libvirt error - "cannot set CPU affinity"

[Dipanjan Das]

On Tue, 31 Jan 2023 at 22:49, Michal Prívozník <mprivozn at 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?
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