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

[Claudio Imbrenda]

On Thu, 4 Aug 2022 17:41:01 +0100
Daniel P. Berrangé <berrange@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 <berrange@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