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

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

[Aaron Tomlin]

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

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

[Aaron Tomlin]

On Tue, Jul 28, 2026 at 04:52:38PM -0400, Aaron Tomlin wrote:
> 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(+)

Hi Peter, Ingo, Juri, Vincent,

Sashiko found [1] pre-existing issues, which are not related to this patch.
Would you prefer that I investigate these concerns in this series?
Thank you. 

[1]: https://sashiko.dev/#/patchset/20260728205238.18447-1-atomlin%40atomlin.com

Kind regards,
-- 
Aaron Tomlin

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

[Aaron Tomlin]

On Wed, Jul 29, 2026 at 09:25:20AM -0400, Aaron Tomlin wrote:
> On Tue, Jul 28, 2026 at 04:52:38PM -0400, Aaron Tomlin wrote:
> > 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(+)
> 
> Hi Peter, Ingo, Juri, Vincent,
> 
> Sashiko found [1] pre-existing issues, which are not related to this patch.
> Would you prefer that I investigate these concerns in this series?
> Thank you. 
> 
> [1]: https://sashiko.dev/#/patchset/20260728205238.18447-1-atomlin%40atomlin.com

Hi Peter, Ingo, Juri, Vincent,

These concerns appear valid.

I'll address them.

Kind regards,
-- 
Aaron Tomlin