Re: [Qemu-ppc] [PATCH] ppc/pnv: fix cores per chip for multiple cpus

[Nikunj A Dadhania]

David Gibson <address@hidden> writes:

> On Wed, Sep 20, 2017 at 12:48:55PM +0530, Nikunj A Dadhania wrote:
>> David Gibson <address@hidden> writes:
>> 
>> > On Wed, Sep 20, 2017 at 12:10:48PM +0530, Nikunj A Dadhania wrote:
>> >> David Gibson <address@hidden> writes:
>> >> 
>> >> > On Wed, Sep 20, 2017 at 10:43:19AM +0530, Nikunj A Dadhania wrote:
>> >> >> David Gibson <address@hidden> writes:
>> >> >> 
>> >> >> > On Wed, Sep 20, 2017 at 09:50:24AM +0530, Nikunj A Dadhania wrote:
>> >> >> >> David Gibson <address@hidden> writes:
>> >> >> >> 
>> >> >> >> > On Fri, Sep 15, 2017 at 02:39:16PM +0530, Nikunj A Dadhania wrote:
>> >> >> >> >> David Gibson <address@hidden> writes:
>> >> >> >> >> 
>> >> >> >> >> > On Fri, Sep 15, 2017 at 01:53:15PM +0530, Nikunj A Dadhania 
>> >> >> >> >> > wrote:
>> >> >> >> >> >> David Gibson <address@hidden> writes:
>> >> >> >> >> >> 
>> >> >> >> >> >> >> 
>> >> >> >> >> >> >> I thought, I am doing the same here for PowerNV, number of 
>> >> >> >> >> >> >> online cores
>> >> >> >> >> >> >> is equal to initial online vcpus / threads per core
>> >> >> >> >> >> >> 
>> >> >> >> >> >> >>    int boot_cores_nr = smp_cpus / smp_threads;
>> >> >> >> >> >> >> 
>> >> >> >> >> >> >> Only difference that I see in PowerNV is that we have 
>> >> >> >> >> >> >> multiple chips
>> >> >> >> >> >> >> (max 2, at the moment)
>> >> >> >> >> >> >> 
>> >> >> >> >> >> >>         cores_per_chip = smp_cpus / (smp_threads * 
>> >> >> >> >> >> >> pnv->num_chips);
>> >> >> >> >> >> >
>> >> >> >> >> >> > This doesn't make sense to me.  Cores per chip should 
>> >> >> >> >> >> > *always* equal
>> >> >> >> >> >> > smp_cores, you shouldn't need another calculation for it.
>> >> >> >> >> >> >
>> >> >> >> >> >> >> And in case user has provided sane smp_cores, we use it.
>> >> >> >> >> >> >
>> >> >> >> >> >> > If smp_cores isn't sane, you should simply reject it, not 
>> >> >> >> >> >> > try to fix
>> >> >> >> >> >> > it.  That's just asking for confusion.
>> >> >> >> >> >> 
>> >> >> >> >> >> This is the case where the user does not provide a 
>> >> >> >> >> >> topology(which is a
>> >> >> >> >> >> valid scenario), not sure we should reject it. So qemu 
>> >> >> >> >> >> defaults
>> >> >> >> >> >> smp_cores/smt_threads to 1. I think it makes sense to 
>> >> >> >> >> >> over-ride.
>> >> >> >> >> >
>> >> >> >> >> > If you can find a way to override it by altering smp_cores 
>> >> >> >> >> > when it's
>> >> >> >> >> > not explicitly specified, then ok.
>> >> >> >> >> 
>> >> >> >> >> Should I change the global smp_cores here as well ?
>> >> >> >> >
>> >> >> >> > I'm pretty uneasy with that option.
>> >> >> >> 
>> >> >> >> Me too.
>> >> >> >> 
>> >> >> >> > It would take a fair bit of checking to ensure that changing 
>> >> >> >> > smp_cores
>> >> >> >> > is safe here. An easier to verify option would be to make the 
>> >> >> >> > generic
>> >> >> >> > logic which splits up an unspecified -smp N into cores and sockets
>> >> >> >> > more flexible, possibly based on machine options for max values.
>> >> >> >> >
>> >> >> >> > That might still be more trouble than its worth.
>> >> >> >> 
>> >> >> >> I think the current approach is the simplest and less intrusive, as 
>> >> >> >> we
>> >> >> >> are handling a case where user has not bothered to provide a 
>> >> >> >> detailed
>> >> >> >> topology, the best we can do is create single threaded cores equal 
>> >> >> >> to
>> >> >> >> number of cores.
>> >> >> >
>> >> >> > No, sorry.  Having smp_cores not correspond to the number of cores 
>> >> >> > per
>> >> >> > chip in all cases is just not ok.  Add an error message if the
>> >> >> > topology isn't workable for powernv by all means.  But users having 
>> >> >> > to
>> >> >> > use a longer command line is better than breaking basic assumptions
>> >> >> > about what numbers reflect what topology.
>> >> >> 
>> >> >> Sorry to ask again, as I am still not convinced, we do similar
>> >> >> adjustment in spapr where the user did not provide the number of cores,
>> >> >> but qemu assumes them as single threaded cores and created
>> >> >> cores(boot_cores_nr) that were not same as smp_cores ?
>> >> >
>> >> > What?  boot_cores_nr has absolutely nothing to do with adjusting the
>> >> > topology, and it certainly doesn't assume they're single threaded.
>> >> 
>> >> When we start a TCG guest and user provides following commandline, e.g.
>> >> "-smp 4", smt_threads is set to 1 by default in vl.c. So the guest boots
>> >> with 4 cores, each having 1 thread.
>> >
>> > Ok.. and what's the problem with that behaviour on powernv?
>> 
>> As smp_thread defaults to 1 in vl.c, similarly smp_cores also has the
>> default value of 1 in vl.c. In powernv, we were setting nr-cores like
>> this:
>> 
>>         object_property_set_int(chip, smp_cores, "nr-cores", &error_fatal);
>> 
>> Even when there were multiple cpus (-smp 4), when the guest boots up, we
>> just get one core (i.e. smp_cores was 1) with single thread(smp_threads
>> was 1), which is wrong as per the command-line that was provided.
>
> Right, so, -smp 4 defaults to 4 sockets, each with 1 core of 1
> thread.  If you can't supply 4 sockets you should error, but you
> shouldn't go and change the number of cores per socket.

OK, that makes sense now. And I do see that smp_cpus is 4 in the above
case. Now looking more into it, i see that powernv has something called
"num_chips", isnt this same as sockets ? Do we need num_chips separately?

Regards,
Nikunj