[PATCH v5 11/12] docs: cgroup-v2: Document kernel-noise isolation via isolated partitions

[Qiliang Yuan <odys.yuan@gmail.com>]

Document that creating a cpuset isolated partition updates the
kernel-noise housekeeping masks (HK_TYPE_KERNEL_NOISE and
HK_TYPE_MANAGED_IRQ) in addition to the sched-domain mask, and
that destroying it restores the boot configuration.

No boot-time kernel parameters such as nohz_full= or rcu_nocbs=
are required; writing "isolated" to cpuset.cpus.partition is the
only mechanism needed.

Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
---
 Documentation/admin-guide/cgroup-v2.rst | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 86a2a0099178e..611c6f7ca4efe 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -2784,6 +2784,23 @@ Cpuset Interface Files
 	kernel boot command line option.  If those CPUs are to be put
 	into a partition, they have to be used in an isolated partition.
 
+	When an isolated partition is created or destroyed, the kernel
+	automatically drives runtime updates of the housekeeping masks
+	for kernel-noise types (nohz_full, RCU NOCB, managed IRQ
+	interrupts).  This extends isolation beyond scheduler domains:
+	the tick is stopped on isolated CPUs, RCU callbacks are
+	offloaded to housekeeping cores, and managed interrupts are
+	migrated away.  No boot-time kernel parameters such as
+	``nohz_full=`` or ``rcu_nocbs=`` are required; writing
+	``isolated`` to ``cpuset.cpus.partition`` is the only mechanism
+	needed.  No additional cgroupfs files are required.
+
+	CPUs with hotplug disabled (typically the boot CPU, CPU 0, on
+	x86-64) cannot be cycled offline for kernel-noise isolation.
+	The kernel emits a one-time warning and keeps those CPUs in
+	the tick and RCU-NOCB housekeeping set, even when they appear
+	in an isolated partition.
+
 
 Device controller
 -----------------

-- 
2.43.0