[SAC] [Hosting] Hypervisor reboots on production Ganeti cluster

Lance Albertson lance at osuosl.org
Thu Sep 6 08:31:55 PDT 2018

Hi all,

I wanted to send you an update on this situation as it seems to keep
happening. Unfortunately it doesn't seem that RHEL has released a new
kernel which resolves this issue. Over the past few days I've been
rebooting nodes to apply the L1TF vulnerability fixes which of course means
these nodes are now running the kernel which has this bug. This has made
the cluster more unstable unfortunately causing multiple reboots a day.

I'm currently working on deploying a new mainline kernel to mitigate this
issue and hope to have it deployed by the end of today.

On Mon, Aug 27, 2018 at 1:01 PM Lance Albertson <lance at osuosl.org> wrote:

> All,
> We've been having issues with our Ganeti cluster hypervisors randomly
> rebooting. So far we've had this happen four times within the last month
> (including this morning) at the following times (in UTC) and duration:
> Event Start Time Event End Time Event Duration
> 2018-07-27 10:25:13 2018-07-27 10:29:29 0d 0h 4m 16s
> 2018-07-28 17:03:19 2018-07-28 17:06:57 0d 0h 3m 38s
> 2018-08-23 08:14:07 2018-08-23 08:18:00 0d 0h 3m 53s
> 2018-08-27 11:46:06 2018-08-27 11:48:44 0d 0h 2m 38s
> The kernel log points to this [1] upstream issue for RHEL and also
> reported on the CentOS forum [2] (We run CentOS 7 on our servers).
> [1] https://access.redhat.com/solutions/3432391
> [2] https://www.centos.org/forums/viewtopic.php?t=67170
> It seems the solution is "In progress" so I expect it to be included in
> the next released kernel in CentOS. Unfortunately the workaround is to
> either use an older kernel or just wait. As of right now we are going to
> opt to wait until the next release happens however if we see increased
> instability we'll look into using an older kernel.
> So far this only affects our Ganeti cluster and hasn't been triggered in
> either of our OpenStack clusters (x86 & ppc64le) as of yet.
> For some of our older VMs which boot using an external kernel we build,
> we've been hitting a different problem with the dracut dropping to an
> emergency shell if there are any fsck warnings on boot. We've had to
> manually fsck those filesystems and reboot the VMs to bring them back
> online. We're hoping to move away from these external kernels soon so let
> us know if you'd like to do that. In the meantime, we're going to look and
> see what changes we can make with the initrd so that it doesn't drop into
> the emergency shell and let's the OS do the fsck instead. We recently
> updated those external kernels to use an initrd which is why this has
> started to happen.
> The RH solution page has the following upstream commit message which
> explains why this bug is happening in the kernel:
>     Because we drop cpu_base->lock around calling hrtimer::function, it is
>     possible for hrtimer_start() to come in between and enqueue the timer.
>     If hrtimer::function then returns HRTIMER_RESTART we'll hit the BUG_ON
>     because HRTIMER_STATE_ENQUEUED will be set.
>     Since the above is a perfectly valid scenario, remove the BUG_ON and
>     make the enqueue_hrtimer() call conditional on the timer not being
>     enqueued already.
>     NOTE: in that concurrent scenario its entirely common for both sites
>     to want to modify the hrtimer, since hrtimers don't provide
>     serialization themselves be sure to provide some such that the
>     hrtimer::function and the hrtimer_start() caller don't both try and
>     fudge the expiration state at the same time.
>     To that effect, add a WARN when someone tries to forward an already
>     enqueued timer, the most common way to change the expiry of self
>     restarting timers. Ideally we'd put the WARN in everything modifying
>     the expiry but most of that is inlines and we don't need the bloat.
> If you have any questions or concerns please let us know.
> Thanks!
> --
> Lance Albertson
> Director
> Oregon State University | Open Source Lab

Lance Albertson
Oregon State University | Open Source Lab
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