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Re: [libvirt] [PATCH 00/18] Re-arrange the way cgroups are setup



On Thu, Apr 04, 2013 at 06:38:29PM +0200, Viktor Mihajlovski wrote:
> On 04/04/2013 03:40 PM, Daniel P. Berrange wrote:
> > /sys/fs/cgroup
> > ├── cpu,cpuacct
> > │   ├── libvirt
> > │   │   ├── lxc
> > │   │   │   └── busy
> > │   │   └── qemu
> > │   │       └── vm1
> > │   │           ├── emulator
> > │   │           └── vcpu0
> 
> It's somehow off-topic but if you do a rework you might also consider a
> conceptual change wrt to $domain/emulator and $domain/vcpu* ...
> 
> Just today I was confronted with a race in
> qemuSetupCgroupForEmulator/virCgroupMoveTask on highly utilized system.
> The problem is that if a QEMU thread terminates during the move from 
> $domain/tasks to $domain/emulator/tasks the virCgroupAddTaskController
> call will fail resulting in a failure to start the domain.
> Another possible issue is that if new QEMU threads are spawned after
> the virCgroupGetValueStr call they will not be moved.

This seems easy enough to fix. Instead of starting in $domain/ and
moving them to $domain/emulator, we should just start QEMU in the
$domain/emulator directory right away.

> So, since the threads in $domain/tasks are 'hypervisor' threads anyway,
> shouldn't we get rid of the emulator directory altogether?

The problem is that any controls you apply at the $domain/ level,
will also affect the $domain/vcpuNN levels. To get controls that
are guaranteed to only affect the emulator threads, you need them
to be in a directory that is parallel  to the vcpuN directories,
not a parent of them.

> 
> -- 
> 
> Mit freundlichen Grüßen/Kind Regards
>    Viktor Mihajlovski
> 
> IBM Deutschland Research & Development GmbH
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