Messages in this thread |  | | Subject | Re: virtio_net: BQL? | From | Jason Wang <> | Date | Mon, 24 May 2021 10:53:08 +0800 |
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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@gmail.com> wrote: >> On Mon, May 17, 2021 at 2:44 PM Dave Taht <dave.taht@gmail.com> 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@redhat.com/ >> >> 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@linux.alibaba.com> wrote: >>>> BUG_ON() uses unlikely in if(), which can be optimized at compile time. >>>> >>>> Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com> >>>> --- >>>> 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 >
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