Re: cgroup2 freezer and kvm_vm_worker_thread()

[Tejun Heo]

Hello,

On Wed, Oct 30, 2024 at 01:05:16PM +0100, Paolo Bonzini wrote:
> On Wed, Oct 30, 2024 at 1:25 AM Tejun Heo <tj@xxxxxxxxxx> wrote:
> > > I'm not sure if the KVM worker thread should process signals.  We want it
> > > to take the CPU time it uses from the guest, but otherwise it's not running
> > > on behalf of userspace in the way that io_wq_worker() is.
> >
> > I see, so io_wq_worker()'s handle signals only partially. It sets
> > PF_USER_WORKER which ignores fatal signals, so the only signals which take
> > effect are STOP/CONT (and friends) which is handled in do_signal_stop()
> > which is also where the cgroup2 freezer is implemented.
> 
> What about SIGKILL? That's the one that I don't want to have for KVM
> workers, because they should only stop when the file descriptor is
> closed.

I don't think SIGKILL does anything for PF_USER_WORKER threads. Those are
all handled in the fatal: label in kernel/signal.c::get_signal() and the
function just returns for PF_USER_WORKER threads. I haven't used it myself
but looking at io_uring usage, it seems pretty straightforward.

> (Replying to Luca: the kthreads are dropping some internal data
> structures that KVM had to "de-optimize" to deal with processor bugs.
> They allow the data structures to be rebuilt in the optimal way using
> large pages).
> 
> > Given that the kthreads are tied to user processes, I think it'd be better
> > to behave similarly to user tasks as possible in this regard if userspace
> > being able to stop/cont these kthreads are okay.
> 
> Yes, I totally agree with you on that, I'm just not sure of the best
> way to do it.
> 
> I will try keeping the kthread and adding allow_signal(SIGSTOP).  That
> should allow me to process the SIGSTOP via get_signal().

I *think* you can just copy what io_wq_worker() is doing.

Thanks.

-- 
tejun