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

[Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>]

David Gibson <david@gibson.dropbear.id.au> writes:

> On Wed, Sep 20, 2017 at 09:50:24AM +0530, Nikunj A Dadhania wrote:
>> David Gibson <david@gibson.dropbear.id.au> writes:
>> 
>> > On Fri, Sep 15, 2017 at 02:39:16PM +0530, Nikunj A Dadhania wrote:
>> >> David Gibson <david@gibson.dropbear.id.au> writes:
>> >> 
>> >> > On Fri, Sep 15, 2017 at 01:53:15PM +0530, Nikunj A Dadhania wrote:
>> >> >> David Gibson <david@gibson.dropbear.id.au> 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 ?

Regards
Nikunj