Re: [patch net-next] virtio_net: add support for Byte Queue Limits

Jason Wang
2024-06-07T06:25:35Z

On Thu, Jun 6, 2024 at 9:45 PM Jiri Pirko <jiri@resnulli.us> wrote:
>
> Thu, Jun 06, 2024 at 09:56:50AM CEST, jasowang@redhat.com wrote:
> >On Thu, Jun 6, 2024 at 2:05 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >>
> >> On Thu, Jun 06, 2024 at 12:25:15PM +0800, Jason Wang wrote:
> >> > > If the codes of orphan mode don't have an impact when you enable
> >> > > napi_tx mode, please keep it if you can.
> >> >
> >> > For example, it complicates BQL implementation.
> >> >
> >> > Thanks
> >>
> >> I very much doubt sending interrupts to a VM can
> >> *on all benchmarks* compete with not sending interrupts.
> >
> >It should not differ too much from the physical NIC. We can have one
> >more round of benchmarks to see the difference.
> >
> >But if NAPI mode needs to win all of the benchmarks in order to get
> >rid of orphan, that would be very difficult. Considering various bugs
> >will be fixed by dropping skb_orphan(), it would be sufficient if most
> >of the benchmark doesn't show obvious differences.
> >
> >Looking at git history, there're commits that removes skb_orphan(), for example:
> >
> >commit 8112ec3b8722680251aecdcc23dfd81aa7af6340
> >Author: Eric Dumazet <edumazet@google.com>
> >Date: Fri Sep 28 07:53:26 2012 +0000
> >
> > mlx4: dont orphan skbs in mlx4_en_xmit()
> >
> > After commit e22979d96a55d (mlx4_en: Moving to Interrupts for TX
> > completions) we no longer need to orphan skbs in mlx4_en_xmit()
> > since skb wont stay a long time in TX ring before their release.
> >
> > Orphaning skbs in ndo_start_xmit() should be avoided as much as
> > possible, since it breaks TCP Small Queue or other flow control
> > mechanisms (per socket limits)
> >
> > Signed-off-by: Eric Dumazet <edumazet@google.com>
> > Acked-by: Yevgeny Petrilin <yevgenyp@mellanox.com>
> > Cc: Or Gerlitz <ogerlitz@mellanox.com>
> > Signed-off-by: David S. Miller <davem@davemloft.net>
> >
> >>
> >> So yea, it's great if napi and hardware are advanced enough
> >> that the default can be changed, since this way virtio
> >> is closer to a regular nic and more or standard
> >> infrastructure can be used.
> >>
> >> But dropping it will go against *no breaking userspace* rule.
> >> Complicated? Tough.
> >
> >I don't know what kind of userspace is broken by this. Or why it is
> >not broken since the day we enable NAPI mode by default.
>
> There is a module option that explicitly allows user to set
> napi_tx=false
> or
> napi_weight=0
>
> So if you remove this option or ignore it, both breaks the user
> expectation.

We can keep them, but I wonder what's the expectation of the user
here? The only thing so far I can imagine is the performance
difference.

> I personally would vote for this breakage. To carry ancient
> things like this one forever does not make sense to me.

Exactly.

> While at it,
> let's remove all virtio net module params. Thoughts?

I tend to

1) drop the orphan mode, but we can have some benchmarks first
2) keep the module parameters

Thanks

>
>
>
> >
> >Thanks
> >
> >>
> >> --
> >> MST
> >>
> >
>