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SubjectRe: [PATCH v9 3/7] cpuset: Add cpuset.sched.load_balance flag to v2
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On 05/31/2018 08:26 AM, Peter Zijlstra wrote:
> On Tue, May 29, 2018 at 09:41:30AM -0400, Waiman Long wrote:
>> The sched.load_balance flag is needed to enable CPU isolation similar to
>> what can be done with the "isolcpus" kernel boot parameter. Its value
>> can only be changed in a scheduling domain with no child cpusets. On
>> a non-scheduling domain cpuset, the value of sched.load_balance is
>> inherited from its parent. This is to make sure that all the cpusets
>> within the same scheduling domain or partition has the same load
>> balancing state.
>>
>> This flag is set by the parent and is not delegatable.
>> + cpuset.sched.domain_root
>> + 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 "1" (on). This flag is set by the parent
>> + and is not delegatable.
>> +
>> + If set, it indicates that the current cgroup is the root of a
>> + new scheduling domain or partition that comprises itself and
>> + all its descendants except those that are scheduling domain
>> + roots themselves and their descendants. The root cgroup is
>> + always a scheduling domain root.
>> +
>> + 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 "cpuset.cpus" is not empty and the list of CPUs are
>> + exclusive, i.e. they are not shared by any of its siblings.
>> + 2) The parent cgroup is also a scheduling domain root.
>> + 3) There is no child cgroups with cpuset enabled. This is
>> + for eliminating corner cases that have to be handled if such
>> + a condition is allowed.
>> +
>> + 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.
>> + Further changes made to "cpuset.cpus" is allowed as long as
>> + the first condition above is still true.
>> +
>> + A parent scheduling domain root cgroup cannot distribute all
>> + its CPUs to its child scheduling domain root cgroups unless
>> + its load balancing flag is turned off.
>> +
>> + cpuset.sched.load_balance
>> + 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 "1" (on). This flag is set by the parent
>> + and is not delegatable. It is on by default in the root cgroup.
>> +
>> + When it is on, tasks within this cpuset will be load-balanced
>> + by the kernel scheduler. Tasks will be moved from CPUs with
>> + high load to other CPUs within the same cpuset with less load
>> + periodically.
>> +
>> + When it is off, there will be no load balancing among CPUs on
>> + this cgroup. Tasks will stay in the CPUs they are running on
>> + and will not be moved to other CPUs.
>> +
>> + The load balancing state of a cgroup can only be changed on a
>> + scheduling domain root cgroup with no cpuset-enabled children.
>> + All cgroups within a scheduling domain or partition must have
>> + the same load balancing state. As descendant cgroups of a
>> + scheduling domain root are created, they inherit the same load
>> + balancing state of their root.
> I still find all that a bit weird.
>
> So load_balance=0 basically changes a partition into a
> 'fully-partitioned partition' with the seemingly random side-effect that
> now sub-partitions are allowed to consume all CPUs.

Are you suggesting that we should allow sub-partition to consume all the
CPUs no matter the load balance state? I can live with that if you think
it is more logical.

> The rationale, only given in the Changelog above, seems to be to allow
> 'easy' emulation of isolcpus.
>
> I'm still not convinced this is a useful knob to have. You can do
> fully-partitioned by simply creating a lot of 1 cpu parititions.

That is certainly true. However, I think there are some additional
overhead in the scheduler side in maintaining those 1-cpu partitions. Right?

> So this one knob does two separate things, both of which seem, to me,
> redundant.
>
> Can we please get better rationale for this?

I am fine getting rid of the load_balance flag if this is the consensus.
However, we do need to come up with a good migration story for those
users that need the isolcpus capability. I think Mike was the one asking
for supporting isolcpus. So Mike, what is your take on that.

Cheers,
Longman


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