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

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

On 2026/1/28 16:17, Masami Hiramatsu (Google) wrote:
> 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" : "");

That would be better, looks good to me ;)