Re: [PATCH net-next] virtio_net: Add TX stop and wake counters

[Jason Wang <jasowang@redhat.com>]

On Sat, Feb 3, 2024 at 12:01 AM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Fri, 2 Feb 2024 14:52:59 +0800 Jason Xing wrote:
> > > Can you say more? I'm curious what's your use case.
> >
> > I'm not working at Nvidia, so my point of view may differ from theirs.
> > From what I can tell is that those two counters help me narrow down
> > the range if I have to diagnose/debug some issues.
>
> right, i'm asking to collect useful debugging tricks, nothing against
> the patch itself :)
>
> > 1) I sometimes notice that if some irq is held too long (say, one
> > simple case: output of printk printed to the console), those two
> > counters can reflect the issue.
> > 2) Similarly in virtio net, recently I traced such counters the
> > current kernel does not have and it turned out that one of the output
> > queues in the backend behaves badly.
> > ...
> >
> > Stop/wake queue counters may not show directly the root cause of the
> > issue, but help us 'guess' to some extent.
>
> I'm surprised you say you can detect stall-related issues with this.
> I guess virtio doesn't have BQL support, which makes it special.

Yes, virtio-net has a legacy orphan mode, this is something that needs
to be dropped in the future. This would make BQL much more easier to
be implemented.

> Normal HW drivers with BQL almost never stop the queue by themselves.
> I mean - if they do, and BQL is active, then the system is probably
> misconfigured (queue is too short). This is what we use at Meta to
> detect stalls in drivers with BQL:
>
> https://lore.kernel.org/all/20240131102150.728960-3-leitao@debian.org/
>
> Daniel, I think this may be a good enough excuse to add per-queue stats
> to the netdev genl family, if you're up for that. LMK if you want more
> info, otherwise I guess ethtool -S is fine for now.
>

Thanks