linux-kernel - Re: [RFC PATCH 0/5] cgroup/cpuset: A new "isolcpus" paritition

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Date:   Wed, 12 Apr 2023 20:55:55 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [RFC PATCH 0/5] cgroup/cpuset: A new "isolcpus" paritition

On 4/12/23 20:33, Tejun Heo wrote:
> Hello,
>
> On Wed, Apr 12, 2023 at 08:26:03PM -0400, Waiman Long wrote:
>>    If the "cpuset.cpus.isolated" isn't set, the existing rules applies. If it
>> is set, the new rule will be used.
>>
>> Does that look reasonable to you?
> Sounds a bit contrived. Does it need to be something defined in the root
> cgroup?

Yes, because we need to take away the isolated CPUs from the effective 
cpus of the root cgroup. So it needs to start from the root. That is 
also why we have the partition rule that the parent of a partition has 
to be a partition root itself. With the new scheme, we don't need a 
special cgroup to hold the isolated CPUs. The new root cgroup file will 
be enough to inform the system what CPUs will have to be isolated.

My current thinking is that the root's "cpuset.cpus.isolated" will start 
with whatever have been set in the "isolcpus" or "nohz_full" boot 
command line and can be extended from there but not shrank below that as 
there can be additional isolation attributes with those isolated CPUs.

Cheers,
Longman

> The only thing that's needed is that a cgroup needs to claim CPUs
> exclusively without using them, right? Let's say we add a new interface
> file, say, cpuset.cpus.reserve which is always exclusive and can be consumed
> by children whichever way they want, wouldn't that be sufficient? Then,
> there would be nothing to describe in the root cgroup.
>
> Thanks.
>