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

[Waiman Long]

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@xxxxxxxxxx> 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