Re: [PATCH v2 2/2] hung_task: Enable runtime reset of hung_task_detect_count

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

On 2025/12/11 11:30, 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.
> 
> Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
> ---
>   Documentation/admin-guide/sysctl/kernel.rst |  2 +-
>   kernel/hung_task.c                          | 30 +++++++++++++++++++--
>   2 files changed, 29 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 6f3fb26378b5..979b7e2fcc19 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -377,6 +377,32 @@ static long hung_timeout_jiffies(unsigned long last_checked,
>   }
>   
>   #ifdef CONFIG_SYSCTL
> +
> +/**
> + * proc_dohung_task_detect_count - proc handler for hung_task_detect_count
> + *
> + * Handles read access for the hung task counter. For write access, it
> + * accepts any successfully parsed value and atomically resets the counter
> + * to zero. Returns the byte count written 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)
> +{
> +	int ret;
> +
> +	if (!write)
> +		return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
> +
> +	ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);

Since the intent is "any write resets to zero", we could skip parsing
the input entirely (untested):

	WRITE_ONCE(sysctl_hung_task_detect_count, 0);
	*ppos += *lenp;
	return 0;

See vmstat_refresh() for a similar pattern :)

> +	if (ret)
> +		return ret;
> +
> +	WRITE_ONCE(sysctl_hung_task_detect_count, 0);
> +
> +	return ret;
> +}

Cheers,
Lance

> +
>   /*
>    * Process updating of timeout sysctl
>    */
> @@ -459,8 +485,8 @@ 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",