Hi Toke,
On Tue, Nov 5, 2024 at 8:24 PM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
>
> Qingfang Deng <dqfext@xxxxxxxxx> 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
>
