linux-kernel - Re: [PATCH v3 6/9] cgroup/cpuset: Add a new isolated cpus.partition type

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Date:   Tue, 27 Jul 2021 13:42:41 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Waiman Long <longman@...hat.com>
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>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Michal Koutný <mkoutny@...e.com>
Subject: Re: [PATCH v3 6/9] cgroup/cpuset: Add a new isolated cpus.partition
 type

On Tue, Jul 20, 2021 at 10:18:31AM -0400, Waiman Long wrote:
> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=TBD
> 
> commit 994fb794cb252edd124a46ca0994e37a4726a100
> Author: Waiman Long <longman@...hat.com>
> Date:   Sat, 19 Jun 2021 13:28:19 -0400
> 
>     cgroup/cpuset: Add a new isolated cpus.partition type
> 
>     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.
> 
>     Signed-off-by: Waiman Long <longman@...hat.com>
> 
> Signed-off-by: Waiman Long <longman@...hat.com>

Nice! And while we are adding a new ABI, can we take advantage of that and
add a specific semantic that if a new isolated partition matches a subset of
"isolcpus=", it automatically maps to it. This means that any further
modification to that isolated partition will also modify the associated
isolcpus= subset.

Or to summarize, when we create a new isolated partition, remove the associated
CPUs from isolcpus= ?

Thanks.