Message-Id: <20210811030607.13824-6-longman@redhat.com>
Date:   Tue, 10 Aug 2021 23:06:06 -0400
From:   Waiman Long <longman@...hat.com>
To:     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>
Cc:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, Phil Auld <pauld@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Michal Koutný <mkoutny@...e.com>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH v4 5/6] cgroup/cpuset: Update description of cpuset.cpus.partition in cgroup-v2.rst

Update Documentation/admin-guide/cgroup-v2.rst on the newly introduced
"isolated" cpuset partition type as well as the ability to create
non-top cpuset partition with no cpu allocated to it.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 Documentation/admin-guide/cgroup-v2.rst | 104 +++++++++++++++---------
 1 file changed, 64 insertions(+), 40 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 5c7377b5bd3e..a8faaba1950e 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -2080,8 +2080,9 @@ Cpuset Interface Files
 	It accepts only the following input values when written to.
 
 	  ========	================================
-	  "root"	a partition root
-	  "member"	a non-root member of a partition
+	  "member"	Non-root member of a partition
+	  "root"	Partition root
+	  "isolated"	Partition root without load balancing
 	  ========	================================
 
 	When set to be a partition root, the current cgroup is the
@@ -2090,9 +2091,14 @@ Cpuset Interface Files
 	partition roots themselves and their descendants.  The root
 	cgroup is always a partition root.
 
-	There are constraints on where a partition root can be set.
-	It can only be set in a cgroup if all the following conditions
-	are true.
+	When set to "isolated", the CPUs in that partition root will
+	be in an isolated state without any load balancing from the
+	scheduler.  Tasks in such a partition must be explicitly bound
+	to each individual CPU.
+
+	There are constraints on where a partition root can be set
+	("root" or "isolated").  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.
@@ -2103,51 +2109,69 @@ Cpuset Interface Files
 	   eliminating corner cases that have to be handled if such a
 	   condition is allowed.
 
-	Setting it to partition root will take the CPUs away from the
-	effective CPUs of the parent cgroup.  Once it is set, this
+	Setting it to a partition root will take the CPUs away from
+	the effective CPUs of the parent cgroup.  Once it is set, this
 	file cannot be reverted back to "member" 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.
+	A parent partition may distribute all its CPUs to its child
+	partitions as long as it is not the root cgroup and there is no
+	task directly associated with that parent partition.  Otherwise,
+	there must be at least one cpu left in the parent partition.
+	A new task cannot be moved to a partition root with no effective
+	cpu.
+
+	Once becoming a partition root, changes to "cpuset.cpus"
+	is generally allowed as long as the first condition above
+	(cpu exclusivity rule) is true.
+
+	Sometimes, changes to "cpuset.cpus" or cpu hotplug may cause
+	the state of the partition root to become invalid when the
+	other constraints of partition root are violated.  Therefore,
+	it is recommended that users should always set "cpuset.cpus"
+	to the proper value first before enabling partition.  In case
+	"cpuset.cpus" has to be modified after partition is enabled,
+	users should check the state of "cpuset.cpus.partition" after
+	making change to it to make sure that the partition is still
+	valid.
+
+	On read, the "cpuset.cpus.partition" file can show the following
+	values.
 
 	  ==============	==============================
 	  "member"		Non-root member of a partition
 	  "root"		Partition root
+	  "isolated"		Partition root without load balancing
 	  "root invalid"	Invalid 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 invalid 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 itself.  In this
-	case, it is not a real partition even though the restriction
-	of the first partition root condition above will still apply.
-	The cpu affinity of all the tasks in the cgroup will then be
-	associated with CPUs in the nearest ancestor partition.
-
-	An invalid 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 cpu
-	affinity of all the tasks in the formerly invalid partition
-	will be associated to the CPUs of the newly formed partition.
-	Changing the partition state of an invalid partition root to
-	"member" is always allowed even if child cpusets are present.
+	A partition root becomes invalid if all the CPUs requested in
+	"cpuset.cpus" become unavailable.  This can happen if all the
+	CPUs have been offlined, or the state of an ancestor partition
+	root become invalid.  In this case, it is not a real partition
+	even though the restriction of the cpu exclusivity rule will
+	still apply.  The cpu affinity of all the tasks in the cgroup
+	will then be associated with CPUs in the nearest ancestor
+	partition.
+
+	In the special case of a parent partition competing with a child
+	partition for the only CPU left, the parent partition wins and
+	the child partition becomes invalid.
+
+	An invalid partition root can be transitioned back to a real
+	partition root if at least one of the requested CPUs become
+	available again. In this case, the cpu affinity of all the tasks
+	in the formerly invalid partition will be associated to the CPUs
+	of the newly formed partition.	Changing the partition state of
+	an invalid partition root to "member" is always allowed even if
+	child cpusets are present. However changing a partition root back
+	to member will not be allowed if child partitions are present.
+
+	Poll and inotify events are triggered whenever the state
+	of "cpuset.cpus.partition" changes.  That includes changes
+	caused by write to "cpuset.cpus.partition" and cpu hotplug.
+	This will allow a user space agent to monitor changes caused
+	by hotplug events.
 
 
 Device controller
-- 
2.18.1

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