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

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Date:   Wed, 10 Nov 2021 10:20:29 -0500
From:   Waiman Long <longman@...hat.com>
To:     Felix Moessbauer <felix.moessbauer@...mens.com>
Cc:     akpm@...ux-foundation.org, cgroups@...r.kernel.org, corbet@....net,
        frederic@...nel.org, guro@...com, hannes@...xchg.org,
        juri.lelli@...hat.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        lizefan.x@...edance.com, mkoutny@...e.com, mtosatti@...hat.com,
        pauld@...hat.com, peterz@...radead.org, shuah@...nel.org,
        tj@...nel.org, jan.kiszka@...mens.com, henning.schild@...mens.com
Subject: Re: [PATCH v8 0/6] cgroup/cpuset: Add new cpuset partition type &
 empty effecitve cpus


On 11/10/21 06:13, Felix Moessbauer wrote:
> 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).

I believe the "isolated" type will have more benefit on non PREEMPT_RT 
kernel. Anyway, having the "isolated" type is just the first step. It 
should be equivalent to "isolcpus=domain". There are other patches 
floating that attempt to move some of the isolcpus=nohz features into 
cpuset as well. It is not there yet, but we should be able to have 
better dynamic cpu isolation down the road.

Cheers,
Longman