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

[Claudio Imbrenda]

On Wed, 3 Aug 2022 18:34:45 +0100
Daniel P. Berrangé <berrange@redhat.com> wrote:

> 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

I don't know the inner details of how libvirt works..

> all processes in the cgroup have exited, including this cloned
> child process.

..but I tested it and it works

my impression is that libvirt by default is only waiting for the
main qemu process.

the only issue I have found is the log file, which stays open as long
as some file descriptors (which the cloned process inherits from the
main qemu process) stay open. A new VM cannot be started if its log file
is still open by the logger process. The close_range() call solves the
issue.

> 
> With regards,
> Daniel