linux-kernel - Re: cgroup2 freezer and kvm_vm_worker

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Message-ID: <CABgObfYR6e0XV94USugVOO5XcOfyctr1rAm+ZWJwfu9AHYPtiA@mail.gmail.com>
Date: Wed, 30 Oct 2024 13:05:16 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Tejun Heo <tj@...nel.org>
Cc: Luca Boccassi <bluca@...ian.org>, Roman Gushchin <roman.gushchin@...ux.dev>, kvm@...r.kernel.org, 
	cgroups@...r.kernel.org, Michal Koutný <mkoutny@...e.com>, 
	linux-kernel@...r.kernel.org
Subject: Re: cgroup2 freezer and kvm_vm_worker_thread()

On Wed, Oct 30, 2024 at 1:25 AM Tejun Heo <tj@...nel.org> 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.

(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().

Paolo