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SubjectRe: [PATCH v8 0/6] cgroup/cpuset: Add new cpuset partition type & empty effecitve cpus
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Hi Weiman,

> v8:
> - Reorganize the patch series and rationalize the features and
> constraints of a partition.
> - Update patch descriptions and documentation accordingly.
>
> v7:
> - Simplify the documentation patch (patch 5) as suggested by Tejun.
> - Fix a typo in patch 2 and improper commit log in patch 3.
>
> v6:
> - Remove duplicated tmpmask from update_prstate() which should fix the
> frame size too large problem reported by kernel test robot.
>
> This patchset makes four enhancements to the cpuset v2 code.
>
> Patch 1: Enable partition with no task to have empty cpuset.cpus.effective.
>
> Patch 2: Refining the features and constraints of a cpuset partition
> clarifying what changes are allowed.
>
> Patch 3: Add a new partition state "isolated" to create a partition
> root without load balancing. This is for handling intermitten workloads
> that have a strict low latency requirement.


I just tested this patch-series and can confirm that it works on 5.15.0-rc7-rt15 (PREEMT_RT).

However, I was not able to see any latency improvements when using
cpuset.cpus.partition=isolated.
The test was performed with jitterdebugger on CPUs 1-3 and the following cmdline:
rcu_nocbs=1-4 nohz_full=1-4 irqaffinity=0,5-6,11 intel_pstate=disable
On the other cpus, stress-ng was executed to generate load.

Just some more general notes:

Even with this new "isolated" type, it is still very tricky to get a similar
behavior as with isolcpus (as long as I don't miss something here):

Consider an RT application that consists of a non-rt thread that should be floating
and a rt-thread that should be placed in the isolated domain.
This requires cgroup.type=threaded on both cgroups and changes to the application
(threads have to be born in non-rt group and moved to rt-group).

Theoretically, this could be done externally, but in case the application sets the
affinity mask manually, you run into a timing issue (setting affinities to CPUs
outside the current cpuset.cpus results in EINVAL).

Best regards,
Felix Moessbauer
Siemens AG

> Patch 4: Enable the "cpuset.cpus.partition" file to show the reason
> that causes invalid partition like "root invalid (No cpu available
> due to hotplug)".
>
> Patch 5 updates the cgroup-v2.rst file accordingly. Patch 6 adds a new
> cpuset test to test the new cpuset partition code.

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