[PATCH v2 2/2] x86/Documentation: Add documentation about cluster

[K Prateek Nayak]

x86 processors map cluster to the L2 cache. Add documentation stating
the same, and provide more information on the values and API related to
CPU clusters exposed by the kernel.

Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
---
o v1->v2
  - Reworded the definition of cluster on x86 based on Peter's
    suggestion.
  - Fixed double spacing before and after the cluster section.
---
 Documentation/x86/topology.rst | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/Documentation/x86/topology.rst b/Documentation/x86/topology.rst
index 7f58010ea86a..5dae8a0327d1 100644
--- a/Documentation/x86/topology.rst
+++ b/Documentation/x86/topology.rst
@@ -33,6 +33,7 @@ historical nature and should be cleaned up.
 The topology of a system is described in the units of:
 
     - packages
+    - cluster
     - cores
     - threads
 
@@ -90,6 +91,23 @@ Package-related topology information in the kernel:
         Cache. In general, it is a number identifying an LLC uniquely on the
         system.
 
+Clusters
+========
+A cluster consists of threads of one or more cores sharing the same L2 cache.
+
+Cluster-related topology information in the kernel:
+
+  - cluster_id:
+
+    A per-CPU variable containing:
+
+      - On Intel, the common upper bits of APIC ID of the list of CPUs sharing
+        the L2 Cache with lower bits set to 0.
+
+      - On AMD and Hygon, with Topology Extension, the common upper bits of the
+        Extended APIC ID of the list of CPUs sharing the L2 Cache, left shifted
+        to remove trailing 0s.
+
 Cores
 =====
 A core consists of 1 or more threads. It does not matter whether the threads
@@ -125,6 +143,11 @@ Thread-related topology information in the kernel:
 
     The number of online threads is also printed in /proc/cpuinfo "siblings."
 
+  - topology_cluster_cpumask():
+
+    The cpumask contains all online threads in the cluster to which a thread
+    belongs.
+
   - topology_sibling_cpumask():
 
     The cpumask contains all online threads in the core to which a thread
@@ -138,6 +161,10 @@ Thread-related topology information in the kernel:
 
     The physical package ID to which a thread belongs.
 
+  - topology_cluster_id();
+
+    The ID of the cluster to which a thread belongs.
+
   - topology_core_id();
 
     The ID of the core to which a thread belongs. It is also printed in /proc/cpuinfo
-- 
2.34.1