linux-kernel - Re: [PATCH v9 4/7] cgroup/cpuset: Add a new isolated cpus.partition type

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Date:   Wed, 12 Jan 2022 10:40:01 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <shuah@...nel.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, Phil Auld <pauld@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Michal Koutný <mkoutny@...e.com>
Subject: Re: [PATCH v9 4/7] cgroup/cpuset: Add a new isolated cpus.partition
 type


On 1/12/22 10:21, Peter Zijlstra wrote:
> On Sun, Dec 05, 2021 at 01:32:17PM -0500, Waiman Long wrote:
>> Cpuset v1 uses the sched_load_balance control file to determine if load
>> balancing should be enabled.  Cpuset v2 gets rid of sched_load_balance
>> as its use may require disabling load balancing at cgroup root.
>>
>> For workloads that require very low latency like DPDK, the latency
>> jitters caused by periodic load balancing may exceed the desired
>> latency limit.
>>
>> When cpuset v2 is in use, the only way to avoid this latency cost is to
>> use the "isolcpus=" kernel boot option to isolate a set of CPUs. After
>> the kernel boot, however, there is no way to add or remove CPUs from
>> this isolated set. For workloads that are more dynamic in nature, that
>> means users have to provision enough CPUs for the worst case situation
>> resulting in excess idle CPUs.
>>
>> To address this issue for cpuset v2, a new cpuset.cpus.partition type
>> "isolated" is added which allows the creation of a cpuset partition
>> without load balancing. This will allow system administrators to
>> dynamically adjust the size of isolated partition to the current need
>> of the workload without rebooting the system.
> you can, ofcourse, create lots of 1 cpu partitions, which is effectively
> what you're doing, except there was a problem with that which you also
> forgot to mention.

Yes, that is a possible workaround. However, it makes cgroup management 
much harder especially in the cgroup v2 environment where multiple 
controllers are likely to be enabled in the same cgroup.

Cheers,
Longman