[libvirt] [PATCH 5/9] qemu_cgroup: put qemu right into emulator sub-cgroup

[Henning Schild]

On Fri, 26 Feb 2016 11:14:31 +0000
"Daniel P. Berrange" <berrange at redhat.com> wrote:

> On Tue, Feb 23, 2016 at 04:58:40PM +0100, Henning Schild wrote:
> > Move qemuSetupCgroupForEmulator up under qemuSetupCgroup. That way
> > we move the one main thread right into the emulator cgroup, instead
> > of moving multiple threads later on. And we do not actually want any
> > threads running in the parent cgroups (cpu cpuacct cpuset).
> > 
> > Signed-off-by: Henning Schild <henning.schild at siemens.com>
> > ---
> >  src/qemu/qemu_process.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)  
> 
> ACK, this is the key part of the fix.  With the old code the QEMU
> pids are only moved /after/ exec(), with this change, the pids are
> moved /before/ exec(), fixing the core problem of threads runing
> int the wrong place between 'exec()' and libvirt querying vCPUs.

If the asserts wont work and we have to live with the task being in the
parent cgroup between fork() and exec() we need to make sure the new
process is truly inactive. We need to make sure we are not just making
the window smaller.

> 
> > 
> > diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
> > index 05cbda2..65f718c 100644
> > --- a/src/qemu/qemu_process.c
> > +++ b/src/qemu/qemu_process.c
> > @@ -4895,6 +4895,10 @@ qemuProcessLaunch(virConnectPtr conn,
> >      if (qemuSetupCgroup(driver, vm, nnicindexes, nicindexes) < 0)
> >          goto cleanup;
> >  
> > +    VIR_DEBUG("Setting cgroup for emulator (if required)");
> > +    if (qemuSetupCgroupForEmulator(vm) < 0)
> > +        goto cleanup;
> > +
> >      /* This must be done after cgroup placement to avoid resetting
> > CPU
> >       * affinity */
> >      if (!vm->def->cputune.emulatorpin &&
> > @@ -4943,10 +4947,6 @@ qemuProcessLaunch(virConnectPtr conn,
> >      if (rv == -1) /* The VM failed to start */
> >          goto cleanup;
> >  
> > -    VIR_DEBUG("Setting cgroup for emulator (if required)");
> > -    if (qemuSetupCgroupForEmulator(vm) < 0)
> > -        goto cleanup;
> > -
> >      VIR_DEBUG("Setting affinity of emulator threads");
> >      if (qemuProcessSetEmulatorAffinity(vm) < 0)
> >          goto cleanup;
> > -- 
> > 2.4.10
> >   
> 
> Regards,
> Daniel