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SubjectRe: [PATCH v8 2/6] cpuset: Add new v2 cpuset.sched.domain flag
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 <>
>> ---
>> 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.


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