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Message-ID: <20251222014210.2032214-3-atomlin@atomlin.com>
Date: Sun, 21 Dec 2025 20:42:10 -0500
From: Aaron Tomlin <atomlin@...mlin.com>
To: akpm@...ux-foundation.org,
lance.yang@...ux.dev,
mhiramat@...nel.org,
gregkh@...uxfoundation.org,
pmladek@...e.com,
joel.granados@...nel.org
Cc: sean@...e.io,
linux-kernel@...r.kernel.org
Subject: [v4 PATCH 2/2] hung_task: Enable runtime reset of hung_task_detect_count
Introduce support for writing to /proc/sys/kernel/hung_task_detect_count.
Writing a value of zero to this file atomically resets the counter of
detected hung tasks. This grants system administrators the ability to
clear the cumulative diagnostic history after resolving an incident,
simplifying monitoring without requiring a system restart.
Signed-off-by: Aaron Tomlin <atomlin@...mlin.com>
---
Documentation/admin-guide/sysctl/kernel.rst | 3 +-
kernel/hung_task.c | 76 ++++++++++++++++++---
2 files changed, 67 insertions(+), 12 deletions(-)
diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 239da22c4e28..68da4235225a 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -418,7 +418,8 @@ hung_task_detect_count
======================
Indicates the total number of tasks that have been detected as hung since
-the system boot.
+the system boot or since the counter was reset. The counter is zeroed when
+a value of 0 is written.
This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 00c3296fd692..70b3db047f5d 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -17,6 +17,7 @@
#include <linux/export.h>
#include <linux/panic_notifier.h>
#include <linux/sysctl.h>
+#include <linux/atomic.h>
#include <linux/suspend.h>
#include <linux/utsname.h>
#include <linux/sched/signal.h>
@@ -36,7 +37,7 @@ static int __read_mostly sysctl_hung_task_check_count = PID_MAX_LIMIT;
/*
* Total number of tasks detected as hung since boot:
*/
-static unsigned long __read_mostly sysctl_hung_task_detect_count;
+static atomic_long_t sysctl_hung_task_detect_count = ATOMIC_LONG_INIT(0);
/*
* Limit number of tasks checked in a batch.
@@ -246,20 +247,26 @@ static inline void hung_task_diagnostics(struct task_struct *t)
}
static void check_hung_task(struct task_struct *t, unsigned long timeout,
- unsigned long prev_detect_count)
+ unsigned long prev_detect_count)
{
- unsigned long total_hung_task;
+ unsigned long total_hung_task, current_detect;
if (!task_is_hung(t, timeout))
return;
/*
* This counter tracks the total number of tasks detected as hung
- * since boot.
+ * since boot. If a reset occurred during the scan, we treat the
+ * current count as the new delta to avoid an underflow error.
+ * Ensure hang details are globally visible before the counter
+ * update.
*/
- sysctl_hung_task_detect_count++;
+ current_detect = atomic_long_inc_return_acquire(&sysctl_hung_task_detect_count);
+ if (current_detect >= prev_detect_count)
+ total_hung_task = current_detect - prev_detect_count;
+ else
+ total_hung_task = current_detect;
- total_hung_task = sysctl_hung_task_detect_count - prev_detect_count;
trace_sched_process_hang(t);
if (sysctl_hung_task_panic && total_hung_task >= sysctl_hung_task_panic) {
@@ -318,7 +325,8 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
int max_count = sysctl_hung_task_check_count;
unsigned long last_break = jiffies;
struct task_struct *g, *t;
- unsigned long prev_detect_count = sysctl_hung_task_detect_count;
+ /* Acquire prevents reordering task checks before this point. */
+ unsigned long prev_detect_count = atomic_long_read_acquire(&sysctl_hung_task_detect_count);
int need_warning = sysctl_hung_task_warnings;
unsigned long si_mask = hung_task_si_mask;
@@ -346,7 +354,9 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
unlock:
rcu_read_unlock();
- if (!(sysctl_hung_task_detect_count - prev_detect_count))
+ /* Ensures we see all hang details recorded during the scan. */
+ if (!(atomic_long_read_acquire(&sysctl_hung_task_detect_count) -
+ prev_detect_count))
return;
if (need_warning || hung_task_call_panic) {
@@ -371,6 +381,51 @@ static long hung_timeout_jiffies(unsigned long last_checked,
}
#ifdef CONFIG_SYSCTL
+
+/**
+ * proc_dohung_task_detect_count - proc handler for hung_task_detect_count
+ * @table: Pointer to the struct ctl_table definition for this proc entry
+ * @dir: Flag indicating the operation
+ * @buffer: User space buffer for data transfer
+ * @lenp: Pointer to the length of the data being transferred
+ * @ppos: Pointer to the current file offset
+ *
+ * This handler is used for reading the current hung task detection count
+ * and for resetting it to zero when a write operation is performed using a
+ * zero value only. Returns 0 on success or a negative error code on
+ * failure.
+ */
+static int proc_dohung_task_detect_count(const struct ctl_table *table, int dir,
+ void *buffer, size_t *lenp, loff_t *ppos)
+{
+ unsigned long detect_count;
+ struct ctl_table proxy_table;
+ int err;
+
+ proxy_table = *table;
+ proxy_table.data = &detect_count;
+
+ if (SYSCTL_KERN_TO_USER(dir)) {
+ detect_count = atomic_long_read(&sysctl_hung_task_detect_count);
+
+ return proc_doulongvec_minmax(&proxy_table, dir, buffer, lenp, ppos);
+ }
+
+ err = proc_doulongvec_minmax(&proxy_table, dir, buffer, lenp, ppos);
+ if (err < 0)
+ return err;
+
+ if (SYSCTL_USER_TO_KERN(dir)) {
+ /* The only valid value for clearing is zero. */
+ if (detect_count)
+ return -EINVAL;
+ atomic_long_set(&sysctl_hung_task_detect_count, 0);
+ }
+
+ *ppos += *lenp;
+ return err;
+}
+
/*
* Process updating of timeout sysctl
*/
@@ -451,10 +506,9 @@ static const struct ctl_table hung_task_sysctls[] = {
},
{
.procname = "hung_task_detect_count",
- .data = &sysctl_hung_task_detect_count,
.maxlen = sizeof(unsigned long),
- .mode = 0444,
- .proc_handler = proc_doulongvec_minmax,
+ .mode = 0644,
+ .proc_handler = proc_dohung_task_detect_count,
},
{
.procname = "hung_task_sys_info",
--
2.51.0
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