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

[Lance Yang <lance.yang@linux.dev>]

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@suse.com>
> Signed-off-by: Feng Tang <feng.tang@linux.alibaba.com>
> ---
>   Documentation/admin-guide/sysctl/kernel.rst |  5 ++
>   kernel/hung_task.c                          | 62 +++++++++++++--------
>   2 files changed, 43 insertions(+), 24 deletions(-)
> 
> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> index a397eeccaea7..45b4408dad31 100644
> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst

[...]

> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index 5ac0e66a1361..5b3a7785d3a2 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -24,6 +24,7 @@
>   #include <linux/sched/sysctl.h>
>   #include <linux/hung_task.h>
>   #include <linux/rwsem.h>
> +#include <linux/sys_info.h>
>   
>   #include <trace/events/sched.h>
>   
> @@ -59,12 +60,17 @@ static unsigned long __read_mostly sysctl_hung_task_check_interval_secs;
>   static int __read_mostly sysctl_hung_task_warnings = 10;
>   
>   static int __read_mostly did_panic;
> -static bool hung_task_show_lock;
>   static bool hung_task_call_panic;
> -static bool hung_task_show_all_bt;
>   
>   static struct task_struct *watchdog_task;
>   
> +/*
> + * A bitmask to control what kinds of system info to be printed when
> + * a hung task is detected, it could be task, memory, lock etc. Refer
> + * include/linux/sys_info.h for detailed bit definition.
> + */
> +static unsigned long hung_task_si_mask;
> +
>   #ifdef CONFIG_SMP
>   /*
>    * Should we dump all CPUs backtraces in a hung task event?
> @@ -217,11 +223,8 @@ 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,
> -		unsigned long prev_detect_count)
> +static void check_hung_task(struct task_struct *t, unsigned long timeout)
>   {
> -	unsigned long total_hung_task;
> -
>   	if (!task_is_hung(t, timeout))
>   		return;
>   
> @@ -231,20 +234,13 @@ 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_show_lock = true;
> -		hung_task_call_panic = true;
> -	}
> -
>   	/*
>   	 * 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
[...]

Cheers,
Lance