On Thu, Aug 04, 2022 at 07:56:49AM +0200, Claudio Imbrenda wrote: > On Wed, 3 Aug 2022 18:34:45 +0100 > Daniel P. Berrangé <berra...@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.
If true, that would be a bug that needs fixing and should not be relied on. > 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 -- |: 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 :|
