linux-kernel - Re: [PATCH v2 2/4] hung_task: Add hung_task_sys_info sysctl to dump sys info on task-hung

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Message-ID: <aRmVVE3klGZuX6aV@U-2FWC9VHC-2323.local>
Date: Sun, 16 Nov 2025 17:11:48 +0800
From: Feng Tang <feng.tang@...ux.alibaba.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: Petr Mladek <pmladek@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lance Yang <ioworker0@...il.com>, linux-kernel@...r.kernel.org,
	Jonathan Corbet <corbet@....net>, paulmck@...nel.org,
	lirongqing@...du.com, leonylgao@...cent.com
Subject: Re: [PATCH v2 2/4] hung_task: Add hung_task_sys_info sysctl to dump
 sys info on task-hung

On Sun, Nov 16, 2025 at 03:58:32PM +0800, Lance Yang wrote:
> 
> 
> On 2025/11/13 19:10, Feng Tang wrote:
> > When task-hung happens, developers may need different kinds of system
> > information (call-stacks, memory info, locks, etc.) to help debugging.
> > 
> > Add 'hung_task_sys_info' sysctl knob to take human readable string like
> > "tasks,mem,timers,locks,ftrace,...", and when task-hung happens, all
> > requested information will be dumped. (refer kernel/sys_info.c for more
> > details).
> > 
> > Meanwhile, the newly introduced sys_info() call is used to unify some
> > existing info-dumping knobs.
> > 
> > Suggested-by: Petr Mladek <pmladek@...e.com>
> > Signed-off-by: Feng Tang <feng.tang@...ux.alibaba.com>
> > ---
> >   Documentation/admin-guide/sysctl/kernel.rst |  5 ++
> >   kernel/hung_task.c                          | 62 +++++++++++++--------
> >   2 files changed, 43 insertions(+), 24 deletions(-)
> >   	 * Ok, the task did not get scheduled for more than 2 minutes,
> >   	 * complain:
> >   	 */
> > -	if (sysctl_hung_task_warnings || hung_task_call_panic) {
> > +	if (sysctl_hung_task_warnings) {
> 
> It seems like the behavior changes when sysctl_hung_task_warnings is
> 0 but a panic is about to be triggered ...
> 
> Looking at the history:
> 
> 1) Commit ("hung_task: ignore hung_task_warnings when hung_task_panic
> is enabled")[1] ensured that hung task information is always dumped
> when a panic is configured, even if the warning counter is exhausted.
> 
> 2) Later, commit ("hung_task: panic when there are more than N hung
> tasks at the same time")[2] refined the logic to trigger a panic based
> on the number of hung tasks found in a single scan.
> 
> To stay consistent with the established behavior, I think we should
> continue to dump the information for hung tasks as long as
> sysctl_hung_task_panic is enabled :)
> 
> [1] https://lore.kernel.org/all/20240613033159.3446265-1-leonylgao@gmail.com
> [2] https://lore.kernel.org/all/20251015063615.2632-1-lirongqing@baidu.com
> [...]

Aha, Petr asked similar question during his review. Thanks for the catch!

How about following fixup patch to restore that part of logic? 

Thanks,
Feng

---
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 5b3a7785d3a2..d2254c91450b 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -223,8 +223,11 @@ static inline void debug_show_blocker(struct task_struct *task, unsigned long ti
 }
 #endif
 
-static void check_hung_task(struct task_struct *t, unsigned long timeout)
+static void check_hung_task(struct task_struct *t, unsigned long timeout,
+		unsigned long prev_detect_count)
 {
+	unsigned long total_hung_task;
+
 	if (!task_is_hung(t, timeout))
 		return;
 
@@ -234,13 +237,19 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 	 */
 	sysctl_hung_task_detect_count++;
 
+	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) {
+		console_verbose();
+		hung_task_call_panic = true;
+	}
+
 	/*
 	 * Ok, the task did not get scheduled for more than 2 minutes,
 	 * complain:
 	 */
-	if (sysctl_hung_task_warnings) {
+	if (sysctl_hung_task_warnings || hung_task_call_panic) {
 		if (sysctl_hung_task_warnings > 0)
 			sysctl_hung_task_warnings--;
 		pr_err("INFO: task %s:%d blocked for more than %ld seconds.\n",
@@ -295,7 +304,6 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
 {
 	int max_count = sysctl_hung_task_check_count;
 	unsigned long last_break = jiffies;
-	unsigned long total_hung_task;
 	struct task_struct *g, *t;
 	unsigned long prev_detect_count = sysctl_hung_task_detect_count;
 	int need_warning = sysctl_hung_task_warnings;
@@ -320,20 +328,14 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
 			last_break = jiffies;
 		}
 
-		check_hung_task(t, timeout);
+		check_hung_task(t, timeout, prev_detect_count);
 	}
  unlock:
 	rcu_read_unlock();
 
-	total_hung_task = sysctl_hung_task_detect_count - prev_detect_count;
-	if (!total_hung_task)
+	if (!(sysctl_hung_task_detect_count - prev_detect_count))
 		return;
 
-	if (sysctl_hung_task_panic && total_hung_task >= sysctl_hung_task_panic) {
-		console_verbose();
-		hung_task_call_panic = true;
-	}
-
 	if (need_warning || hung_task_call_panic) {
 		si_mask |= SYS_INFO_LOCKS;