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

[David Hildenbrand]

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