[PATCH v2 1/2] kernel/watchdog: add /sys/kernel/{hard,soft}lockup_count

[Max Kellermann <max.kellermann@ionos.com>]

There is /proc/sys/kernel/hung_task_detect_count,
/sys/kernel/warn_count and /sys/kernel/oops_count but there is no
userspace-accessible counter for hard/soft lockups.  Having this is
useful for monitoring tools.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
v1 -> v2: added documentation; added patch set cover letter with
  justification
---
 .../ABI/testing/sysfs-kernel-hardlockup_count |  7 +++
 .../ABI/testing/sysfs-kernel-softlockup_count |  7 +++
 kernel/watchdog.c                             | 53 +++++++++++++++++++
 3 files changed, 67 insertions(+)
 create mode 100644 Documentation/ABI/testing/sysfs-kernel-hardlockup_count
 create mode 100644 Documentation/ABI/testing/sysfs-kernel-softlockup_count

diff --git a/Documentation/ABI/testing/sysfs-kernel-hardlockup_count b/Documentation/ABI/testing/sysfs-kernel-hardlockup_count
new file mode 100644
index 000000000000..dfdd4078b077
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-kernel-hardlockup_count
@@ -0,0 +1,7 @@
+What:		/sys/kernel/hardlockup_count
+Date:		May 2025
+KernelVersion:	6.16
+Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
+Description:
+		Shows how many times the system has detected a hard lockup since last boot.
+		Available only if CONFIG_HARDLOCKUP_DETECTOR is enabled.
diff --git a/Documentation/ABI/testing/sysfs-kernel-softlockup_count b/Documentation/ABI/testing/sysfs-kernel-softlockup_count
new file mode 100644
index 000000000000..337ff5531b5f
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-kernel-softlockup_count
@@ -0,0 +1,7 @@
+What:		/sys/kernel/softlockup_count
+Date:		May 2025
+KernelVersion:	6.16
+Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
+Description:
+		Shows how many times the system has detected a soft lockup since last boot.
+		Available only if CONFIG_SOFTLOCKUP_DETECTOR is enabled.
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 9fa2af9dbf2c..09994bfb47af 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -63,6 +63,29 @@ int __read_mostly sysctl_hardlockup_all_cpu_backtrace;
  */
 unsigned int __read_mostly hardlockup_panic =
 			IS_ENABLED(CONFIG_BOOTPARAM_HARDLOCKUP_PANIC);
+
+#ifdef CONFIG_SYSFS
+
+static unsigned int hardlockup_count;
+
+static ssize_t hardlockup_count_show(struct kobject *kobj, struct kobj_attribute *attr,
+				     char *page)
+{
+	return sysfs_emit(page, "%u\n", hardlockup_count);
+}
+
+static struct kobj_attribute hardlockup_count_attr = __ATTR_RO(hardlockup_count);
+
+static __init int kernel_hardlockup_sysfs_init(void)
+{
+	sysfs_add_file_to_group(kernel_kobj, &hardlockup_count_attr.attr, NULL);
+	return 0;
+}
+
+late_initcall(kernel_hardlockup_sysfs_init);
+
+#endif // CONFIG_SYSFS
+
 /*
  * We may not want to enable hard lockup detection by default in all cases,
  * for example when running the kernel as a guest on a hypervisor. In these
@@ -169,6 +192,10 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
 		unsigned int this_cpu = smp_processor_id();
 		unsigned long flags;
 
+#ifdef CONFIG_SYSFS
+		++hardlockup_count;
+#endif
+
 		/* Only print hardlockups once. */
 		if (per_cpu(watchdog_hardlockup_warned, cpu))
 			return;
@@ -311,6 +338,28 @@ unsigned int __read_mostly softlockup_panic =
 static bool softlockup_initialized __read_mostly;
 static u64 __read_mostly sample_period;
 
+#ifdef CONFIG_SYSFS
+
+static unsigned int softlockup_count;
+
+static ssize_t softlockup_count_show(struct kobject *kobj, struct kobj_attribute *attr,
+				     char *page)
+{
+	return sysfs_emit(page, "%u\n", softlockup_count);
+}
+
+static struct kobj_attribute softlockup_count_attr = __ATTR_RO(softlockup_count);
+
+static __init int kernel_softlockup_sysfs_init(void)
+{
+	sysfs_add_file_to_group(kernel_kobj, &softlockup_count_attr.attr, NULL);
+	return 0;
+}
+
+late_initcall(kernel_softlockup_sysfs_init);
+
+#endif // CONFIG_SYSFS
+
 /* Timestamp taken after the last successful reschedule. */
 static DEFINE_PER_CPU(unsigned long, watchdog_touch_ts);
 /* Timestamp of the last softlockup report. */
@@ -742,6 +791,10 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 	touch_ts = __this_cpu_read(watchdog_touch_ts);
 	duration = is_softlockup(touch_ts, period_ts, now);
 	if (unlikely(duration)) {
+#ifdef CONFIG_SYSFS
+		++softlockup_count;
+#endif
+
 		/*
 		 * Prevent multiple soft-lockup reports if one cpu is already
 		 * engaged in dumping all cpu back traces.
-- 
2.47.2