Re: virtio_net: BQL?

[Jason Wang]

在 2021/5/18 上午5:48, Dave Taht 写道:
> On Mon, May 17, 2021 at 1:23 PM Willem de Bruijn
> <willemdebruijn.kernel@xxxxxxxxx> wrote:
> > On Mon, May 17, 2021 at 2:44 PM Dave Taht <dave.taht@xxxxxxxxx> wrote:
> > > Not really related to this patch, but is there some reason why virtio
> > > has no support for BQL?
> > There have been a few attempts to add it over the years.
> >
> > Most recently, https://lore.kernel.org/lkml/20181205225323.12555-2-mst@xxxxxxxxxx/
> >
> > That thread has a long discussion. I think the key open issue remains
> >
> > "The tricky part is the mode switching between napi and no napi."
> Oy, vey.
>
> I didn't pay any attention to that discussion, sadly enough.
>
> It's been about that long (2018) since I paid any attention to
> bufferbloat in the cloud and my cloudy provider (linode) switched to
> using virtio when I wasn't looking. For over a year now, I'd been
> getting reports saying that comcast's pie rollout wasn't working as
> well as expected, that evenroute's implementation of sch_cake and sqm
> on inbound wasn't working right, nor pf_sense's and numerous other
> issues at Internet scale.
>
> Last week I ran a string of benchmarks against starlink's new services
> and was really aghast at what I found there, too. but the problem
> seemed deeper than in just the dishy...
>
> Without BQL, there's no backpressure for fq_codel to do its thing.
> None. My measurement servers aren't FQ-codeling
> no matter how much load I put on them. Since that qdisc is the default
> now in most linux distributions, I imagine that the bulk of the cloud
> is now behaving as erratically as linux was in 2011 with enormous
> swings in throughput and latency from GSO/TSO hitting overlarge rx/tx
> rings, [1], breaking various rate estimators in codel, pie and the tcp
> stack itself.
>
> See:
>
> http://fremont.starlink.taht.net/~d/virtio_nobql/rrul_-_evenroute_v3_server_fq_codel.png
>
> See the swings in latency there? that's symptomatic of tx/rx rings
> filling and emptying.
>
> it wasn't until I switched my measurement server temporarily over to
> sch_fq that I got a rrul result that was close to the results we used
> to get from the virtualized e1000e drivers we were using in 2014.
>
> http://fremont.starlink.taht.net/~d/virtio_nobql/rrul_-_evenroute_v3_server_fq.png
>
> While I have long supported the use of sch_fq for tcp-heavy workloads,
> it still behaves better with bql in place, and fq_codel is better for
> generic workloads... but needs bql based backpressure to kick in.
>
> [1] I really hope I'm overreacting but, um, er, could someone(s) spin
> up a new patch that does bql in some way even half right for this
> driver and help test it? I haven't built a kernel in a while.


I think it's time to obsolete skb_orphan() for virtio-net to get rid of 
a brunch of tricky codes in the current virtio-net driver.

Then we can do BQL on top.

I will prepare some patches to do this (probably with Michael's BQL patch).

Thanks


> > > On Mon, May 17, 2021 at 11:41 AM Xianting Tian
> > > <xianting.tian@xxxxxxxxxxxxxxxxx> wrote:
> > > > BUG_ON() uses unlikely in if(), which can be optimized at compile time.
> > > >
> > > > Signed-off-by: Xianting Tian <xianting.tian@xxxxxxxxxxxxxxxxx>
> > > > ---
> > > >    drivers/net/virtio_net.c | 5 ++---
> > > >    1 file changed, 2 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > > index c921ebf3ae82..212d52204884 100644
> > > > --- a/drivers/net/virtio_net.c
> > > > +++ b/drivers/net/virtio_net.c
> > > > @@ -1646,10 +1646,9 @@ static int xmit_skb(struct send_queue *sq, struct
> > > > sk_buff *skb)
> > > >          else
> > > >                  hdr = skb_vnet_hdr(skb);
> > > >
> > > > -       if (virtio_net_hdr_from_skb(skb, &hdr->hdr,
> > > > +       BUG_ON(virtio_net_hdr_from_skb(skb, &hdr->hdr,
> > > >                                      virtio_is_little_endian(vi->vdev), false,
> > > > -                                   0))
> > > > -               BUG();
> > > > +                                   0));
> > > >
> > > >          if (vi->mergeable_rx_bufs)
> > > >                  hdr->num_buffers = 0;
> > > > --
> > > > 2.17.1
> > >
> > > --
> > > Latest Podcast:
> > > https://www.linkedin.com/feed/update/urn:li:activity:6791014284936785920/
> > >
> > > Dave Täht CTO, TekLibre, LLC
>
>
> --
> Latest Podcast:
> https://www.linkedin.com/feed/update/urn:li:activity:6791014284936785920/
>
> Dave Täht CTO, TekLibre, LLC