On Thu, 4 Aug 2022 17:41:01 +0100 Daniel P. Berrangé <berra...@redhat.com> wrote:
> On Thu, Aug 04, 2022 at 04:49:29PM +0200, Claudio Imbrenda wrote: > > On Thu, 4 Aug 2022 09:29:39 +0100 > > Daniel P. Berrangé <berra...@redhat.com> wrote: > > > > > On Wed, Aug 03, 2022 at 06:34:45PM +0100, Daniel P. Berrangé 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 > > > > all processes in the cgroup have exited, including this cloned > > > > child process. > > > > > > Also, have you disabled use of seccomp with QEMU when testing this, > > > as the seccomp filter that libivrt enables is supposed to block > > > any use of clone() except for the creation of threads. > > > > it was just a vanilla libvirt 8.0.0 as found on ubuntu 22.04; I have no > > idea how it is configured by default > > Ok, so the reason it is working is because the extra process is > cloned() right in middle of processing argv. This is before the > seccomp filter is applied to the process, so clone() is not blocked. > > One think I note about this in practice is that (unsurprisingly) > if you do a process listing, users now see 2 QEMU processes instead > of one. > > I wonder if we should consider overwriting argv in the child > process with "[qemu async teardown]" to give users a hint as to > why this duplicate process exists. sounds like a good idea > > With regards, > Daniel
