Re: [PATCH V2] cpuset: Add knob to make allowed masks hotplug invariant on legacy hierarchy

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

On 04/11/2015 02:05 PM, Peter Zijlstra wrote:
> On Fri, Apr 10, 2015 at 07:41:52PM +0530, Preeti U Murthy wrote:
>> The cpus_allowed and mems_allowed masks of a cpuset get overwritten
>> after each hotplug operation on the legacy hierarchy of cgroups so as to
>> remain in sync with the online mask. But there are use cases which
>> expect user configured masks to remain unchanged.
>>
>> For instance, when hotplugged out CPUs are brought back online, they
>> remain idle with none of the existing tasks allowed to run on them since
>> the cpus_allowed mask was overwritten to not include them when they were
>> offlined.
>>
>> We cannot change the legacy hierarchy design now to keep the allowed
>> masks hotplug invariant since it is a user visible change. It was
>> suggested instead to add a knob in the root cpuset directory which
>> allows the user to specify if he wants the user configured masks to be
>> hotplug invariant [1]. This knob will enforce the choice throughout the
>> hierarchy. If the knob is set, the allowed maks will not be varied on
>> hotplug. It is also to to be noted that this knob will appear in the
>> root cgroup mounted on the legacy hierarchy alone since the default
>> hierarchy does not overwrite the allowed masks anyway.
>>
>> Having said this, there are fair reasons to argue that the kernel is not
>> responsible for taking care of user configurations in the face of
>> hotplug. But one of the consequences of the current legacy hierarchy
>> design, is that CPUs are left out from being used at all on online
>> operations. The reason for this is not very obvious at first and several
>> users have raised the issue as a bug. Hence the patch was strongly
>> called for.
>>
>> Moreover the default hierarchy keeps the allowed masks hotplug invariant
>> too. So the patch is not bringing about a fundamental change in the
>> design of cgroups.
> 
> What you've not explained is why you can use this knob but not use the
> other new mode?

It is claimed that the existing software which work with the legacy
hierarchy break when cgroups are mounted on the default hierarchy due to
poor backward compatibility. I will include this point in the changelog.

Thanks

Regards
Preeti U Murthy

>