linux-kernel - Re: [PATCH v2] KVM: Move VM's worker kthreads back to the original cgroups before exiting.

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Message-ID: <20220105180420.GC6464@blackbody.suse.cz>
Date:   Wed, 5 Jan 2022 19:04:20 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Vipin Sharma <vipinsh@...gle.com>
Cc:     pbonzini@...hat.com, seanjc@...gle.com, tj@...nel.org,
        lizefan.x@...edance.com, hannes@...xchg.org, dmatlack@...gle.com,
        jiangshanlai@...il.com, kvm@...r.kernel.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: Move VM's worker kthreads back to the original
 cgroups before exiting.

Hi Vipin.

On Wed, Dec 22, 2021 at 10:53:50PM +0000, Vipin Sharma <vipinsh@...gle.com> wrote:
> VM worker kthreads can linger in the VM process's cgroup for sometime
> after KVM terminates the VM process.

Why is it a problem? And how long are we talking about?

> A VM process can terminate between the time window of exit_mm() to
> cgroup_exit(), leaving only worker kthreads in the cgroup.

Even kthreads should eventually have PF_EXITING set, they shouldd be
treated as "user-space" zombies by cgroups, i.e. mostly invisible (e.g.
it doesn't prevent rmdir'ing the cgroup).

(And after the last task_struct reference is gone, the cgroup structs
can be released too. Maybe the cause is holding the reference to the KVM
worker thread somewhere for too long.)

> Moving worker kthreads back to the original cgroup (kthreadd_task's
> cgroup) makes sure that cgroup is empty as soon as the main VM process
> is terminated.

BTW this used to be done for "user-space" tasks too (migrate to root
cgroup) but it was replaced with the less transactional "ignore zombies"
approach. So this change seems inconsistent.

Regards,
Michal