Migration with "--p2p --tunnelled" hanging in v6.9.0

[Daniel P. Berrangé]

On Wed, Nov 25, 2020 at 02:33:44PM +0100, Christian Ehrhardt wrote:
> On Wed, Nov 25, 2020 at 1:38 PM Daniel P. Berrangé <berrange at redhat.com> wrote:
> >
> > On Wed, Nov 25, 2020 at 01:28:09PM +0100, Christian Ehrhardt wrote:
> > > On Wed, Nov 25, 2020 at 10:55 AM Christian Ehrhardt
> > > <christian.ehrhardt at canonical.com> wrote:
> > > >
> > > > On Tue, Nov 24, 2020 at 4:30 PM Peter Krempa <pkrempa at redhat.com> wrote:
> > > > >
> > > > > On Tue, Nov 24, 2020 at 16:05:53 +0100, Christian Ehrhardt wrote:
> > > > > > Hi,
> > > > >
> > > > > [...]
> > > >
> > > > BTW to reduce the scope what to think about - I have rebuilt 6.8 as
> > > > well it works.
> > > > Thereby I can confirm that the offending change should be in between
> > > > 6.8.0 -> 6.9.0.
> > >
> > > I was able to get this working in git bisect builds from git between
> > > v6.8 / v6.9.
> > > I identified the following offending commit:
> > >   7d959c30  rpc: Fix virt-ssh-helper detection
> > >
> > > Ok that makes a bit of sense, first we had in 6.8
> > >   f8ec7c84 rpc: use new virt-ssh-helper binary for remote tunnelling
> > > That makes it related to tunneling which matches our broken use-case.
> > >
> > > The identified commit "7d959c30 rpc: Fix virt-ssh-helper detection" might
> > > finally really enable the new helper and that is then broken?
> > >
> > > With that knowledge I was able to confirm that it really is the native mode
> > >
> > > $ virsh migrate --unsafe --live --p2p --tunnelled h-migr-test
> > > qemu+ssh://testkvm-hirsute-to/system?proxy=netcat
> > > <works>
> > > $ virsh migrate --unsafe --live --p2p --tunnelled h-migr-test
> > > qemu+ssh://testkvm-hirsute-to/system?proxy=native
> > > <hangs>
> > >
> > > I recently discussed with Andrea if we'd need apparmor rules for
> > > virt-ssh-helper,
> > > but there are no denials nor libvirt log entries related to virt-ssh-helper.
> > > But we don't need such rules since it is spawned on the ssh login and
> > > not under libvirtd itself.
> > >
> > > PS output of the hanging receiving virt-ssh-helper (looks not too unhappy):
> > > Source:
> > > 4     0   41305       1  20   0 1627796 23360 poll_s Ssl ?
> > > 0:05 /usr/sbin/libvirtd
> > > 0     0   41523   41305  20   0   9272  4984 poll_s S    ?
> > > 0:02  \_ ssh -T -e none -- testkvm-hirsute-to sh -c 'virt-ssh-helper
> > > 'qemu:///system''
> > > Target
> > > 4     0     213       1  20   0  13276  4132 poll_s Ss   ?
> > > 0:00 sshd: /usr/sbin/sshd -D [listener] 0 of 250-500 startups
> > > 4     0   35148     213  20   0  19048 11320 poll_s Ss   ?
> > > 0:02  \_ sshd: root at notty
> > > 4     0   35206   35148  20   0   2584   544 do_wai Ss   ?
> > > 0:00      \_ sh -c virt-ssh-helper qemu:///system
> > > 0     0   35207   35206  20   0  81348 26684 -      R    ?
> > > 0:34          \_ virt-ssh-helper qemu:///system
> > >
> > > I've looked at it with strace [1] and gdb for backtraces [2] - it is
> > > not dead or stuck and keeps working.
> > > Could it be just so slow that it appears to hang until it times out?
> > > Or is the event mechanism having issues and it wakes up too rarely?
> >
> > Lets take migration out of the picture. What if you simply do
> >
> >     virsh -c qemu+ssh://testkvm-hirsute-to/system?proxy=native  list
> >
> > does that work ?
> 
> Yes it does, no hang and proper results

Ok, so that shows  virt-ssh-helper is not completely broken at least.

Makes me think there is possibly something related to streams code
that causes the issue.

You might try the virsh "console" or "vol-upload" commands to test
the streams stuff in isolation. If that also works, then the problem
is specific to migration, and we'll probably wnt to colllect debug
level logs from src+dst hosts.

Regards,
Daniel
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