Re: [v7 PATCH 1/2] hung_task: Refactor detection logic and atomicise detection count

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

On 2026/1/25 21:58, Aaron Tomlin wrote:
> The check_hung_task() function currently conflates two distinct
> responsibilities: validating whether a task is hung and handling the
> subsequent reporting (printing warnings, triggering panics, or
> tracepoints).
> 
> This patch refactors the logic by introducing hung_task_info(), a
> function dedicated solely to reporting. The actual detection check,
> task_is_hung(), is hoisted into the primary loop within
> check_hung_uninterruptible_tasks(). This separation clearly decouples
> the mechanism of detection from the policy of reporting.
> 
> Furthermore, to facilitate future support for concurrent hung task
> detection, the global sysctl_hung_task_detect_count variable is
> converted from unsigned long to atomic_long_t. Consequently, the
> counting logic is updated to accumulate the number of hung tasks locally
> (this_round_count) during the iteration. The global counter is then
> updated atomically via atomic_long_cmpxchg_relaxed() once the loop
> concludes, rather than incrementally during the scan.
> 
> These changes are strictly preparatory and introduce no functional
> change to the system's runtime behaviour.
> 
> Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
> ---
>   kernel/hung_task.c | 58 ++++++++++++++++++++++++++--------------------
>   1 file changed, 33 insertions(+), 25 deletions(-)
> 
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index d2254c91450b..df10830ed9ef 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -36,7 +36,7 @@ static int __read_mostly sysctl_hung_task_check_count = PID_MAX_LIMIT;
>   /*
>    * Total number of tasks detected as hung since boot:
>    */
> -static unsigned long __read_mostly sysctl_hung_task_detect_count;
> +static atomic_long_t sysctl_hung_task_detect_count = ATOMIC_LONG_INIT(0);
>   
>   /*
>    * Limit number of tasks checked in a batch.
> @@ -223,31 +223,29 @@ 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)
> +/**
> + * hung_task_info - Print diagnostic details for a hung task
> + * @t: Pointer to the detected hung task.
> + * @timeout: Timeout threshold for detecting hung tasks
> + * @this_round_count: Count of hung tasks detected in the current iteration
> + *
> + * Print structured information about the specified hung task, if warnings
> + * are enabled or if the panic batch threshold is exceeded.
> + */
> +static void hung_task_info(struct task_struct *t, unsigned long timeout,
> +			   unsigned long this_round_count)
>   {
> -	unsigned long total_hung_task;
> -
> -	if (!task_is_hung(t, timeout))
> -		return;
> -
> -	/*
> -	 * This counter tracks the total number of tasks detected as hung
> -	 * since boot.
> -	 */
> -	sysctl_hung_task_detect_count++;

Previously, the global detect count updated immediately when a hung task
was found. BUT now, it only updates after the full scan finishes ...

Ideally, the count should update as soon as possible, so that userspace
can react in time :)

For example, by migrating critical containers away from the node before
the situation gets worse - something we already do.

Cheers,
Lance