linux-kernel - Re: [PATCH v14 10/12] cpuset: Add documentation about the new "cpuset.sched.partition" flag

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Date:   Tue, 6 Nov 2018 12:50:45 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kernel-team@...com, pjt@...gle.com, luto@...capital.net,
        Mike Galbraith <efault@....de>, torvalds@...ux-foundation.org,
        Roman Gushchin <guro@...com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Tom Hromatka <tom.hromatka@...cle.com>
Subject: Re: [PATCH v14 10/12] cpuset: Add documentation about the new
 "cpuset.sched.partition" flag

On Mon, Oct 15, 2018 at 04:29:35PM -0400, Waiman Long wrote:
> The cgroup-v2.rst file is updated to document the purpose of the new
> "cpuset.sched.partition" flag and how its usage.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  Documentation/admin-guide/cgroup-v2.rst | 66 +++++++++++++++++++++++++
>  1 file changed, 66 insertions(+)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 533e85cb851b..178cda473a26 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1686,6 +1686,72 @@ Cpuset Interface Files
>  
>  	Its value will be affected by memory nodes hotplug events.
>  
> +  cpuset.sched.partition
> +	A read-write single value file which exists on non-root
> +	cpuset-enabled cgroups.  It accepts either "0" (off) or "1"
> +	(on) when written to.  

> +     This flag is set and owned by the
> +	parent cgroup.

What does that mean? The parent cgroup doesn't 'set' anything at all.
The user will.

> +
> +	If set, it indicates that the current cgroup is the root of a
> +	new partition or scheduling domain that comprises itself and
> +	all its descendants except those that are separate partition
> +	roots themselves and their descendants.  The root cgroup is
> +	always a partition root.
> +
> +	There are constraints on where this flag can be set.  It can
> +	only be set in a cgroup if all the following conditions are true.
> +
> +	1) The "cpuset.cpus" is not empty and the list of CPUs are
> +	   exclusive, i.e. they are not shared by any of its siblings.
> +	2) The parent cgroup is a partition root.
> +	3) The "cpuset.cpus" is also a proper subset of the parent's
> +	   "cpuset.cpus.effective".
> +	4) There is no child cgroups with cpuset enabled.  This is for
> +	   eliminating corner cases that have to be handled if such a
> +	   condition is allowed.
> +
> +	Setting this flag will take the CPUs away from the effective
> +	CPUs of the parent cgroup.  Once it is set, this flag cannot
> +	be cleared if there are any child cgroups with cpuset enabled.
> +
> +	A parent partition cannot distribute all its CPUs to its
> +	child partitions.  There must be at least one cpu left in the
> +	parent partition.
> +
> +	Once becoming a partition root, changes to "cpuset.cpus" is
> +	generally allowed as long as the first condition above is true,
> +	the change will not take away all the CPUs from the parent
> +	partition and the new "cpuset.cpus" value is a superset of its
> +	children's "cpuset.cpus" values.

> +	Sometimes, external factors like changes to ancestors'
> +	"cpuset.cpus" or cpu hotplug can cause the state of the partition
> +	root to change.  On read, the "cpuset.sched.partition" file
> +	can show the following values.

Are those the only conditions under which that -1 can happen? Parent
taking away CPUs it previously granted and hotplug?

> +
> +	"0"  Not a partition root
> +	"1"  Partition root
> +	"-1" Erroneous partition root
> +
> +	It is a partition root if the first 2 partition root conditions
> +	above are true and at least one CPU from "cpuset.cpus" is
> +	granted by the parent cgroup.
> +
> +	A partition root can become an erroneous partition root if none
> +	of CPUs requested in "cpuset.cpus" can be granted by the parent
> +	cgroup or the parent cgroup is no longer a partition root.
> +	In this case, it is not a real partition even though the
> +	restriction of the first partition root condition above will
> +	still apply.  All the tasks in the cgroup will be migrated to
> +	the nearest ancestor partition.

Effectively or actual? Actual migrating tasks out of the cgroup is
irreversible.

> +	An erroneous partition root can be transitioned back to a real
> +	partition root if at least one of the requested CPUs can now be
> +	granted by its parent.	In this case, the tasks will be migrated
> +	back to the newly created partition.  Clearing the partition
> +	flag of an erroneous partition root is always allowed even if
> +	child cpusets are present.

So you need to clarify the above point (I think it is effectively),
because otherwise you don't know which tasks to put back.