linux-kernel - Re: cgroup2 freezer and kvm_vm_worker

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Message-ID: <CABgObfYrX8O-Vj5g7t8DD=6_twedXToajNtLedLgtKR6+G9_Hg@mail.gmail.com>
Date: Thu, 7 Nov 2024 00:22:33 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Luca Boccassi <bluca@...ian.org>
Cc: Tejun Heo <tj@...nel.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 Thu, Nov 7, 2024 at 12:21 AM Luca Boccassi <bluca@...ian.org> wrote:
>
> On Wed, 30 Oct 2024 at 18:14, Tejun Heo <tj@...nel.org> wrote:
> >
> > 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@...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.
> >
> > 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.
>
> Any update on this? We keep getting reports of this issue, so it would
> be great if there was a fix for 6.12. Thanks!

No, did not have time yet.

Paolo