_change_tx_queue_len is currently the only
> implementation of change_tx_queue_len, would it be reasonable to
> handle partial failures solely within pfifo_fast_change_tx_queue_len
> (which in turn leads to skb_array_resize_multiple_bh)? In other words,
> is it sufficient to modify only the underlying low level
> implementation of pfifo_fast_change_tx_queue_len for partial failures,
> given that it's the sole implementation of change_tx_queue_len?

Thanks for your patch.

As you noticed it is tricky to handle the failure elegantly here, which
was also the reason why I didn't do it. Did you observe any real issue?

To answer your question above: I am not sure if we can do it in pfifo
fast implementation since struct netdev_queue is not explicitly exposed to
the lower Qdisc.

On the other hand, although dev_qdisc_change_tx_queue_len() is generic,
it is only called for this very specific code path, so changing it won't
impact other code paths, IMHO.

Regards,
Cong Wang