Bug#784688: Thousands of "xen:balloon: Cannot add additional memory (-17) messages" despite dom0 ballooning disabled

[Ian Campbell <ijc@debian.org>]

On Mon, 2016-01-25 at 20:36 +0200, KSB wrote:
> > Do you have a package version which you know to be good? How confident
> > are
> > you that it is ok (sometimes the problem is intermittent)?
> > 
> > Lastly, is there any chance you upgraded the Xen packages at the same
> > time?
> > I'm starting to wonder if maybe this is not a kernel issue.
> > 
> Sorry, but there is chance, sadly.

This is actually useful, because it shows that the issue occurs even with
Xen 4.6, which I think rules out a Xen side issue (otherwise we'd have had
lots more reports from 4.4 through to 4.6) and points to a kernel side
issue somewhere.

> But I checked logs more thoroughly and found it even on more recent
> kernels:
> 1) Lot of messages on 3.14-2-amd64 with xen-4.6, 13 domU's.

Just to be clear, "Lots" here means "hundreds or thousands"? I think it is
expected to see one or two around the time a VM is started or stopped, so
with 13 domUs a couple of dozen messages wouldn't seem out of line to me.

> 2) 4.3.0-1-amd64 xen-4.6, only two messages shortly after boot, only 1 
> domU running:
> [   12.473778] xen:balloon: Cannot add additional memory (-17)
> [   21.673298] xen:balloon: Cannot add additional memory (-17)
> uptime 17 days.
> 
> Previous on same machine was 4.2.0-1-amd64 with more (-17)'s

Was it running xen-4.6 when it was running 4.2.0 or was that also older?

Also 4.2.0-1-amd64 is the ABI, not the package version. The package
versions is either in dpkg or you can find it in /proc/version:

Linux version 4.1.0-2-amd64 (debian-kernel@lists.debian.org) (gcc version 4.9.3 (Debian 4.9.3-3) ) #1 SMP Debian 4.1.6-1 (2015-08-23)
              ^^^ABI^^^^^^^                                                                               ^^^VERSION^^^^

> 3) 4.3.0-1-amd64, one month, several reboots, average 4 domU's, and no 
> messages

Any idea which Xen?

> 4) 3.16.0-4-amd64, xen-4.1, 22 domU's, uptime 188 days, in last month I 
> see only
> Jan 7 14:12:08
> Jan 7 14:12:08
> Jan 7 14:12:08
> Jan 7 14:12:08
> Jan 7 14:27:47
> Jan 7 14:27:47
> Jan 7 14:27:47
> Jan 7 14:27:48
> and this is roughly the time last machine was created(started).
> 
> 
>