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

[Lance Yang]

On 2026/1/28 16:17, Masami Hiramatsu (Google) wrote:
> On Sun, 25 Jan 2026 15:39:05 -0500
> Aaron Tomlin <atomlin@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> > ---
> >   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" : "");

That would be better, looks good to me ;)