[PATCH 1/7] rteval: Add CoreSiblings class for CPU core topology queries

Add a new CoreSiblings class in rteval/sysinfo/coresiblings.py that
provides a simple interface for querying which CPUs share physical cores.

The class reads /sys/devices/system/cpu/cpu*/topology/thread_siblings_list
to build a mapping of CPU core siblings and provides three methods:

- share_core(cpu1, cpu2): Check if two CPUs share a physical core
- get_siblings(cpu): Get all CPUs that share a core with a given CPU
- get_core_groups(): Get all unique core sibling groups

The implementation is generic and works with any SMT configuration:
- No SMT (1 CPU per core)
- 2-way SMT (typical Intel hyperthreading)
- 4-way SMT (some IBM POWER processors)
- 8-way SMT (IBM POWER8/9)

This provides a foundation for cpuset management features where we need
to understand CPU core topology for proper isolation.

Assisted-by: Claude Sonnet 4.5 <[email protected]>
Signed-off-by: John Kacur <[email protected]>
---
 rteval/sysinfo/coresiblings.py | 110 +++++++++++++++++++++++++++++++++
 1 file changed, 110 insertions(+)
 create mode 100644 rteval/sysinfo/coresiblings.py

diff --git a/rteval/sysinfo/coresiblings.py b/rteval/sysinfo/coresiblings.py
new file mode 100644
index 000000000000..3c97439d0a39
--- /dev/null
+++ b/rteval/sysinfo/coresiblings.py
@@ -0,0 +1,110 @@
+# -*- coding: utf-8 -*-
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+#   Copyright 2026   John Kacur <[email protected]>
+#
+
+import os
+from rteval.cpulist_utils import expand_cpulist
+
+class CoreSiblings:
+    """Query CPU core topology to determine which CPUs share physical cores"""
+
+    def __init__(self, root="/"):
+        self.sysdir = os.path.join(root, 'sys', 'devices', 'system', 'cpu')
+        self.core_map = {}  # Maps cpu -> set of sibling cpus
+        self._parse()
+
+    def _parse(self):
+        """Parse thread_siblings_list for each CPU"""
+        for dirname in os.listdir(self.sysdir):
+            # Only parse cpu<integer> directories
+            if dirname.startswith('cpu') and os.path.isdir(os.path.join(self.sysdir, dirname)):
+                try:
+                    cpu_id = int(dirname[3:])
+                except ValueError:
+                    continue
+
+                siblings_file = os.path.join(self.sysdir, dirname, 'topology', 'thread_siblings_list')
+                if os.path.exists(siblings_file):
+                    with open(siblings_file, 'r') as f:
+                        siblings_str = f.read().strip()
+                        # expand_cpulist returns a list of cpu numbers
+                        siblings = set(expand_cpulist(siblings_str))
+                        self.core_map[cpu_id] = siblings
+
+    def share_core(self, cpu1, cpu2):
+        """
+        Check if two CPUs share the same physical core.
+
+        Args:
+            cpu1: First CPU ID (int)
+            cpu2: Second CPU ID (int)
+
+        Returns:
+            True if CPUs share a core, False otherwise
+        """
+        if cpu1 not in self.core_map:
+            return False
+        return cpu2 in self.core_map[cpu1]
+
+    def get_siblings(self, cpu):
+        """
+        Get all CPUs that share a core with the given CPU.
+
+        Args:
+            cpu: CPU ID (int)
+
+        Returns:
+            Set of CPU IDs that share a core with cpu (includes cpu itself)
+        """
+        return self.core_map.get(cpu, set())
+
+    def get_core_groups(self):
+        """
+        Get all unique core sibling groups.
+
+        Returns:
+            List of sets, where each set contains CPUs that share a core
+        """
+        seen = set()
+        groups = []
+
+        for cpu, siblings in self.core_map.items():
+            # Use frozenset as a hashable representation
+            group_key = frozenset(siblings)
+            if group_key not in seen:
+                seen.add(group_key)
+                groups.append(siblings)
+
+        return groups
+
+
+def unit_test():
+    """Simple unit test"""
+    try:
+        cs = CoreSiblings()
+
+        print("Core Sibling Groups:")
+        for i, group in enumerate(cs.get_core_groups()):
+            print(f"  Core {i}: {sorted(group)}")
+
+        # Test share_core with first two CPUs if they exist
+        if len(cs.core_map) >= 2:
+            cpus = sorted(cs.core_map.keys())
+            cpu1, cpu2 = cpus[0], cpus[1]
+            print(f"\nDo CPU {cpu1} and CPU {cpu2} share a core? {cs.share_core(cpu1, cpu2)}")
+            print(f"CPU {cpu1} siblings: {sorted(cs.get_siblings(cpu1))}")
+            print(f"CPU {cpu2} siblings: {sorted(cs.get_siblings(cpu2))}")
+
+        return 0
+    except Exception as e:
+        print(f"** EXCEPTION: {e}")
+        import traceback
+        traceback.print_exc()
+        return 1
+
+
+if __name__ == '__main__':
+    import sys
+    sys.exit(unit_test())
-- 
2.53.0