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

From: Waiman Long
Date: Sun May 28 2023 - 17:19:47 EST

On 5/22/23 15:49, Tejun Heo wrote:
Hello, Waiman.

Sorry for the late reply as I had been off for almost 2 weeks due to PTO.

On Sun, May 07, 2023 at 09:03:44PM -0400, Waiman Long wrote:
    A read-write multiple values file which exists only on root

    It lists all the CPUs that are reserved for adjacent and remote
    partitions created in the system.  See the next section for
    more information on what an adjacent or remote partitions is.

    Creation of adjacent partition does not require touching this
    control file as CPU reservation will be done automatically.
    In order to create a remote partition, the CPUs needed by the
    remote partition has to be written to this file first.

    A "+" prefix can be used to indicate a list of additional
    CPUs that are to be added without disturbing the CPUs that are
    originally there.  For example, if its current value is "3-4",
    echoing ""+5" to it will change it to "3-5".

    Once a remote partition is destroyed, its CPUs have to be
    removed from this file or no other process can use them.  A "-"
    prefix can be used to remove a list of CPUs from it.  However,
    removing CPUs that are currently used in existing partitions
    may cause those partitions to become invalid.  A single "-"
    character without any number can be used to indicate removal
    of all the free CPUs not allocated to any partitions to avoid
    accidental partition invalidation.
Why is the syntax different from .cpus? Wouldn't it be better to keep them
the same?

Unlike cpuset.cpus, cpuset.cpus.reserve is supposed to contains CPUs that are used in multiple partitions. Also automatic reservation of adjacent partitions can happen in parallel. That is why I think it will be safer if we allow incremental increase or decrease of reserve CPUs to be used for remote partitions. I will include this reasoning into the doc file.

    A read-write single value file which exists on non-root
    cpuset-enabled cgroups.  This flag is owned by the parent cgroup
    and is not delegatable.

    It accepts only the following input values when written to.

      ==========    =====================================
      "member"    Non-root member of a partition
      "root"    Partition root
      "isolated"    Partition root without load balancing
      ==========    =====================================

    A cpuset partition is a collection of cgroups with a partition
    root at the top of the hierarchy and its descendants except
    those that are separate partition roots themselves and their
    descendants.  A partition has exclusive access to the set of
    CPUs allocated to it.  Other cgroups outside of that partition
    cannot use any CPUs in that set.

    There are two types of partitions - adjacent and remote.  The
    parent of an adjacent partition must be a valid partition root.
    Partition roots of adjacent partitions are all clustered around
    the root cgroup.  Creation of adjacent partition is done by
    writing the desired partition type into "cpuset.cpus.partition".

    A remote partition does not require a partition root parent.
    So a remote partition can be formed far from the root cgroup.
    However, its creation is a 2-step process.  The CPUs needed
    by a remote partition ("cpuset.cpus" of the partition root)
    has to be written into "cpuset.cpus.reserve" of the root
    cgroup first.  After that, "isolated" can be written into
    "cpuset.cpus.partition" of the partition root to form a remote
    isolated partition which is the only supported remote partition
    type for now.

    All remote partitions are terminal as adjacent partition cannot
    be created underneath it.
Can you elaborate this extra restriction a bit further?

Are you referring to the fact that only remote isolated partitions are supported? I do not preclude the support of load balancing remote partitions. I keep it to isolated partitions for now for ease of implementation and I am not currently aware of a use case where such a remote partition type is needed.

If you are talking about remote partition being terminal. It is mainly because it can be more tricky to support hierarchical adjacent partitions underneath it especially if it is not isolated. We can certainly support it if a use case arises. I just don't want to implement code that nobody is really going to use.

BTW, with the current way the remote partition is created, it is not possible to have another remote partition underneath it.

In general, I think it'd be really helpful if the document explains the
reasoning behind the design decisions. ie. Why is reserving for? What
purpose does it serve that the regular isolated ones cannot? That'd help
clarifying the design decisions.

I understand your concern. If you think it is better to support both types of remote partitions or hierarchical adjacent partitions underneath it for symmetry purpose, I can certain do that. It just needs to take a bit more time.