Re: [PATCH v1 3/7] util: Introduce ThreadContext user-creatable object

[David Hildenbrand <david@redhat.com>]

>>>> +##
>>>> +# @ThreadContextProperties:
>>>> +#
>>>> +# Properties for thread context objects.
>>>> +#
>>>> +# @cpu-affinity: the CPU affinity for all threads created in the thread
>>>> +#                context (default: QEMU main thread affinity)
>>>> +#
>>>> +# Since: 7.2
>>>> +##
>>>> +{ 'struct': 'ThreadContextProperties',
>>>> +  'data': { '*cpu-affinity': ['uint16'] } }
>>>
>>> I understand this is a list of affinities.  What I poor ignorant me
>>> doesn't understand is the meaning of the list index.  Or in other words,
>>> the list maps some range [0:N] to affinities, but what are the numbers
>>> being mapped there?
>>
>> Assume you have 8 physical CPUs.
>>
>> $ lscpu
>> ...
>>
>> NUMA:
>>    NUMA node(s):          1
>>    NUMA node0 CPU(s):     0-7
>> ...
>>
>> You will provide the CPU IDs here, for example as in patch #7 example:
>>
>> qemu-system-x86_64 -m 1G \
>>   -object thread-context,id=tc1,cpu-affinity=3-4 \
>>   -object memory-backend-ram,id=pc.ram,size=1G,prealloc=on,prealloc-threads=2,prealloc-context=tc1
>> \
>>   -machine memory-backend=pc.ram \
>>   -S -monitor stdio -sandbox enable=on,resourcecontrol=deny
>>
>>
>> Details about CPU affinities in general can be found in the man page of taskset:
>>
>> https://man7.org/linux/man-pages/man1/taskset.1.html
> 
> Is @cpu-affinity a set of CPU numbers?

Yes! For now I added to the description:

...
     General information about CPU affinities can be found in the man page of
     taskset:
         CPU affinity is a scheduler property that "bonds" a process to a given
         set of CPUs on the system. The Linux scheduler will honor the given CPU
         affinity and the process will not run on any other CPUs.
...
     A simple QEMU example to set the CPU affinity to CPU 0,1,6,7 would be:
         qemu-system-x86_64 -S \
           -object thread-context,id=tc1,cpu-affinity=0-1,cpu-affinity=6-7
     
     And we can query it via HMP/QMP:
         (qemu) qom-get tc1 cpu-affinity
         [
             0,
             1,
             6,
             7
         ]
     
     But note that due to dynamic library loading this example will not work
     before we actually make use of thread_context_create_thread() in QEMU
     code, because the type will otherwise not get registered.



> 
>> Please let me know how I can further clarify this, that would help, thanks!
> 
> What happens when you try to create a thread context object with CPU
> affinities on a host system that doesn't support CPU affinities?

qemu_thread_set_affinity() will fail and setting the attribute will result
in a "Setting CPU affinity failed".

Thanks!

-- 
Thanks,

David / dhildenb