On Mon, Dec 06, 2021 at 12:43:12PM +0100, Claudio Imbrenda wrote: > On Mon, 6 Dec 2021 11:21:10 +0000 > Daniel P. Berrangé <berra...@redhat.com> wrote: > > > On Mon, Dec 06, 2021 at 12:06:11PM +0100, 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 using the clone syscall, so that it will share its > > > address space with qemu. > > > > > > The new process will then 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. > > > > If this suggested workaround has any benefit to the shutdown of a VM > > with libvirt, then it is a bug in libvirt IMHO. > > > > When libvirt tears down a QEMU VM, it should be waiting for *every* > > process in the VM's cgroup to be terminated before it reports that > > the VM is shutoff. IOW, the fact that this workaround lets the main > > QEMU process exit quickly should not matter. libvirt should still > > be blocked in exactly the same place in its code, waiting for the > > "async" cleanup process to exit. IOW, this should not be async at > > all from libvirt's POV. > > interesting, I did not know that about libvirt. > > maybe libvirt could be fixed/improved to allow this patch to work?
That would not be desirable. When libvirt reports a VM as shutoff, it is expected that all resources associated with the VM huave been fully released, such that they are available for launching a new VM. We can't allow resources to be asynchronously released as that violates app's expectation that the resources are released once the VM is shutoff. > surely without this patch an asynchronous teardown will not be possible > at all I appreciate that the current slow teardown is a pain, but async teardown does not sound like an appealing alternative given that the app can't use the resources again until the teardown is complete. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|