Date:   Wed, 12 Jan 2022 16:21:00 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Shuah Khan <shuah@kernel.org>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kselftest@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>, Phil Auld <pauld@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
Subject: Re: [PATCH v9 4/7] cgroup/cpuset: Add a new isolated cpus.partition
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References: <20211205183220.818872-1-longman@redhat.com>
 <20211205183220.818872-5-longman@redhat.com>
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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.