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[Bug 1413540] Re: qemu-kvm package enables KSM on VMs

To: kernel-packages@xxxxxxxxxxxxxxxxxxx
From: Chris J Arges <1413540@xxxxxxxxxxxxxxxxxx>
Date: Thu, 22 Jan 2015 16:46:08 -0000
Reply-to: Bug 1413540 <1413540@xxxxxxxxxxxxxxxxxx>
Sender: bounces@xxxxxxxxxxxxx

@serge-hallyn:

First step is to debug this issue and see if its a reasonable fix. If
that's possible then we should leave qemu alone.

Second if we want to disable KSM for nested VMs relying on a 'QEMU' cpu isn't sufficient if someone uses a different -cpu flag. We could do:
grep -q "Booting paravirtualized kernel on KVM"  /var/log/kern.log && echo 0 > /sys/kernel/mm/ksm/run
Or use tools like virt-what.

Regardless I'll work on step 1.

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https://bugs.launchpad.net/bugs/1413540

Title:
  qemu-kvm package enables KSM on VMs

Status in linux package in Ubuntu:
  Incomplete
Status in qemu package in Ubuntu:
  Confirmed

Bug description:
  When installing qemu-kvm on a VM, KSM is enabled.

  I have encountered this problem in trusty:$ lsb_release -a
  Distributor ID: Ubuntu
  Description:    Ubuntu 14.04.1 LTS
  Release:        14.04
  Codename:       trusty
  $ uname -a
  Linux juju-gema-machine-2 3.13.0-40-generic #69-Ubuntu SMP Thu Nov 13 17:53:56 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

  The way to see the behaviour:
  1) $ more /sys/kernel/mm/ksm/run
  0
  2) $ sudo apt-get install qemu-kvm
  3) $ more /sys/kernel/mm/ksm/run
  1

  To see the soft lockups, deploy a cloud on a virtualised env like ctsstack, run tempest on it, the compute nodes of the virtualised deployment will eventually stop responding with (run tempest 2 times at least):
   24096.072003] BUG: soft lockup - CPU#0 stuck for 23s! [qemu-system-x86:24791]
  [24124.072003] BUG: soft lockup - CPU#0 stuck for 23s! [qemu-system-x86:24791]
  [24152.072002] BUG: soft lockup - CPU#0 stuck for 22s! [qemu-system-x86:24791]
  [24180.072003] BUG: soft lockup - CPU#0 stuck for 22s! [qemu-system-x86:24791]
  [24208.072004] BUG: soft lockup - CPU#0 stuck for 22s! [qemu-system-x86:24791]
  [24236.072004] BUG: soft lockup - CPU#0 stuck for 22s! [qemu-system-x86:24791]
  [24264.072003] BUG: soft lockup - CPU#0 stuck for 22s! [qemu-system-x86:24791]

  I am not sure whether the problem is that we are enabling KSM on a VM
  or the problem is that nested KSM is not behaving properly. Either way
  I can easily reproduce, please contact me if you need further details.

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