[PATCH v2] sched/debug: Introduce per-CPU debugfs files

[Aaron Tomlin <atomlin@atomlin.com>]

Currently, accessing scheduler debugging details for a specific CPU
requires reading /sys/kernel/debug/sched/debug, which outputs
information for all online CPUs. When investigating a latency anomaly or
scheduling issue isolated to a specific CPU, accessing
/sys/kernel/debug/sched/cpu/cpu<N>/debug provides an immediate, targeted
view of that runqueue.

Add support for per-CPU debug files under:
/sys/kernel/debug/sched/cpu/cpu<N>/debug. Reading
/sys/kernel/debug/sched/cpu/cpu<N>/debug calls print_cpu() specifically
for CPU <N>, exposing CPU-specific runqueue details on demand. If the
target CPU is currently offline, reading its file returns -ENODEV.

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
Changes since v1:

 - Reframed commit message motivation around targeted interactive
   debugging on large SMP topologies (Peter Zijlstra and Zhan Xusheng)

 - Gated sched_debug_cpu_show() with a cpu_online(cpu) check
   returning -ENODEV when target CPU is offline (Zhan Xusheng)

 - Linked to v1: https://lore.kernel.org/lkml/20260728020309.6169-1-atomlin@atomlin.com/
---
 kernel/sched/debug.c | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 40584b27ea0c..af6f68f73e5f 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -356,6 +356,7 @@ static const struct file_operations sched_verbose_fops = {
 };
 
 static const struct seq_operations sched_debug_sops;
+static void print_cpu(struct seq_file *m, int cpu);
 
 static int sched_debug_open(struct inode *inode, struct file *filp)
 {
@@ -633,6 +634,47 @@ static void debugfs_fair_server_init(void)
 	}
 }
 
+static int sched_debug_cpu_show(struct seq_file *m, void *v)
+{
+	unsigned long cpu = (unsigned long) m->private;
+
+	if (!cpu_online(cpu))
+		return -ENODEV;
+
+	print_cpu(m, cpu);
+	return 0;
+}
+
+static int sched_debug_cpu_open(struct inode *inode, struct file *filp)
+{
+	return single_open(filp, sched_debug_cpu_show, inode->i_private);
+}
+
+static const struct file_operations sched_debug_cpu_fops = {
+	.open		= sched_debug_cpu_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
+static __init void debugfs_cpu_init(void)
+{
+	struct dentry *d_cpu_dir;
+	unsigned long cpu;
+	char buf[16];
+
+	d_cpu_dir = debugfs_create_dir("cpu", debugfs_sched);
+
+	for_each_possible_cpu(cpu) {
+		struct dentry *d_cpu;
+
+		snprintf(buf, sizeof(buf), "cpu%lu", cpu);
+		d_cpu = debugfs_create_dir(buf, d_cpu_dir);
+
+		debugfs_create_file("debug", 0444, d_cpu, (void *) cpu, &sched_debug_cpu_fops);
+	}
+}
+
 static __init int sched_init_debug(void)
 {
 	struct dentry __maybe_unused *numa, *llc;
@@ -690,6 +732,7 @@ static __init int sched_init_debug(void)
 #ifdef CONFIG_SCHED_CLASS_EXT
 	debugfs_ext_server_init();
 #endif
+	debugfs_cpu_init();
 
 	return 0;
 }
-- 
2.55.0