Date:   Tue, 2 May 2023 17:26:17 -0400
From:   Waiman Long <longman@...hat.com>
To:     Michal Koutný <mkoutny@...e.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>, 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 5/2/23 14:01, Michal Koutný wrote:
> Hello.
>
> The previous thread arrived incomplete to me, so I respond to the last
> message only. Point me to a message URL if it was covered.
>
> On Fri, Apr 14, 2023 at 03:06:27PM -0400, Waiman Long <longman@...hat.com> wrote:
>> Below is a draft of the new cpuset.cpus.reserve cgroupfs file:
>>
>>    cpuset.cpus.reserve
>>          A read-write multiple values file which exists on all
>>          cpuset-enabled cgroups.
>>
>>          It lists the reserved CPUs to be used for the creation of
>>          child partitions.  See the section on "cpuset.cpus.partition"
>>          below for more information on cpuset partition.  These reserved
>>          CPUs should be a subset of "cpuset.cpus" and will be mutually
>>          exclusive of "cpuset.cpus.effective" when used since these
>>          reserved CPUs cannot be used by tasks in the current cgroup.
>>
>>          There are two modes for partition CPUs reservation -
>>          auto or manual.  The system starts up in auto mode where
>>          "cpuset.cpus.reserve" will be set automatically when valid
>>          child partitions are created and users don't need to touch the
>>          file at all.  This mode has the limitation that the parent of a
>>          partition must be a partition root itself.  So child partition
>>          has to be created one-by-one from the cgroup root down.
>>
>>          To enable the creation of a partition down in the hierarchy
>>          without the intermediate cgroups to be partition roots,
> Why would be this needed? Owning a CPU (a resource) must logically be
> passed all the way from root to the target cgroup, i.e. this is
> expressed by valid partitioning down to given level.
>
>>          one
>>          has to turn on the manual reservation mode by writing directly
>>          to "cpuset.cpus.reserve" with a value different from its
>>          current value.  By distributing the reserve CPUs down the cgroup
>>          hierarchy to the parent of the target cgroup, this target cgroup
>>          can be switched to become a partition root if its "cpuset.cpus"
>>          is a subset of the set of valid reserve CPUs in its parent.
> level n
> `- level n+1
>     cpuset.cpus	// these are actually configured by "owner" of level n
>     cpuset.cpus.partition // similrly here, level n decides if child is a partition
>
> I.e. what would be level n/cpuset.cpus.reserve good for when it can
> directly control level n+1/cpuset.cpus?

In the new scheme, the available cpus are still directly passed down to 
a descendant cgroup. However, isolated CPUs (or more generally CPUs 
dedicated to a partition) have to be exclusive. So what the 
cpuset.cpus.reserve does is to identify those exclusive CPUs that can be 
excluded from the effective_cpus of the parent cgroups before they are 
claimed by a child partition. Currently this is done automatically when 
a child partition is created off a parent partition root. The new scheme 
will break it into 2 separate steps without the requirement that the 
parent of a partition has to be a partition root itself.

Cheers,
Longman

claimed by a partition and will be excluded from the effective_cpus of 
the parent

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