Re: [Qemu-devel] [RFC PATCH v2] PPC: smp: autodetect numbers of threads per core

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 01/10/2014 10:40 AM, Alexander Graf wrote:
> 
> 
>> Am 09.01.2014 um 23:12 schrieb Alexey Kardashevskiy <aik@ozlabs.ru>:
>>
>>> On 01/10/2014 08:00 AM, Mike Day wrote:
>>>
>>> Alexey Kardashevskiy <aik@ozlabs.ru> writes:
>>>
>>>>         /* compute missing values, prefer sockets over cores over threads */
>>>>         if (cpus == 0 || sockets == 0) {
>>>>             sockets = sockets > 0 ? sockets : 1;
>>>>             cores = cores > 0 ? cores : 1;
>>>> -            threads = threads > 0 ? threads : 1;
>>>> +            if (threads_max) {
>>>> +                if (threads > 0) {
>>>> +                    fprintf(stderr, "Use either threads or threads_max\n");
>>>> +                    exit(1);
>>>
>>> If you went ahead with the threads="max" string option you wouldn't need
>>> to check here for mutual excusivity and the user wouldn't need to worry
>>> about an extra command options.
>>
>>
>> Is this the only concern and the rest is fine and can go to upstream? If
>> so, I'll fix it and repost.
> 

> What if we make the max thread count a property of our cpu class? The
> we
can add a threads=max option which will be identical between kvm and tcg.


You lost me here :)
Right now the sequence is:
1. smp_parse
2. config_accelerator
3. machine_init

I proposed
1. config_accelerator - reads max threads from KVM (and initializes "host"
type)
2. smp_parse - does the parsing using smp_threads tweaked in 1)
3. machine_init - creates CPUs which may or may be not "host".

>From what you said I conclude that we add this "max" property only to the
"host" CPU type and then in smp_parse() we look if "host" CPU is going to
be used and if so, then we read "max" property from it and use it. But we
cannot read property for a class, only from an instance and it is not
created yet. And we also need to tweak the number depending on "compat".

And we still need "-smp ...threads_max" or "-smp ...threads=max" property
for "-smp".

Where am I wrong here?

> Overall I'm not yet fully convinced this whole idea is eventually going
> to improve the situation though.


My goal is to have a script which would automatically set threads to the
maximum value supported by the host hardware. Reading /proc/cpuinfo and
parsing for POWER6/7/8 and setting threads to 2/4/8 is lame for my taste
because what was the point of adding cap_ppc_smt at the first place then?



> 
> 
> Alex
> 
>>
>>
>>>> +                }
>>>> +                threads = smp_threads > 0 ? smp_threads : 1;
>>>> +            } else {
>>>> +                threads = threads > 0 ? threads : 1;
>>>> +            }
>>>>             if (cpus == 0) {
>>>>                 cpus = cores * threads * sockets;
>>>>             }
>>
>>
>> -- 
>> Alexey


-- 
Alexey