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

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Date:   Wed, 19 Jan 2022 08:30:43 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vipin Sharma <vipinsh@...gle.com>,
        Michal Koutný <mkoutny@...e.com>,
        seanjc@...gle.com, 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.

On Wed, Jan 19, 2022 at 07:02:53PM +0100, Paolo Bonzini wrote:
> On 1/18/22 21:39, Tejun Heo wrote:
> > So, these are normally driven by the !populated events. That's how everyone
> > else is doing it. If you want to tie the kvm workers lifetimes to kvm
> > process, wouldn't it be cleaner to do so from kvm side? ie. let kvm process
> > exit wait for the workers to be cleaned up.
> 
> It does.  For example kvm_mmu_post_init_vm's call to
> kvm_vm_create_worker_thread is matched with the call to
> kthread_stop in kvm_mmu_pre_destroy_vm.
> According to Vpin, the problem is that there's a small amount of time
> between the return from kthread_stop and the point where the cgroup
> can be removed.  My understanding of the race is the following:

Okay, this is because kthread_stop piggy backs on vfork_done to wait for the
task exit intead of the usual exit notification, so it only waits till
exit_mm(), which is uhh... weird. So, migrating is one option, I guess,
albeit a rather ugly one. It'd be nicer if we can make kthread_stop()
waiting more regular but I couldn't find a good existing place and routing
the usual parent signaling might be too complicated. Anyone has better
ideas?

Thanks.

-- 
tejun