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SubjectRe: [PATCH v8 2/6] cpuset: Add new v2 cpuset.sched.domain flag
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On 05/24/2018 11:41 AM, Peter Zijlstra wrote:
> On Thu, May 17, 2018 at 04:55:41PM -0400, Waiman Long wrote:
>> A new cpuset.sched.domain boolean flag is added to cpuset v2. This new
>> flag indicates that the CPUs in the current cpuset should be treated
>> as a separate scheduling domain.
> The traditional name for this is a partition.

Do you want to call it cpuset.sched.partition? That name sounds strange
to me.

>> This new flag is owned by the parent
>> and will cause the CPUs in the cpuset to be removed from the effective
>> CPUs of its parent.
> This is a significant departure from existing behaviour, but one I can
> appreciate. I don't immediately see something terribly wrong with it.
>
>> This is implemented internally by adding a new isolated_cpus mask that
>> holds the CPUs belonging to child scheduling domain cpusets so that:
>>
>> isolated_cpus | effective_cpus = cpus_allowed
>> isolated_cpus & effective_cpus = 0
>>
>> This new flag can only be turned on in a cpuset if its parent is either
>> root or a scheduling domain itself with non-empty cpu list. The state
>> of this flag cannot be changed if the cpuset has children.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>> Documentation/cgroup-v2.txt | 22 ++++
>> kernel/cgroup/cpuset.c | 237 +++++++++++++++++++++++++++++++++++++++++++-
>> 2 files changed, 256 insertions(+), 3 deletions(-)
>>
>> diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
>> index cf7bac6..54d9e22 100644
>> --- a/Documentation/cgroup-v2.txt
>> +++ b/Documentation/cgroup-v2.txt
>> @@ -1514,6 +1514,28 @@ Cpuset Interface Files
>> it is a subset of "cpuset.mems". Its value will be affected
>> by memory nodes hotplug events.
>>
>> + cpuset.sched.domain
>> + A read-write single value file which exists on non-root
>> + cpuset-enabled cgroups. It is a binary value flag that accepts
>> + either "0" (off) or a non-zero value (on).
> I would be conservative and only allow 0/1.

I stated that because echoing other integer value like 2 into the flag
file won't return any error. I will modify it to say just 0 and 1.

>> This flag is set
>> + by the parent and is not delegatable.
>> +
>> + If set, it indicates that the CPUs in the current cgroup will
>> + be the root of a scheduling domain. The root cgroup is always
>> + a scheduling domain. There are constraints on where this flag
>> + can be set. It can only be set in a cgroup if all the following
>> + conditions are true.
>> +
>> + 1) The parent cgroup is also a scheduling domain with a non-empty
>> + cpu list.
> Ah, so initially I was confused by the requirement for root to have it
> always set, but you'll allow child domains to steal _all_ CPUs, such
> that root ends up with an empty effective set?
>
> What about the (kernel) threads that cannot be moved out of the root
> group?

Actually, the current code won't allow you to take all the CPUs from a
scheduling domain cpuset with load balancing on. So there must be at
least 1 cpu left. You can take all away if load balancing is off.

>> + 2) The list of CPUs are exclusive, i.e. they are not shared by
>> + any of its siblings.
> Right.
>
>> + 3) There is no child cgroups with cpuset enabled.
>> +
>> + Setting this flag will take the CPUs away from the effective
>> + CPUs of the parent cgroup. Once it is set, this flag cannot be
>> + cleared if there are any child cgroups with cpuset enabled.
> This I'm not clear on. Why?
>
That is for pragmatic reason as it is easier to code this way. We could
remove this restriction but that will make the code more complex.

Cheers,
Longman


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