linux-kernel - Re: [PATCH 2/5] cgroup/cpuset: Add new cpus.partition type with no load balancing

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Message-ID: <820aff72-fce2-ac2f-88e6-787249e04308@redhat.com>
Date:   Thu, 10 Jun 2021 15:16:29 -0400
From:   Waiman Long <llong@...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>
Subject: Re: [PATCH 2/5] cgroup/cpuset: Add new cpus.partition type with no
 load balancing

On 6/10/21 2:50 PM, Peter Zijlstra wrote:
> On Thu, Jun 03, 2021 at 05:24:13PM -0400, 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
>> "root-nolb" is added which allows the creation of a cpuset partition with
>> no load balancing. This will allow system administrators to dynamically
>> adjust the size of the no load balancing partition to the current need
>> of the workload without rebooting the system.
> I'm confused, why do you need this? Just create a parition for each cpu.
>
 From a management point of view, it is more cumbersome to do one cpu 
per partition. I have suggested this idea of 1 cpu per partition to the 
container developers, but they don't seem to like it.

Cheers,
Longman