Re: [PATCH v4 1/1] os-posix: asynchronous teardown for shutdown on Linux

[Paolo Bonzini <pbonzini@redhat.com>]

On 8/12/22 15:34, 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 have the
> name "cleanup/<QEMU_PID>". It will wait until qemu terminates
> completely, 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 cleanup 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 cleanup
> process will share the same cgroups as qemu, so both memory usage and
> cpu time will be accounted properly.
> 
> If possible, close_range will be used in the cleanup process to close
> all open file descriptors. If it is not available or if it fails, /proc
> will be used to determine which file descriptors to close.
> 
> If the cleanup process is forcefully killed with SIGKILL before the
> main qemu process has terminated completely, the mechanism is defeated
> and the teardown will not be asynchronous.
> 
> This feature can already be used with libvirt by adding the following
> to the XML domain definition to pass the parameter to qemu directly:
> 
>    <commandline xmlns="http://libvirt.org/schemas/domain/qemu/1.0">
>    <arg value='-async-teardown'/>
>    </commandline>
> 
> Signed-off-by: Claudio Imbrenda<imbrenda@linux.ibm.com>
> Reviewed-by: Murilo Opsfelder Araujo<muriloo@linux.ibm.com>
> Tested-by: Murilo Opsfelder Araujo<muriloo@linux.ibm.com>
> ---

Nice trick indeed!

The only question I have is whether it would make sense to do this in 
Libvirt instead.

Having a new independent one-off option like this is not great, but I 
think it's fine because it's not a very reusable grouping.

Paolo