linux-kernel - Re: cgroup2 freezer and kvm_vm_worker

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Message-ID: <Zy0njoaJGS9310eR@google.com>
Date: Thu, 7 Nov 2024 12:48:14 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Tejun Heo <tj@...nel.org>
Cc: "Michal Koutný" <mkoutny@...e.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Luca Boccassi <bluca@...ian.org>, Roman Gushchin <roman.gushchin@...ux.dev>, kvm@...r.kernel.org, 
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: cgroup2 freezer and kvm_vm_worker_thread()

On Thu, Nov 07, 2024, Tejun Heo wrote:
> Hello,
> 
> On Thu, Nov 07, 2024 at 07:05:46PM +0100, Michal Koutný wrote:
> ...
> > I'd first ask why the kvm_vm_worker_thread needs to be in the KVM task's
> > cgroup (and copy its priority at creation time but no later adjustments)?
> > 
> > If it can remain inside root cgroup (like any other good kthread) its
> > job may be even chunked into periodic/deferred workqueue pieces with no
> > kthread per KVM at all.
> 
> That'd be better but I suppose kvm wants them in the same cgroup for a
> reason.

Yes.  The one and only user of kvm_vm_create_worker_thread() does non-trivial
work on behalf of the VM, and so we want all of the CPU time consumed by that
work to be charged to the VM's container, e.g. so that a VM that is generating
a lot of work doesn't negatively impact other VMs on the same host (the amount
of work done is directly affected by how the guest is accessing its memory).