[PATCH v3 0/2] docs: cgroup-v2: Correct cpuset CPU descriptions

[PATCH v3 0/2] docs: cgroup-v2: Correct cpuset CPU descriptions

[Guopeng Zhang]

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

The cpuset interface documentation says cpuset.cpus.isolated is empty
unless an isolated partition exists, omitting CPUs isolated at boot with
isolcpus=domain. Its cpuset.cpus.effective description also assumes CPUs
come from the immediate parent and are normally a subset of
cpuset.cpus, which is not always true for valid non-root partition roots.

Update both descriptions to match the existing behavior.

Changes since v2:
- Update the valid partition root example to show that an offline CPU
  can remain in cpuset.cpus.exclusive.effective while being excluded
  from cpuset.cpus.effective.
- Add the Reviewed-by tag to patch 2.
- Patch 1 is unchanged.

Guopeng Zhang (2):
  docs: cgroup-v2: Describe boot-isolated CPUs in cpuset.cpus.isolated
  docs: cgroup-v2: Describe effective CPUs of partition roots

 Documentation/admin-guide/cgroup-v2.rst | 62 ++++++++++++++++++++-----
 1 file changed, 51 insertions(+), 11 deletions(-)

-- 
2.43.0

[PATCH v3 1/2] docs: cgroup-v2: Describe boot-isolated CPUs in cpuset.cpus.isolated

[Guopeng Zhang]

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

cpuset.cpus.isolated is documented as empty when no isolated partition
exists. This is not true when CPUs are isolated at boot with
isolcpus=domain. cpuset_init() adds those CPUs to the isolated mask,
and isolcpus=<cpu-list> uses domain isolation by default. They remain
listed after a partition using them is released.

Document the boot-isolated CPUs as well.

Suggested-by: Ridong Chen <ridong.chen@linux.dev>
Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
---
 Documentation/admin-guide/cgroup-v2.rst | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 2ec582985b5b..18a01ce4c334 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -2626,8 +2626,9 @@ Cpuset Interface Files
 	A read-only and root cgroup only multiple values file.
 
 	This file shows the set of all isolated CPUs used in existing
-	isolated partitions. It will be empty if no isolated partition
-	is created.
+	isolated partitions or isolated at boot time via the "domain"
+	flag of "isolcpus". The latter remain listed even after any
+	partition using them is released.
 
   cpuset.cpus.partition
 	A read-write single value file which exists on non-root
-- 
2.43.0

[PATCH v3 2/2] docs: cgroup-v2: Describe effective CPUs of partition roots

[Guopeng Zhang]

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

The cpuset.cpus.effective description says that the effective CPUs are
granted by the parent and are normally a subset of cpuset.cpus. This
does not always hold for a valid non-root partition root. Its effective
CPUs are the active CPUs in cpuset.cpus.exclusive.effective after CPUs
assigned to valid child partition roots are removed.

If cpuset.cpus.exclusive differs from cpuset.cpus, the effective CPUs
may therefore be outside cpuset.cpus. Remote partitions also do not
necessarily get their CPUs from their immediate parent.

Suggested-by: Ridong Chen <ridong.chen@linux.dev>
Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
---
 Documentation/admin-guide/cgroup-v2.rst | 59 ++++++++++++++++++++-----
 1 file changed, 49 insertions(+), 10 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 18a01ce4c334..8f7263eeed4d 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -2505,16 +2505,55 @@ Cpuset Interface Files
 	A read-only multiple values file which exists on all
 	cpuset-enabled cgroups.
 
-	It lists the onlined CPUs that are actually granted to this
-	cgroup by its parent.  These CPUs are allowed to be used by
-	tasks within the current cgroup.
-
-	If "cpuset.cpus" is empty, the "cpuset.cpus.effective" file shows
-	all the CPUs from the parent cgroup that can be available to
-	be used by this cgroup.  Otherwise, it should be a subset of
-	"cpuset.cpus" unless none of the CPUs listed in "cpuset.cpus"
-	can be granted.  In this case, it will be treated just like an
-	empty "cpuset.cpus".
+	It lists the active CPUs available to tasks in this cgroup.
+
+	For a cgroup that is not a valid partition root, an empty
+	"cpuset.cpus" makes "cpuset.cpus.effective" show all CPUs
+	available from the parent cgroup. Otherwise, it is a subset
+	of "cpuset.cpus" unless none of the requested CPUs can be
+	granted. In that case, it is treated like an empty
+	"cpuset.cpus".
+
+	For example::
+
+	  P (cpuset.cpus.effective=0-3)
+	  |
+	  +-- C0 (member):       cpuset.cpus=1-2
+	  |                      cpuset.cpus.effective=1-2
+	  +-- C1 (invalid root): cpuset.cpus=4-5
+	  |                      cpuset.cpus.effective=0-3
+	  \-- C2 (member):       cpuset.cpus=<empty>
+	                         cpuset.cpus.effective=0-3
+
+	C0 gets CPUs 1-2 because they are available from P.
+	None of C1's requested CPUs 4-5 are available from P, so C1 uses P's
+	effective CPUs 0-3 instead.
+	C2 has an empty "cpuset.cpus" and therefore also uses P's effective
+	CPUs 0-3.
+
+	For a valid non-root partition root, "cpuset.cpus.effective" contains
+	the active CPUs from "cpuset.cpus.exclusive.effective" except
+	those granted to valid child partition roots. If
+	"cpuset.cpus.exclusive" is set to a value different from
+	"cpuset.cpus", the effective CPUs need not be a subset of
+	"cpuset.cpus".
+
+	For example, if CPUs 2-5 are allocated to P for exclusive use,
+	CPUs 2-4 are active, CPU 5 is offline, and none of CPUs 2-5 are
+	assigned to a valid child partition root::
+
+	  top (root)
+	  |
+	  \-- P (valid partition root)
+	      cpuset.cpus=0-3
+	      cpuset.cpus.exclusive=2-5
+	      cpuset.cpus.exclusive.effective=2-5
+	      cpuset.cpus.effective=2-4
+
+	P has 2-5 in "cpuset.cpus.exclusive.effective", including the
+	offline CPU 5. CPU 5 is excluded from "cpuset.cpus.effective",
+	leaving 2-4. The result also shows that "cpuset.cpus.effective"
+	need not be a subset of "cpuset.cpus", which is 0-3.
 
 	Its value will be affected by CPU hotplug events.
 
-- 
2.43.0