linux-kernel - Re: [PATCH] KVM: x86: switch hugepage recovery thread to vhost

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Message-ID: <CABgObfbUzKswAjPuq_+KL9jyQegXgjSRQmc6uSm1cAXifNo_Xw@mail.gmail.com>
Date: Mon, 18 Nov 2024 13:42:27 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Michal Koutný <mkoutny@...e.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	michael.christie@...cle.com, Tejun Heo <tj@...nel.org>, 
	Luca Boccassi <bluca@...ian.org>
Subject: Re: [PATCH] KVM: x86: switch hugepage recovery thread to vhost_task

On Fri, Nov 15, 2024 at 5:59 PM Michal Koutný <mkoutny@...e.com> wrote:
>
> On Fri, Nov 08, 2024 at 08:07:37AM GMT, Paolo Bonzini <pbonzini@...hat.com> wrote:
> > Since the worker kthread is tied to a user process, it's better if
> > it behaves similarly to user tasks as much as possible, including
> > being able to send SIGSTOP and SIGCONT.
>
> Do you mean s/send/receive/?

I mean being able to send it to the threads with an effect.

> Consequently, it's OK if a (possibly unprivileged) user stops this
> thread forever (they only harm themselves, not the rest of the system),
> correct?

Yes, they will run with fewer huge pages and worse TLB performance.

Paolo

> > In fact, vhost_task is all that kvm_vm_create_worker_thread() wanted
> > to be and more: not only it inherits the userspace process's cgroups,
> > it has other niceties like being parented properly in the process
> > tree.  Use it instead of the homegrown alternative.
>
> It is nice indeed.
> I think the bugs we saw are not so serious to warrant
> Fixes: c57c80467f90e ("kvm: Add helper function for creating VM worker threads")
> .
> (But I'm posting it here so that I can find the reference later.)