Re: [PATCH v2 1/1] osdep: asynchronous teardown for shutdown on Linux

[Daniel P . Berrangé]

On Wed, Aug 03, 2022 at 07:31:41PM +0200, Claudio Imbrenda wrote:
> This patch adds support for asynchronously tearing down a VM on Linux.
> 
> When qemu terminates, either naturally or because of a fatal signal,
> the VM is torn down. If the VM is huge, it can take a considerable
> amount of time for it to be cleaned up. In case of a protected VM, it
> might take even longer than a non-protected VM (this is the case on
> s390x, for example).
> 
> Some users might want to shut down a VM and restart it immediately,
> without having to wait. This is especially true if management
> infrastructure like libvirt is used.
> 
> This patch implements a simple trick on Linux to allow qemu to return
> immediately, with the teardown of the VM being performed
> asynchronously.
> 
> If the new commandline option -async-teardown is used, a new process is
> spawned from qemu at startup, using the clone syscall, in such way that
> it will share its address space with qemu.
> 
> The new process will then simpy wait until qemu terminates, and then it
> will exit itself.
> 
> This allows qemu to terminate quickly, without having to wait for the
> whole address space to be torn down. The teardown process will exit
> after qemu, so it will be the last user of the address space, and
> therefore it will take care of the actual teardown.
> 
> The teardown process will share the same cgroups as qemu, so both
> memory usage and cpu time will be accounted properly.
> 
> This feature can already be used with libvirt by adding the following
> to the XML domain definition:
> 
>   <commandline xmlns="http://libvirt.org/schemas/domain/qemu/1.0";>
>   <arg value='-async-teardown'/>
>   </commandline>

How does this work in practice ?  Libvirt should be blocking until
all processes in the cgroup have exited, including this cloned
child process.

With regards,
Daniel
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