tools/sched: Add root_domains_dump.py which dumps root domains info

Root domains information is somewhat hard to access at runtime. Even
with sched_debug and sched_verbose, such information is only printed
on kernel console when domains are modified.

Add a simple drgn script to more easily retrieve root domains
information at runtime.

Since tools/sched is a new directory, add it to MAINTAINERS as well.

Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Marcel Ziswiler <marcel.ziswiler@codethink.co.uk> # nuc & rock5b
Link: https://lore.kernel.org/r/20250627115118.438797-5-juri.lelli@redhat.com

Author: jlelli

MAINTAINERS

@@ -22165,6 +22165,7 @@ F:	include/linux/wait.h
 F:	include/uapi/linux/sched.h
 F:	kernel/fork.c
 F:	kernel/sched/
+F:	tools/sched/
 
 SCHEDULER - SCHED_EXT
 R:	Tejun Heo <tj@kernel.org>

tools/sched/root_domains_dump.py

@@ -0,0 +1,68 @@
+#!/usr/bin/env drgn
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2025 Juri Lelli <juri.lelli@redhat.com>
+# Copyright (C) 2025 Red Hat, Inc.
+
+desc = """
+This is a drgn script to show the current root domains configuration. For more
+info on drgn, visit https://github.com/osandov/drgn.
+
+Root domains are only printed once, as multiple CPUs might be attached to the
+same root domain.
+"""
+
+import os
+import argparse
+
+import drgn
+from drgn import FaultError
+from drgn.helpers.common import *
+from drgn.helpers.linux import *
+
+def print_root_domains_info():
+
+    # To store unique root domains found
+    seen_root_domains = set()
+
+    print("Retrieving (unique) Root Domain Information:")
+
+    runqueues = prog['runqueues']
+    def_root_domain = prog['def_root_domain']
+
+    for cpu_id in for_each_possible_cpu(prog):
+        try:
+            rq = per_cpu(runqueues, cpu_id)
+
+            root_domain = rq.rd
+
+            # Check if we've already processed this root domain to avoid duplicates
+            # Use the memory address of the root_domain as a unique identifier
+            root_domain_cast = int(root_domain)
+            if root_domain_cast in seen_root_domains:
+                continue
+            seen_root_domains.add(root_domain_cast)
+
+            if root_domain_cast == int(def_root_domain.address_):
+                print(f"\n--- Root Domain @ def_root_domain ---")
+            else:
+                print(f"\n--- Root Domain @ 0x{root_domain_cast:x} ---")
+
+            print(f"  From CPU: {cpu_id}") # This CPU belongs to this root domain
+
+            # Access and print relevant fields from struct root_domain
+            print(f"  Span       : {cpumask_to_cpulist(root_domain.span[0])}")
+            print(f"  Online     : {cpumask_to_cpulist(root_domain.span[0])}")
+
+        except drgn.FaultError as fe:
+            print(f"  (CPU {cpu_id}: Fault accessing kernel memory: {fe})")
+        except AttributeError as ae:
+            print(f"  (CPU {cpu_id}: Missing attribute for root_domain (kernel struct change?): {ae})")
+        except Exception as e:
+            print(f"  (CPU {cpu_id}: An unexpected error occurred: {e})")
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser(description=desc,
+                                     formatter_class=argparse.RawTextHelpFormatter)
+    args = parser.parse_args()
+
+    print_root_domains_info()