Re: [PATCH] hung_task: Increment the global counter immediately

[Aaron Tomlin]

On Wed, Feb 04, 2026 at 12:04:54PM +0100, Petr Mladek wrote:
> A recent change allowed to reset the global counter of hung tasks using
> the sysctl interface. A potential race with the regular check has been
> solved by updating the global counter only once at the end of the check.
> 
> However, the hung task check can take a significant amount of time,
> particularly when task information is being dumped to slow serial
> consoles. Some users monitor this global counter to trigger immediate
> migration of critical containers. Delaying the increment until the
> full check completes postpones these high-priority rescue operations.
> 
> Update the global counter as soon as a hung task is detected. Since
> the value is read asynchronously, a relaxed atomic operation is
> sufficient.
> 
> Reported-by: Lance Yang <lance.yang@xxxxxxxxx>
> Closes: https://lore.kernel.org/r/f239e00f-4282-408d-b172-0f9885f4b01b@xxxxxxxxx
> Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
> ---
> This is a followup patch for
> https://lore.kernel.org/r/20260125135848.3356585-1-atomlin@xxxxxxxxxxx
> 
> Note that I could not use commit IDs because the original
> patchset is not in a stable tree yet. In fact, it seems
> that it is not even in linux-next at the moment.
> 
> Best Regards,
> Petr
> 
>  kernel/hung_task.c | 23 ++++++++---------------
>  1 file changed, 8 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index 350093de0535..8bc043fbe89c 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -302,15 +302,10 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
>  	int max_count = sysctl_hung_task_check_count;
>  	unsigned long last_break = jiffies;
>  	struct task_struct *g, *t;
> -	unsigned long total_count, this_round_count;
> +	unsigned long this_round_count;
>  	int need_warning = sysctl_hung_task_warnings;
>  	unsigned long si_mask = hung_task_si_mask;
>  
> -	/*
> -	 * The counter might get reset. Remember the initial value.
> -	 * Acquire prevents reordering task checks before this point.
> -	 */
> -	total_count = atomic_long_read_acquire(&sysctl_hung_task_detect_count);
>  	/*
>  	 * If the system crashed already then all bets are off,
>  	 * do not report extra hung tasks:
> @@ -330,6 +325,13 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
>  		}
>  
>  		if (task_is_hung(t, timeout)) {
> +			/*
> +			 * Increment the global counter so that userspace could
> +			 * start migrating tasks ASAP. But count the current
> +			 * round separately because userspace could reset
> +			 * the global counter at any time.
> +			 */
> +			atomic_long_inc(&sysctl_hung_task_detect_count);
>  			this_round_count++;
>  			hung_task_info(t, timeout, this_round_count);
>  		}
> @@ -340,15 +342,6 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
>  	if (!this_round_count)
>  		return;
>  
> -	/*
> -	 * Do not count this round when the global counter has been reset
> -	 * during this check. Release ensures we see all hang details
> -	 * recorded during the scan.
> -	 */
> -	atomic_long_cmpxchg_release(&sysctl_hung_task_detect_count,
> -				    total_count, total_count +
> -				    this_round_count);
> -
>  	if (need_warning || hung_task_call_panic) {
>  		si_mask |= SYS_INFO_LOCKS;
>  
> -- 
> 2.52.0
> 

Agreed.
This is correct given the architectural shift from "Batched" to "Immediate"
updates.

Reviewed-by: Aaron Tomlin <atomlin@xxxxxxxxxxx>

-- 
Aaron Tomlin