Re: [Qemu-devel] [Qemu-ppc] [PATCH 1/1] spapr: Ensure CPU cores are added contiguously and removed in LIFO order

[Greg Kurz <groug@kaod.org>]

On Wed, 13 Jul 2016 09:42:54 +0200
Igor Mammedov <imammedo@redhat.com> wrote:

> On Wed, 13 Jul 2016 12:20:20 +0530
> Bharata B Rao <bharata@linux.vnet.ibm.com> wrote:
> 
> > If CPU core addition or removal is allowed in random order leading to
> > holes in the core id range (and hence in the cpu_index range), migration
> > can fail as migration with holes in cpu_index range isn't yet handled
> > correctly.
> > 
> > Prevent this situation by enforcing the addition in contiguous order
> > and removal in LIFO order so that we never end up with holes in
> > cpu_index range.  
> Adding this limitation looks better than adding migration_id as
> it will allow libvirt to use the current -numa cpus=... while
> doing the same amount of guess work as it does now.
> 
> Similar patch for x86 won't be so simple as cpu-add can add cpus
> with gaps (breaking migration at that), so I'd need to keep it
> that way with some compat code, but that shouldn't be issue.
> 

And does libvirt take care not to add cpus with gaps ? If so, maybe it
can do the same with PPC, and we'd simply warn the user that migration
is broken if we introduce gaps. Makes sense ?

> 
> > Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> > ---
> > While there is work in progress to support migration when there are holes
> > in cpu_index range resulting from out-of-order plug or unplug, this patch
> > is intended as a last resort if no easy, risk-free and elegant solution
> > emerges before 2.7 dev cycle ends.
> > 
> >  hw/ppc/spapr_cpu_core.c | 21 ++++++++++++++++++++-
> >  1 file changed, 20 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/ppc/spapr_cpu_core.c b/hw/ppc/spapr_cpu_core.c
> > index bc52b3c..4bfc96b 100644
> > --- a/hw/ppc/spapr_cpu_core.c
> > +++ b/hw/ppc/spapr_cpu_core.c
> > @@ -126,12 +126,23 @@ static void spapr_core_release(DeviceState *dev, void *opaque)
> >  void spapr_core_unplug(HotplugHandler *hotplug_dev, DeviceState *dev,
> >                         Error **errp)
> >  {
> > +    sPAPRMachineState *spapr = SPAPR_MACHINE(OBJECT(hotplug_dev));
> >      CPUCore *cc = CPU_CORE(dev);
> >      sPAPRDRConnector *drc =
> >          spapr_dr_connector_by_id(SPAPR_DR_CONNECTOR_TYPE_CPU, cc->core_id);
> >      sPAPRDRConnectorClass *drck;
> >      Error *local_err = NULL;
> > +    int smt = kvmppc_smt_threads();
> > +    int index = cc->core_id / smt;
> > +    int spapr_max_cores = max_cpus / smp_threads;
> > +    int i;
> >  
> > +    for (i = spapr_max_cores - 1; i > index; i--) {
> > +        if (spapr->cores[i]) {
> > +            error_setg(errp, "core-id %d should be removed first", i * smt);
> > +            return;
> > +        }
> > +    }
> >      g_assert(drc);
> >  
> >      drck = SPAPR_DR_CONNECTOR_GET_CLASS(drc);
> > @@ -214,7 +225,7 @@ void spapr_core_pre_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
> >      sPAPRMachineClass *smc = SPAPR_MACHINE_GET_CLASS(OBJECT(hotplug_dev));
> >      sPAPRMachineState *spapr = SPAPR_MACHINE(OBJECT(hotplug_dev));
> >      int spapr_max_cores = max_cpus / smp_threads;
> > -    int index;
> > +    int index, i;
> >      int smt = kvmppc_smt_threads();
> >      Error *local_err = NULL;
> >      CPUCore *cc = CPU_CORE(dev);
> > @@ -252,6 +263,14 @@ void spapr_core_pre_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
> >          goto out;
> >      }
> >  
> > +    for (i = 0; i < index; i++) {
> > +        if (!spapr->cores[i]) {
> > +            error_setg(&local_err, "core-id %d should be added first",
> > +                       i * smt);
> > +            goto out;
> > +        }
> > +    }
> > +
> >  out:
> >      g_free(base_core_type);
> >      error_propagate(errp, local_err);  
> 
>