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

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Message-ID: <f81f4867-1fea-03fc-639a-2783fbd68abf@redhat.com>
Date:   Thu, 10 Jun 2021 15:21:51 -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 3:00 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.
> Also, can we change isolcpus to create a default cgroup hierarchy
> instead of being the fugly hack that it is? I really hate isolcpus with
> a passion, it needs to die.
>
That is probably doable assuming that we can allow cpuset v2 to have a 
non-load balanced partition.

Depending on which cpuset version is set up, we can automatically set up 
a isolated subdirectory under / to contain cpus that are isolated. 
However, that will be a follow-on patch after this one.

Cheers,
Longman