[ovs-dev] [PATCH 2/2] debian: force-reload-kmod while package upgrading.

[Ben Pfaff]

On Thu, Apr 11, 2013 at 04:40:13PM -0700, Ben Pfaff wrote:
> On Wed, Apr 10, 2013 at 03:41:23PM -0700, Gurucharan Shetty wrote:
> > Currently, when we upgrade openvswitch packages, we do restart
> > of userspace daemons. This does not replace the kernel module.
> > 
> > But almost everytime, we want to also use the new kernel module
> > that comes with the new version. This means that we need to
> > manually do a "force-reload-kmod". This step, along with reloading
> > kernel module also restarts userspace daemons. This gives us a
> > total of 2 restarts. This is quite expensive in a hypervisor with
> > hundreds of VMs sending real traffic. This also hurts the controller
> > as it gets 2 reconnections in a short amount of time.
> > 
> > With this patch, during a package upgrade, we will automatically do
> > a force-reload-kmod while openvswitch-switch is installed. Usually,
> > when openvswitch is upgraded, both kernel package and userspace
> > package is upgraded together with a "dpkg -i" on command line. This
> > results in both packages unpacked first and then configured. This
> > ensures that we use the new kernel module during the postinst of
> > openvswitch-switch package. One can also install the kernel package
> > first and then install the userspace package in 2 separate steps.
> > 
> > If anyone wants to just restart the userspace package instead of
> > force-reload-kmod, they can set the value of OVS_FORCE_RELOAD_KMOD=no
> > while installing the package.
> > Ex: OVS_FORCE_RELOAD_KMOD=no dpkg -i openvswitch-switch*
> > 
> > Signed-off-by: Gurucharan Shetty <gshetty at nicira.com>
> 
> Did you consider using dpkg "triggers" to accomplish this instead?
> Installing either openvswitch-switch or a kernel module package would
> fire the appropriate trigger, but the implementation of the triggers
> would only do either "force-reload-kmod" or "restart" even if both
> triggers had fired.

Guru told me off-list that he'd tried triggers but they didn't work
for this purpose.

Given that, the patch seems OK, but I worry about a few things:

        * We used to be very conservative about doing
          force-reload-kmod, doing it only when the user told us to,
          because we were worried that it would be disruptive.  I
          don't think we've heard about big problems with it, so maybe
          we can ignore that, but I thought I'd point it out just in
          case.

        * force-reload-kmod is no better than restart, and might be
          worse, when the kernel module hasn't actually changed.  For
          example, if dhclient is running, then it is definitely
          worse.

        * force-reload-kmod is definitely worse if the kernel module
          got removed.

I think we could mitigate the second and third problems by only doing
force-reload-kmod if the module has changed.  Maybe with logic that
compares the srcversion from "/sbin/modinfo openvswitch" against
/sys/module/openvswitch/srcversion, so that we only reload the module
if the on-disk version is different from the loaded version.

What do you think?

Thanks,

Ben.