netdev - Re: [RFC PATCH net-next] net: ppp: convert to IFF_NO

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Message-ID: <CALW65jbz=3JNTx-SWk21DT4yc+oD3Dsz49z__zgDXF7TjUV7Lw@mail.gmail.com>
Date: Tue, 5 Nov 2024 22:25:23 +0800
From: Qingfang Deng <dqfext@...il.com>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-ppp@...r.kernel.org
Subject: Re: [RFC PATCH net-next] net: ppp: convert to IFF_NO_QUEUE

Hi Toke,

On Tue, Nov 5, 2024 at 8:24 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Qingfang Deng <dqfext@...il.com> writes:
>
> > When testing the parallel TX performance of a single PPPoE interface
> > over a 2.5GbE link with multiple hardware queues, the throughput could
> > not exceed 1.9Gbps, even with low CPU usage.
> >
> > This issue arises because the PPP interface is registered with a single
> > queue and a tx_queue_len of 3. This default behavior dates back to Linux
> > 2.3.13, which was suitable for slower serial ports. However, in modern
> > devices with multiple processors and hardware queues, this configuration
> > can lead to congestion.
> >
> > For PPPoE/PPTP, the lower interface should handle qdisc, so we need to
> > set IFF_NO_QUEUE.
>
> This bit makes sense - the PPPoE and PPTP channel types call through to
> the underlying network stack, and their start_xmit() ops never return
> anything other than 1 (so there's no pushback against the upper PPP
> device anyway). The same goes for the L2TP PPP channel driver.
>
> > For PPP over a serial port, we don't benefit from a qdisc with such a
> > short TX queue, so handling TX queueing in the driver and setting
> > IFF_NO_QUEUE is more effective.
>
> However, this bit is certainly not true. For the channel drivers that
> do push back (which is everything apart from the three mentioned above,
> AFAICT), we absolutely do want a qdisc to store the packets, instead of
> this arbitrary 32-packet FIFO inside the driver. Your comment about the
> short TX queue only holds for the pfifo_fast qdisc (that's the only one
> that uses the tx_queue_len for anything), anything else will do its own
> thing.
>
> (Side note: don't use pfifo_fast!)
>
> I suppose one option here could be to set the IFF_NO_QUEUE flag
> conditionally depending on whether the underlying channel driver does
> pushback against the PPP device or not (add a channel flag to indicate
> this, or something), and then call the netif_{wake,stop}_queue()
> functions conditionally depending on this. But setting the noqueue flag
> unconditionally like this patch does, is definitely not a good idea!

I agree. Then the problem becomes how to test if a PPP device is a PPPoE.
It seems like PPPoE is the only one that implements
ops->fill_forward_path, should I use that? Or is there a better way?

- Qingfang

>
> -Toke
>