Re: [Qemu-devel] [RFC PATCH v2 3/5] spapr: Set stable_cpu_id for threads of CPU cores

[Greg Kurz <groug@kaod.org>]

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On Fri, 8 Jul 2016 17:59:07 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Fri, Jul 08, 2016 at 09:46:47AM +0200, Greg Kurz wrote:
> > On Fri, 8 Jul 2016 15:25:33 +1000
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> > > On Thu, Jul 07, 2016 at 06:11:31PM +0200, Greg Kurz wrote:  
> > > > On Thu,  7 Jul 2016 20:20:23 +0530
> > > > Bharata B Rao <bharata@linux.vnet.ibm.com> wrote:
> > > >     
> > > > > Conditonally set stable_cpu_id for CPU threads that are created as part
> > > > > of spapr CPU cores. The use of stable_cpu_id is enabled for pseries-2.7
> > > > > onwards.
> > > > >     
> > > > 
> > > > The last sentence is a bit confusing since the enablement actually happens
> > > > in patch 5/5.
> > > >     
> > > > > Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> > > > > ---
> > > > >  hw/ppc/spapr_cpu_core.c | 7 +++++++
> > > > >  1 file changed, 7 insertions(+)
> > > > > 
> > > > > diff --git a/hw/ppc/spapr_cpu_core.c b/hw/ppc/spapr_cpu_core.c
> > > > > index b104778..0ec3513 100644
> > > > > --- a/hw/ppc/spapr_cpu_core.c
> > > > > +++ b/hw/ppc/spapr_cpu_core.c
> > > > > @@ -293,8 +293,15 @@ static void spapr_cpu_core_realize(DeviceState *dev, Error **errp)
> > > > >      for (i = 0; i < cc->nr_threads; i++) {
> > > > >          char id[32];
> > > > >          obj = sc->threads + i * size;
> > > > > +        CPUState *cs;
> > > > >  
> > > > >          object_initialize(obj, size, typename);
> > > > > +        cs = CPU(obj);
> > > > > +
> > > > > +        /* Use core_id (which is actually cpu_dt_id) as stable CPU id */    
> > > > 
> > > > It isn't what I had in mind. More something like below:
> > > > 
> > > > In ppc_spapr_init():
> > > > 
> > > >     for (i = 0; i < spapr_max_cores; i++) {
> > > >         spapr->cores[i]->stable_id = i * smp_threads;
> > > >     }
> > > > 
> > > > 
> > > > In spapr_cpu_core_realize():
> > > > 
> > > >     for (j = 0; j < cc->nr_threads; j++) {
> > > >         stable_cpu_id = cc->stable_id + j;
> > > >     }
> > > > 
> > > > So we need to introduce cc->stable_id.    
> > > 
> > > No, we don't.  Cores have had a stable ID since they were introduced.
> > >   
> > 
> > I agree core_dt_id is stable but it is a DT concept.  
> 
> There is no core_dt_id.  There's just core-id, which is machine
> assigned (via the query hotpluggable cpus interface) and stable.
> 
> > static void ppc_spapr_init(MachineState *machine)
> > {
> > [...]
> >         for (i = 0; i < spapr_max_cores; i++) {
> >             int core_dt_id = i * smt;  
> 
> ..uh, ok, except for that poorly named variable.  But that's because
> this is in the machine type, and it knows it's going to use the same
> ids to give to the core object and to put in the device tree.
> 

It is core_id everywhere else.

$ git grep core_id hw/ppc/
hw/ppc/spapr.c:        cpu_props->has_core_id = true;
hw/ppc/spapr.c:        cpu_props->core_id = i * smt;
hw/ppc/spapr_cpu_core.c:    spapr->cores[cc->core_id / smt] = NULL;
hw/ppc/spapr_cpu_core.c:    drc = spapr_dr_connector_by_id(SPAPR_DR_CONNECTOR_TYPE_CPU, cc->core_id);
hw/ppc/spapr_cpu_core.c:    index = cc->core_id / smt;
hw/ppc/spapr_cpu_core.c:    if (cc->core_id % smt) {
hw/ppc/spapr_cpu_core.c:        error_setg(&local_err, "invalid core id %d\n", cc->core_id);
hw/ppc/spapr_cpu_core.c:    index = cc->core_id / smt;
hw/ppc/spapr_cpu_core.c:        error_setg(&local_err, "core id %d out of range", cc->core_id);
hw/ppc/spapr_cpu_core.c:        error_setg(&local_err, "core %d already populated", cc->core_id);

$ git grep core_dt_id hw/ppc/
hw/ppc/spapr.c:            int core_dt_id = i * smt;
hw/ppc/spapr.c:                                       SPAPR_DR_CONNECTOR_TYPE_CPU, core_dt_id);
hw/ppc/spapr.c:                object_property_set_int(core, core_dt_id, CPU_CORE_PROP_CORE_ID,

I got confused because the current code still puts cpu_dt_id of thread0 in the
device tree. And since cpu_dt_id is still being computed on cpu_index, it is
a different beast (which needs to be killed since it even crashes simple
hotplug/unplug scenarios).

> > [...]
> >                 object_property_set_int(core, core_dt_id, CPU_CORE_PROP_CORE_ID,
> >                                         &error_fatal);
> > 
> > This patch produces stable_cpu_id in the [0...smt * smp_cores) range. I find it
> > awkward it depends on the host setup.  
> 
> True.  Possibly we should set these as i * (maximum plausible number
> of threads).
> 
> The gotcha is that currently we're using the same "dt_id" to control
> KVM's cpu id and that in turn controls the SMT level.  That's a poor
> interface on the kernel side (my bad), but we have to live with it
> now.  However we could de-couple that KVM id from the core-id.  It'd
> no doubt cause some complications with kvm-xics, but we can probably
> handle it.
> 
> > I'm suggesting we introduce cc->stable_id to be able to compute a simple
> > stable_cpu_id in the range [0...max_cpus), like x86 and ARM.  
> 
> I really don't see what properties this is supposed to have that are
> different from the existing core-id.
> 

Simplicity of always having CPU0, 1, 2, 3... max_cpus in QEMU, and try
to hide the "poor interface on the kernel side" from the code that
does not need it... but maybe that is not that important.

> >   
> > > Instead we should be setting the thread stable ids based on the core
> > > stable id.
> > >   
> > > > I think stable_cpu_id is the prerequisite to compute both cpu_dt_id and instance_id.
> > > > 
> > > > Makes sense ?
> > > >     
> > > > > +        if (cs->has_stable_cpu_id) {
> > > > > +            cs->stable_cpu_id = cc->core_id + i;
> > > > > +        }
> > > > >          snprintf(id, sizeof(id), "thread[%d]", i);
> > > > >          object_property_add_child(OBJECT(sc), id, obj, &local_err);
> > > > >          if (local_err) {    
> > > >     
> > >   
> >   
> 
> 
> 


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