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[Benjamin Drung <bdrung@xxxxxxxxx>]

> I'm still not convinced that we don't want to make the change only for
> powerpc systemx - x86 systems AFAIK don't hotplug like drunken sailors.

We do on amd64. We run Ubuntu as virtual machine guests and do allow
hot-plugging CPUs. We do not have cgmanager installed.

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https://bugs.launchpad.net/bugs/1392176

Title:
  mounts cgroups unconditionally which causes undesired effects with cpu
  hotplug

Status in cgmanager package in Ubuntu:
  Confirmed
Status in linux package in Ubuntu:
  Confirmed
Status in systemd package in Ubuntu:
  Incomplete

Bug description:
  == Comment: #0 - Preeti U. Murthy <preeti.murthy@xxxxxxxxxx> - 2014-10-20 04:40:12 ==
  ---Problem Description---
  Systemd mounts cgroups explicitly every boot. Since the user had no say in it, undesired consequences are observed in reaction to cpu hotplug operations.  Here is how.

  Systemd moves the tasks to the cgroup mounted by it. This cgroup automatically becomes the child of the root cgroup which is present by default. The children cgroups are not expected to remember their configured cpusets after hotplug operations in the kernel. Hence when cpus are taken offline and brought back online they are no longer used for load balancing of tasks and hence remain unused. 
     This is an undesired consequence because the user had not even asked for cgroups to be mounted, yet is not able to use the full capacity of the system.

  Only when the user himself creates cgroup hierarchies, should he be
  exposed to the side effects of cpu hotplug on cpusets. Else all online
  cpus must be made available to him which is not happening since
  systemd mounts cgroups on every boot.

  Hence please revert this feature or provide an explaination as to why this is being done.
   
  ---uname output---
  Linux tul181p1 3.16.0-18-generic #25-Ubuntu SMP Fri Sep 26 02:39:53 UTC 2014 ppc64le ppc64le ppc64le GNU/Linux
   
  Machine Type = Tuleta 8286-42A 
   ---Debugger---
  A debugger was configured, however the system did not enter into the debugger
   
  ---Steps to Reproduce---
   $ taskset -p $$
  $ 0-127
  $ echo 0 > /sys/devices/system/cpu/cpu7/online
  $ taskset -p $$
  $ 0-6,8-127
  $ echo 1 > /sys/devices/system/cpu/cpu7/online
  $ taskset -p $$
  $ 0-6,8-127
   
   
  Userspace tool common name: systemd 
   
  The userspace tool has the following bit modes: 64-bit 

  Userspace rpm: systemd_208-8ubuntu8_ppc64el.deb

  Userspace tool obtained from project website:   208-8ubuntu8

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