Re: [PATCH v3 2/2] hung_task: Enable runtime reset of hung_task_detect_count

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

On 2025/12/16 11:00, Aaron Tomlin wrote:
> Introduce support for writing to /proc/sys/kernel/hung_task_detect_count.
> 
> Writing any value to this file atomically resets the counter of detected
> hung tasks to zero. This grants system administrators the ability to clear
> the cumulative diagnostic history after resolving an incident, simplifying
> monitoring without requiring a system restart.

Thanks!

> 
> Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
> ---
>   Documentation/admin-guide/sysctl/kernel.rst |  2 +-
>   kernel/hung_task.c                          | 29 +++++++++++++++++++--
>   2 files changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> index 239da22c4e28..43c17b919969 100644
> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst
> @@ -418,7 +418,7 @@ hung_task_detect_count
>   ======================
>   
>   Indicates the total number of tasks that have been detected as hung since
> -the system boot.
> +the system boot. The counter can be reset to zero when written to.
>   
>   This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
>   
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index 5902573200c0..01ce46a107b0 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -375,6 +375,31 @@ 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
> + * @write: 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.
> + * Returns 0 on success or a negative error code on failure.
> + */
> +static int proc_dohung_task_detect_count(const struct ctl_table *table, int write,
> +					 void *buffer, size_t *lenp, loff_t *ppos)
> +{
> +	if (!write)
> +		return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
> +
> +	WRITE_ONCE(sysctl_hung_task_detect_count, 0);

The reset uses WRITE_ONCE() but the increment in check_hung_task()
is plain ++, it's just a stat cunter so fine though ;)

Tested-by: Lance Yang <lance.yang@linux.dev>

Reviewed-by: Lance Yang <lance.yang@linux.dev>

Cheers,
Lance

> +	*ppos += *lenp;
> +
> +	return 0;
> +}
> +
>   /*
>    * Process updating of timeout sysctl
>    */
> @@ -457,8 +482,8 @@ 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",