Re: virtio_net: BQL?

From: Jason Wang
Date: Sun May 23 2021 - 22:53:25 EST

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,

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 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.

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).


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)
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;

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