Re: [v2 PATCH 1/1] hung_task: Explicitly report I/O wait state in log output

[Petr Mladek <pmladek@suse.com>]

On Wed 2026-01-28 15:45:16, Aaron Tomlin wrote:
> Currently, the hung task reporting mechanism indiscriminately labels all
> TASK_UNINTERRUPTIBLE (D) tasks as "blocked", irrespective of whether they
> are awaiting I/O completion or kernel locking primitives. This ambiguity
> compels system administrators to manually inspect stack traces to discern
> whether the delay stems from an I/O wait (typically indicative of
> hardware or filesystem anomalies) or software contention. Such detailed
> analysis is not always immediately accessible to system administrators
> or support engineers.
> 
> To address this, this patch utilises the existing in_iowait field within
> struct task_struct to augment the failure report. If the task is blocked
> due to I/O (e.g., via io_schedule_prepare()), the log message is updated
> to explicitly state "blocked in I/O wait".
> 
> Examples:
>         - Standard Block: "INFO: task bash:123 blocked for more than 120
>           seconds".
> 
>         - I/O Block: "INFO: task dd:456 blocked in I/O wait for more than
>           120 seconds".
> 
> Accessing in_iowait is safe in this context. The detector holds
> rcu_read_lock() within check_hung_uninterruptible_tasks(), ensuring the
> task structure remains valid in memory.

This part is true.

> Furthermore, as the task is
> confirmed to be in a persistent TASK_UNINTERRUPTIBLE state, it cannot
> modify its own in_iowait flag, rendering the read operation stable and
> free from data races.

Strictly speaking, this is not true. IMHO, nothing prevents the task
from waking up in parallel. Just the chance is small.

I would say that the information will be valid in 99.99% of situations
because the message is printed only when the task has been stuck
in the state for a long time. A possible mistake should be
visible from the later printed backtrace.

> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -250,8 +250,9 @@ static void hung_task_info(struct task_struct *t, unsigned long timeout,
>  	if (sysctl_hung_task_warnings || hung_task_call_panic) {
>  		if (sysctl_hung_task_warnings > 0)
>  			sysctl_hung_task_warnings--;
> -		pr_err("INFO: task %s:%d blocked for more than %ld seconds.\n",
> -		       t->comm, t->pid, (jiffies - t->last_switch_time) / HZ);
> +		pr_err("INFO: task %s:%d blocked %s for more than %ld seconds.\n",

s/blocked %s for/blocked%s for/

> +		       t->comm, t->pid, t->in_iowait ? "in I/O wait" : "",

and here: " in I/O wait".

Otherwise, it would print two spaces in the non-io case, like"

  "INFO: task bash:123 blocked  for more than 120 seconds"

> +		       (jiffies - t->last_switch_time) / HZ);
>  		pr_err("      %s %s %.*s\n",
>  			print_tainted(), init_utsname()->release,
>  			(int)strcspn(init_utsname()->version, " "),

Otherwise, it looks good.

Best Regards,
Petr