[PATCH v2.1 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.

Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx> # cluster_id description
Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
---
o v2->v2.1
  - Reword the cluster_id description based on Dave's suggestions.
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 | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/Documentation/x86/topology.rst b/Documentation/x86/topology.rst
index 7f58010ea86a..9de14f3f7783 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 +91,22 @@ 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:
+
+      - Upper bits extracted from the APIC ID.  CPUs which have the same value
+        in these bits share an L2 and have the same cluster_id.
+
+        CPUs for which cluster information is unavailable will show 65535
+        (BAD_APICID) as the cluster_id.
+
 Cores
 =====
 A core consists of 1 or more threads. It does not matter whether the threads
@@ -125,6 +142,11 @@ 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 +160,10 @@ 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