On Wed, Jun 10, 2020 at 06:18:32PM +0200, Eugenio Perez Martin wrote:
> On Wed, Jun 10, 2020 at 5:13 PM Michael S. Tsirkin <m...@redhat.com> wrote:
> >
> > On Wed, Jun 10, 2020 at 02:37:50PM +0200, Eugenio Perez Martin wrote:
> > > > +/* This function returns a value > 0 if a descriptor was found, or 0
> > > > if none were found.
> > > > + * A negative code is returned on error. */
> > > > +static int fetch_descs(struct vhost_virtqueue *vq)
> > > > +{
> > > > + int ret;
> > > > +
> > > > + if (unlikely(vq->first_desc >= vq->ndescs)) {
> > > > + vq->first_desc = 0;
> > > > + vq->ndescs = 0;
> > > > + }
> > > > +
> > > > + if (vq->ndescs)
> > > > + return 1;
> > > > +
> > > > + for (ret = 1;
> > > > + ret > 0 && vq->ndescs <= vhost_vq_num_batch_descs(vq);
> > > > + ret = fetch_buf(vq))
> > > > + ;
> > >
> > > (Expanding comment in V6):
> > >
> > > We get an infinite loop this way:
> > > * vq->ndescs == 0, so we call fetch_buf() here
> > > * fetch_buf gets less than vhost_vq_num_batch_descs(vq); descriptors. ret
> > > = 1
> > > * This loop calls again fetch_buf, but vq->ndescs > 0 (and avail_vq ==
> > > last_avail_vq), so it just return 1
> >
> > That's what
> > [PATCH RFC v7 08/14] fixup! vhost: use batched get_vq_desc version
> > is supposed to fix.
> >
>
> Sorry, I forgot to include that fixup.
>
> With it I don't see CPU stalls, but with that version latency has
> increased a lot and I see packet lost:
> + ping -c 5 10.200.0.1
> PING 10.200.0.1 (10.200.0.1) 56(84) bytes of data.
> >From 10.200.0.2 icmp_seq=1 Destination Host Unreachable
> >From 10.200.0.2 icmp_seq=2 Destination Host Unreachable
> >From 10.200.0.2 icmp_seq=3 Destination Host Unreachable
> 64 bytes from 10.200.0.1: icmp_seq=5 ttl=64 time=6848 ms
>
> --- 10.200.0.1 ping statistics ---
> 5 packets transmitted, 1 received, +3 errors, 80% packet loss, time 76ms
> rtt min/avg/max/mdev = 6848.316/6848.316/6848.316/0.000 ms, pipe 4
> --
>
> I cannot even use netperf.
OK so that's the bug to try to find and fix I think.
> If I modify with my proposed version:
> + ping -c 5 10.200.0.1
> PING 10.200.0.1 (10.200.0.1) 56(84) bytes of data.
> 64 bytes from 10.200.0.1: icmp_seq=1 ttl=64 time=7.07 ms
> 64 bytes from 10.200.0.1: icmp_seq=2 ttl=64 time=0.358 ms
> 64 bytes from 10.200.0.1: icmp_seq=3 ttl=64 time=5.35 ms
> 64 bytes from 10.200.0.1: icmp_seq=4 ttl=64 time=2.27 ms
> 64 bytes from 10.200.0.1: icmp_seq=5 ttl=64 time=0.426 ms
Not sure which version this is.
> [root@localhost ~]# netperf -H 10.200.0.1 -p 12865 -l 10 -t TCP_STREAM
> MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to
> 10.200.0.1 () port 0 AF_INET
> Recv Send Send
> Socket Socket Message Elapsed
> Size Size Size Time Throughput
> bytes bytes bytes secs. 10^6bits/sec
>
> 131072 16384 16384 10.01 4742.36
> [root@localhost ~]# netperf -H 10.200.0.1 -p 12865 -l 10 -t UDP_STREAM
> MIGRATED UDP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to
> 10.200.0.1 () port 0 AF_INET
> Socket Message Elapsed Messages
> Size Size Time Okay Errors Throughput
> bytes bytes secs # # 10^6bits/sec
>
> 212992 65507 10.00 9214 0 482.83
> 212992 10.00 9214 482.83
>
> I will compare with the non-batch version for reference, but the
> difference between the two is noticeable. Maybe it's worth finding a
> good value for the if() inside fetch_buf?
>
> Thanks!
>
I don't think it's performance, I think it's a bug somewhere,
e.g. maybe we corrupt a packet, or stall the queue, or
something like this.
Let's do this, I will squash the fixups and post v8 so you can bisect
and then debug cleanly.
> > --
> > MST
> >
