[systemd-devel] machined: after CPU offline then online, vcpupin KVM guest failed to start

[Dr. Werner Fink]

On Fri, Aug 05, 2016 at 11:07:50AM +0200, Lennart Poettering wrote:
> On Thu, 04.08.16 16:19, Cedric Bosdonnat (cbosdonnat at suse.com) wrote:
> 
> > Hi Lennart and Werner,
> > 
> > On Wed, 2016-08-03 at 16:56 +0200, Lennart Poettering wrote:
> > > On Wed, 03.08.16 14:46, Dr. Werner Fink (werner at suse.de) wrote:
> > > > problem with v228 (and I guess this is also later AFAICS from logs of
> > > > current git) that repeating CPU hotplug events (offline/online). The
> > > > root cause is that cpuset.cpus become not restored by machined.
> > > > Please note that libvirt can not do this as it is not allowed to do
> > > > so.
> > > 
> > > This is a limitation of the kernel cpuset interface, and it's one of
> > > the reasons we do not expose cpusets at all in systemd right
> > > now. Thankfully, there's an alternative to cpusets, which is the CPU
> > > affinity controls exposed via CPUAffinity= in systemd, which do much
> > > of the same, but have less borked semantics.
> > > 
> > > We'd like to support cpusets directly in systemd, but we don't do this
> > > as long as the kernel interfaces are as borked as they are. For
> > > example, cpusets are flushed out entirely currently when the system
> > > goes through a suspend/resume cycle.
> > > 
> > > If libvirt has hook-ups with cpuset, then it bypasses systemd for
> > > that.
> > 
> > I guess by CPU affinity you mean sched_setaffinity and friends. If that is
> > the case, then this is constrained by cpuset too as mentioned here:
> > 
> > http://www.mjmwired.net/kernel/Documentation/cpusets.txt#53
> > 
> > As long as the machine.slice cpuset isn't restored after onlining a CPU again,
> > then libvirt won't be able to set either the affinity or the cpuset if it
> > contains that CPU.
> > 
> > May be the kernel's behaviour is weird and can be discussed, but libvirt can't
> > do anything on that bug.
> 
> Yeah, to make this clear: I do not blame libvirt for this borkedness
> at all. I blame the kernel.

Hmmm ... IMHO it is useless to pass the buck from kernel to user space
as well do the same from user space back to kernel.  I've an open bug
from a customer and this bug requires a solution.  AFAICS libvirt can
not do this but machined could do.

Werner

-- 
  "Having a smoking section in a restaurant is like having
          a peeing section in a swimming pool." -- Edward Burr
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