Re: [PATCH v1] KVM: isolation: retain initial mask for kthread VM worker

[Marcelo Tosatti]

On Tue, Oct 05, 2021 at 07:15:43PM +0200, Paolo Bonzini wrote:
> On 05/10/21 15:21, Marcelo Tosatti wrote:
> > > The QEMU I/O thread is not hogging the CPU 100% of the time, and
> > > therefore the nx-recovery thread should be able to run on that CPU.
> > 
> > 1) The cpumask of the parent thread is not inherited
> > 
> > 	set_cpus_allowed_ptr(task, housekeeping_cpumask(HK_FLAG_KTHREAD));
> > 
> > On __kthread_create_on_node should fail (because its cgroup, the one
> > inherited from QEMU, contains only isolated CPUs).
> > 
> > (The QEMU I/O thread runs on an isolated CPU, and is moved by libvirt
> > to HK-cgroup as mentioned before).
> 
> Ok, that's the part that I missed.  So the core issue is that libvirt moves
> the I/O thread out of the isolated-CPU cgroup too late?  This in turn is
> because libvirt gets access to the QEMU monitor too late (the KVM file
> descriptor is created when QEMU processes the command line).

Actually, what i wrote was incorrect: set_cpus_allowed_ptr should 
succeed (kthread creation), but cgroup_attach_task_all will
override the kthread mask with the cgroup mask
(which contains isolated CPUs).

Paolo: about your suggestion to use the same CPU for nx-recovery thread
as the I/O thread one: I believe the cpumask of the userspace parent (QEMU I/O
thread) is not inherited by the kernel thread.

We will resend the patchset once more data to justify this is available
(or will if an userspace solution is not possible).