Bug#785557: perl: FTBFS on i386 and amd64: itimer problems on buildds?

[Apollon Oikonomopoulos]

On 16:38 Sun 24 May     , Ben Hutchings wrote:
> On Sun, 2015-05-24 at 14:09 +0300, Niko Tyni wrote:
> > On Sun, May 24, 2015 at 02:55:00PM +0800, Paul Wise wrote:
> > > On Sat, 2015-05-23 at 19:10 +0200, Dominic Hargreaves wrote:
> > > 
> > > > This is rather strange; any ideas from DSA?
> > > 
> > > The underlying hosts do not have the same issue.
> > > 
> > > All of the guests use the same virtual CPU version/flags.
> > > 
> > > All of the guests use the same Linux kernel version.
> > 
> > Thanks for the update.
> > 
> > > I guess diving into the Linux implementation of times(2) for clues would
> > > be the next step for figuring out what the issue is here.
> > 
> > I'm taking the kernel maintainers in the loop. The status here is that
> > times(2) seems to be misbehaving on some i386 and amd64 debian.org virtual
> > hosts running jessie (under ganeti/qemu, with jessie on the underlying
> > hosts too). These hosts include at least barriere and x86-grnet-01.
> > 
> > The misbehaviour is that user time stays at zero all the time, as seen
> > for example with 'time yes'. This is making perl fail to build from
> > source due to test failures, and I'd expect it to affect other things too.
> > 
> > Any help is appreciated.
> 
> I can't reproduce this, but wonder if it's related to #784960?

There seems to be something fundamentally broken in 
barriere.debian.org's CPU time accounting, not related to times(2) per 
se. Just issuing

  yes >/dev/null

and firing up top -d1 gives the following interesting results:

  - `yes' shows up taking 100% CPU time as expected, but
  - pressing `1' shows that all CPUs are idle (!)

htop OTOH displays all CPUs as constantly 100% busy, which is 
inconsistent with the system's load average (~0.8 at that point).

Also watching the output of `cat /proc/$(pidof yes)/stat | awk '{ print 
$14, $15 }'' ($14 is utime, $15 is stime per proc(5)) indeed shows 100% 
system time and 0 user time.

If you look at the `top' stats for all CPUs of barriere.debian.org, it 
looks as if the only thing that's correctly being accounted for is 
iowait time.

Cheers,
Apollon