Re: [PATCH] hung_task: Differentiate between I/O and Lock/Resource waits

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Sun, 25 Jan 2026 15:39:05 -0500
Aaron Tomlin <atomlin@atomlin.com> wrote:

> Currently, the hung task reporting mechanism does not differentiate
> between the underlying causes of a D state, labelling all such tasks
> merely as "blocked". Consequently, administrators must perform manual
> stack trace inspection to ascertain if the delay stems from an I/O wait
> (indicative of hardware or filesystem issues) or a lock wait (indicative
> of software contention).
> 
> This change utilises the in_iowait field from struct task_struct to
> distinguish between two distinct failure modes in the log output:
> 
>         1. D state "Disk I/O": The task is waiting in io_schedule().
>            This typically implies a storage device, filesystem, or
>            network filesystem (e.g., NFS) is unresponsive.
> 
>         2. D state "Lock/Resource": The task is waiting on a kernel
>            primitive (e.g., mutex). This typically implies a software
>            bug, deadlock, or resource starvation.
> 
> It is safe to read in_iowait in this manner because
> check_hung_uninterruptible_tasks() holds the RCU read lock, preserving
> the task structure. Moreover, the task is effectively quiescent (in a
> persistent TASK_UNINTERRUPTIBLE state) and thus cannot update its own
> in_iowait status, guaranteeing a stable, race-free value.
> 
> Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
> ---
>  kernel/hung_task.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index 350093de0535..608731c7ccba 100644
> --- 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 in %s state for more than %ld seconds.\n",
> +		       t->comm, t->pid, t->in_iowait ? "D (Disk I/O)" : "D (Lock/Resource)",

If this is only for human readability, I rather like just adding
"in iowait" at the end. "D" state seems redundant, and "Lock/Resource"
can mislead. What about something like below?

	pr_err("INFO: task %s:%d blocked for more than %ld seconds%s.\n",
		..., t->in_iowait ? " in I/O wait" : "");

Thank you,

> +		       (jiffies - t->last_switch_time) / HZ);
>  		pr_err("      %s %s %.*s\n",
>  			print_tainted(), init_utsname()->release,
>  			(int)strcspn(init_utsname()->version, " "),
> -- 
> 2.51.0
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>