[PATCH 2/7] rteval: Add core sharing validation for CPU isolation

[John Kacur <jkacur@redhat.com>]

Add validation to detect and warn when CPUs sharing physical cores are
assigned to different workload types (housekeeping, measurement, load).

When SMT/hyperthreading is enabled, CPUs sharing a physical core also
share L1/L2 caches, execution units, TLB, and other core-level resources.
Running different workload types on sibling threads defeats CPU isolation
and can corrupt real-time measurements or create unreproducible results.

Key features:
- Uses CoreSiblings class to detect which CPUs share physical cores
- Only warns when at least one CPU in a pair is isolated (via isolcpus)
- Checks all three workload type combinations:
  * Housekeeping vs Measurement
  * Housekeeping vs Load
  * Measurement vs Load
- Warning format clearly indicates which CPUs are isolated with "(isol)"
  marker

Example warning:
  Warning: Housekeeping CPU 0 (isol) shares core with measurement CPU 8 (isol)

Implementation:
- coresiblings.py: Add comment explaining SMT-enabled/disabled behavior
- systopology.py: Add validate_core_sharing() function
- rteval-cmd: Call validation after CPU lists are finalized and log warnings

This validation is purely informational - it warns users but does not
prevent execution, allowing users to make informed decisions about their
CPU configurations.

Assisted-by: Claude Sonnet 4.5 <noreply@anthropic.com>
Signed-off-by: John Kacur <jkacur@redhat.com>
---
 rteval-cmd                     |  8 ++++-
 rteval/sysinfo/coresiblings.py |  8 ++++-
 rteval/systopology.py          | 55 ++++++++++++++++++++++++++++++++++
 3 files changed, 69 insertions(+), 2 deletions(-)

diff --git a/rteval-cmd b/rteval-cmd
index a903b537c891..f0efb0117726 100755
--- a/rteval-cmd
+++ b/rteval-cmd
@@ -31,7 +31,7 @@ from rteval.modules.loads import LoadModules
 from rteval.modules.measurement import MeasurementModules
 from rteval import cpupower
 from rteval.version import RTEVAL_VERSION
-from rteval.systopology import SysTopology, parse_cpulist_from_config, validate_housekeeping_cpus
+from rteval.systopology import SysTopology, parse_cpulist_from_config, validate_housekeeping_cpus, validate_core_sharing
 from rteval.modules.loads.kcompile import ModuleParameters
 from rteval.cpulist_utils import CpuList, is_relative, collapse_cpulist
 
@@ -446,6 +446,12 @@ if __name__ == '__main__':
             logger.log(Log.DEBUG, f"measurement cpulist: {msrcfg.cpulist}")
         logger.log(Log.DEBUG, f"workdir: {rtevcfg.workdir}")
 
+        # Validate core sharing between housekeeping, measurement, and load CPUs
+        if housekeeping_cpus or msrcfg_cpus or ldcfg_cpus:
+            core_warnings = validate_core_sharing(housekeeping_cpus, msrcfg_cpus, ldcfg_cpus)
+            for warning in core_warnings:
+                logger.log(Log.WARN, warning)
+
         # if --summarize was specified then just parse the XML, print it and exit
         if cmd_opts.rteval___summarize:
             for xmlfile in cmd_opts.rteval___summarize:
diff --git a/rteval/sysinfo/coresiblings.py b/rteval/sysinfo/coresiblings.py
index 3c97439d0a39..d55a4b699149 100644
--- a/rteval/sysinfo/coresiblings.py
+++ b/rteval/sysinfo/coresiblings.py
@@ -8,7 +8,13 @@ import os
 from rteval.cpulist_utils import expand_cpulist
 
 class CoreSiblings:
-    """Query CPU core topology to determine which CPUs share physical cores"""
+    """
+    Query CPU core topology to determine which CPUs share physical cores
+
+    Note: This class works correctly whether SMT/hyperthreading is enabled or disabled.
+    When SMT is disabled, each CPU's thread_siblings_list contains only itself,
+    so the class will correctly report that no CPUs share cores.
+    """
 
     def __init__(self, root="/"):
         self.sysdir = os.path.join(root, 'sys', 'devices', 'system', 'cpu')
diff --git a/rteval/systopology.py b/rteval/systopology.py
index 5f2bc291f608..0ee81d5c5e36 100644
--- a/rteval/systopology.py
+++ b/rteval/systopology.py
@@ -294,6 +294,61 @@ def parse_cpulist_from_config(cpulist, run_on_isolcpus=False):
     return result
 
 
+def validate_core_sharing(housekeeping_cpus, measurement_cpus, load_cpus):
+    """
+    Check for CPUs sharing physical cores across different workload types.
+    Warns if isolated CPUs share cores between housekeeping, measurement, and load groups.
+
+    :param housekeeping_cpus: List of housekeeping CPU integers
+    :param measurement_cpus: List of measurement CPU integers
+    :param load_cpus: List of load CPU integers
+    :return: List of warning messages (empty if no issues)
+    """
+    from rteval.sysinfo.coresiblings import CoreSiblings
+
+    warnings = []
+
+    # Get isolated CPUs
+    isolated_cpus = set(SysTopology().isolated_cpus())
+
+    # Create CoreSiblings instance
+    try:
+        cs = CoreSiblings()
+    except Exception:
+        # If we can't read topology, skip validation
+        return warnings
+
+    def check_pair(cpu1, cpu2, type1, type2):
+        """Check if two CPUs share a core and generate warning if at least one is isolated"""
+        if cs.share_core(cpu1, cpu2):
+            cpu1_isol = cpu1 in isolated_cpus
+            cpu2_isol = cpu2 in isolated_cpus
+
+            # Only warn if at least one CPU is isolated
+            if cpu1_isol or cpu2_isol:
+                cpu1_str = f"{type1} CPU {cpu1}{' (isol)' if cpu1_isol else ''}"
+                cpu2_str = f"{type2} CPU {cpu2}{' (isol)' if cpu2_isol else ''}"
+                warnings.append(f"Warning: {cpu1_str} shares core with {cpu2_str}")
+
+    # Check all three combinations
+    # 1. Housekeeping vs Measurement
+    for hk_cpu in housekeeping_cpus:
+        for msr_cpu in measurement_cpus:
+            check_pair(hk_cpu, msr_cpu, "Housekeeping", "measurement")
+
+    # 2. Housekeeping vs Load
+    for hk_cpu in housekeeping_cpus:
+        for ld_cpu in load_cpus:
+            check_pair(hk_cpu, ld_cpu, "Housekeeping", "load")
+
+    # 3. Measurement vs Load
+    for msr_cpu in measurement_cpus:
+        for ld_cpu in load_cpus:
+            check_pair(msr_cpu, ld_cpu, "Measurement", "load")
+
+    return warnings
+
+
 if __name__ == "__main__":
 
     def unit_test():
-- 
2.53.0