Re: [PATCH v2] KVM: Move VM's worker kthreads back to the original cgroups before exiting.

[Vipin Sharma]

On Wed, Jan 19, 2022 at 10:30 AM Tejun Heo <tj@xxxxxxxxxx> wrote:
>
> On Wed, Jan 19, 2022 at 07:02:53PM +0100, Paolo Bonzini wrote:
> > On 1/18/22 21:39, Tejun Heo wrote:
> > > So, these are normally driven by the !populated events. That's how everyone
> > > else is doing it. If you want to tie the kvm workers lifetimes to kvm
> > > process, wouldn't it be cleaner to do so from kvm side? ie. let kvm process
> > > exit wait for the workers to be cleaned up.
> >
> > It does.  For example kvm_mmu_post_init_vm's call to
> > kvm_vm_create_worker_thread is matched with the call to
> > kthread_stop in kvm_mmu_pre_destroy_vm.
> > According to Vpin, the problem is that there's a small amount of time
> > between the return from kthread_stop and the point where the cgroup
> > can be removed.  My understanding of the race is the following:
>
> Okay, this is because kthread_stop piggy backs on vfork_done to wait for the
> task exit intead of the usual exit notification, so it only waits till
> exit_mm(), which is uhh... weird. So, migrating is one option, I guess,
> albeit a rather ugly one. It'd be nicer if we can make kthread_stop()
> waiting more regular but I couldn't find a good existing place and routing
> the usual parent signaling might be too complicated. Anyone has better
> ideas?
>
Sean suggested that we can use the real_parent of the kthread task
which will always be kthreadd_task, this will also not require any
changes in the cgroup API. I like that approach, I will give it a try.
This will avoid changes in cgroup APIs completely.