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: <cf6c6442-e160-4aad-9c12-a49225b73501@linux.dev>
Date: Sun, 16 Nov 2025 21:22:43 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Feng Tang <feng.tang@...ux.alibaba.com>
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 2025/11/16 17:11, Feng Tang wrote:
> 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;

Looks good to me now! I assume v3 would be expected, can you
post a new version?

Cheers,
Lance