On Tue, 31 Jan 2023 at 22:49, Michal Prívozník <mprivozn@redhat.com> wrote:

On 1/31/23 13:43, Dipanjan Das wrote:
> Hello,
>
> I am getting the following error message when trying to run a VM with
> the emulatorpin cpuset set to a value -
>
> "cannot set CPU affinity on process 78: Invalid argument".
>
> Note that removing the emulatorpin leads to the VM running successfully.
>
> The core that I am setting for the emulatorpin cpuset is present on the
> system and is not in the isolcpus set of the host.

The question is, whether the container that libvirtd runs in is allowed
to run on the selected cpuset. I mean, you can have a cgroup that allows
a subset of physical CPUs, and when a process running within the cgroup
tries to set affinity to a CPU that's not allowed it gets EINVAL.

Makes sense.

I am now wondering whether we can check the cgroups on the host or inside the containers to confirm that this is the case. Since I am setting the emulator pin thread to a known cpu, being able to check whether the cpu is in the cpu subset of the cgroup might be a quick way to find the problem.

The function this error message comes from is: virProcessSetAffinity()
(the linux version)

https://gitlab.com/libvirt/libvirt/-/blob/master/src/util/virprocess.c#L445

Another reason for the error might be insufficient permissions (though
the man page of sched_setaffinity() says you'd get an EPERM so maybe not).

>
> Any suggestions on what could be the problem? Any pointers to how to
> debug the issue would help greatly too.

Turning on debug logs is a good start. From there you'd need a trained
eye to see the code path, but if you post a link to the logs here I'll
try to help.

Thank you very much. Should I enable debug logs for any specific module(s) or for all of them?