tools/sched: Add dl_bw_dump.py for printing bandwidth accounting info

dl_rq bandwidth accounting information is crucial for the correct
functioning of SCHED_DEADLINE.

Add a drgn script for accessing that information at runtime, so that
it's easier to check and debug issues related to it.

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/20250627115118.438797-6-juri.lelli@redhat.com

tools/sched/dl_bw_dump.py

@@ -0,0 +1,57 @@
+#!/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 dl_rq bandwidth accounting information. For more
+info on drgn, visit https://github.com/osandov/drgn.
+
+Only online CPUs are reported.
+"""
+
+import os
+import argparse
+
+import drgn
+from drgn import FaultError
+from drgn.helpers.common import *
+from drgn.helpers.linux import *
+
+def print_dl_bws_info():
+
+    print("Retrieving dl_rq bandwidth accounting information:")
+
+    runqueues = prog['runqueues']
+
+    for cpu_id in for_each_possible_cpu(prog):
+        try:
+            rq = per_cpu(runqueues, cpu_id)
+
+            if rq.online == 0:
+                continue
+
+            dl_rq = rq.dl
+
+            print(f"  From CPU: {cpu_id}")
+
+            # Access and print relevant fields from struct dl_rq
+            print(f"  running_bw : {dl_rq.running_bw}")
+            print(f"  this_bw    : {dl_rq.this_bw}")
+            print(f"  extra_bw   : {dl_rq.extra_bw}")
+            print(f"  max_bw     : {dl_rq.max_bw}")
+            print(f"  bw_ratio   : {dl_rq.bw_ratio}")
+
+        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_dl_bws_info()