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

[Cédric Le Goater]

On 09/21/2017 05:54 AM, Nikunj A Dadhania wrote:
> 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?

yes that would do for cpus, but how do we retrieve the number of 
sockets ? I don't see a smp_sockets. 

If we start looking at such issues, we should also take into account 
memory distribution :

      -numa node[,mem=size][,cpus=firstcpu[-lastcpu]][,nodeid=node]

would allow us to define a set of cpus per node, cpus should be evenly 
distributed on the nodes though, and also define memory per node, but 
some nodes could be without memory.   

C.