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

[David Hildenbrand <david@redhat.com>]

On 29.09.22 13:12, Markus Armbruster wrote:
> David Hildenbrand <david@redhat.com> writes:
> 
>> Setting the CPU affinity of QEMU threads is a bit problematic, because
>> QEMU doesn't always have permissions to set the CPU affinity itself,
>> for example, with seccomp after initialized by QEMU:
>>      -sandbox enable=on,resourcecontrol=deny
>>
>> While upper layers are already aware how to handl;e CPU affinities for
> 
> Typo in handle.

Thanks!

> 
>> long-lived threads like iothreads or vcpu threads, especially short-lived
>> threads, as used for memory-backend preallocation, are more involved to
>> handle. These threads are created on demand and upper layers are not even
>> able to identify and configure them.
>>
>> Introduce the concept of a ThreadContext, that is essentially a thread
>> used for creating new threads. All threads created via that context
>> thread inherit the configured CPU affinity. Consequently, it's
>> sufficient to create a ThreadContext and configure it once, and have all
>> threads created via that ThreadContext inherit the same CPU affinity.
>>
>> The CPU affinity of a ThreadContext can be configured two ways:
>>
>> (1) Obtaining the thread id via the "thread-id" property and setting the
>>      CPU affinity manually.
>>
>> (2) Setting the "cpu-affinity" property and letting QEMU try set the
>>      CPU affinity itself. This will fail if QEMU doesn't have permissions
>>      to do so anymore after seccomp was initialized.
> 
> Could you provide usage examples?

Patch #7 and the cover letter contain examples. I can add another 
example here.

>> +##
>> +# @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


Please let me know how I can further clarify this, that would help, thanks!

-- 
Thanks,

David / dhildenb