Re: [PATCH v8 0/6] cgroup/cpuset: Add new cpuset partition type & empty effecitve cpus

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.