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

[Aaron Tomlin]

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)",
+		       (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