[PATCH v2] hung_task: Dump blocker task if it is not hung

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

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

Dump the lock blocker task if it is not hung because if the blocker
task is also hung, it should be dumped by the detector. This will
de-duplicate the same stackdumps if the blocker task is also blocked
by another task (and hung).

Suggested-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Lance Yang <lance.yang@linux.dev>
---
 Changes in v2:
  - Rebase on next-20250730
  - Merge task state checks.
  - Add Lance's ack.
---
 kernel/hung_task.c |   78 +++++++++++++++++++++++++++-------------------------
 1 file changed, 41 insertions(+), 37 deletions(-)

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 8708a1205f82..b2c1f14b8129 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -95,9 +95,41 @@ static struct notifier_block panic_block = {
 	.notifier_call = hung_task_panic,
 };
 
+static bool task_is_hung(struct task_struct *t, unsigned long timeout)
+{
+	unsigned long switch_count = t->nvcsw + t->nivcsw;
+	unsigned int state = READ_ONCE(t->__state);
+
+	/*
+	 * skip the TASK_KILLABLE tasks -- these can be killed
+	 * skip the TASK_IDLE tasks -- those are genuinely idle
+	 * skip the TASK_FROZEN task -- it reasonably stops scheduling by freezer
+	 */
+	if (!(state & TASK_UNINTERRUPTIBLE) ||
+	    (state & (TASK_WAKEKILL | TASK_NOLOAD | TASK_FROZEN)))
+		return false;
+
+	/*
+	 * When a freshly created task is scheduled once, changes its state to
+	 * TASK_UNINTERRUPTIBLE without having ever been switched out once, it
+	 * musn't be checked.
+	 */
+	if (unlikely(!switch_count))
+		return false;
+
+	if (switch_count != t->last_switch_count) {
+		t->last_switch_count = switch_count;
+		t->last_switch_time = jiffies;
+		return false;
+	}
+	if (time_is_after_jiffies(t->last_switch_time + timeout * HZ))
+		return false;
+
+	return true;
+}
 
 #ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
-static void debug_show_blocker(struct task_struct *task)
+static void debug_show_blocker(struct task_struct *task, unsigned long timeout)
 {
 	struct task_struct *g, *t;
 	unsigned long owner, blocker, blocker_type;
@@ -174,41 +206,21 @@ static void debug_show_blocker(struct task_struct *task)
 			       t->pid, rwsem_blocked_by);
 			break;
 		}
-		sched_show_task(t);
+		/* Avoid duplicated task dump, skip if the task is also hung. */
+		if (!task_is_hung(t, timeout))
+			sched_show_task(t);
 		return;
 	}
 }
 #else
-static inline void debug_show_blocker(struct task_struct *task)
+static inline void debug_show_blocker(struct task_struct *task, unsigned long timeout)
 {
 }
 #endif
 
 static void check_hung_task(struct task_struct *t, unsigned long timeout)
 {
-	unsigned long switch_count = t->nvcsw + t->nivcsw;
-
-	/*
-	 * Ensure the task is not frozen.
-	 * Also, skip vfork and any other user process that freezer should skip.
-	 */
-	if (unlikely(READ_ONCE(t->__state) & TASK_FROZEN))
-		return;
-
-	/*
-	 * When a freshly created task is scheduled once, changes its state to
-	 * TASK_UNINTERRUPTIBLE without having ever been switched out once, it
-	 * musn't be checked.
-	 */
-	if (unlikely(!switch_count))
-		return;
-
-	if (switch_count != t->last_switch_count) {
-		t->last_switch_count = switch_count;
-		t->last_switch_time = jiffies;
-		return;
-	}
-	if (time_is_after_jiffies(t->last_switch_time + timeout * HZ))
+	if (!task_is_hung(t, timeout))
 		return;
 
 	/*
@@ -243,7 +255,7 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 		pr_err("\"echo 0 > /proc/sys/kernel/hung_task_timeout_secs\""
 			" disables this message.\n");
 		sched_show_task(t);
-		debug_show_blocker(t);
+		debug_show_blocker(t, timeout);
 		hung_task_show_lock = true;
 
 		if (sysctl_hung_task_all_cpu_backtrace)
@@ -299,7 +311,6 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
 	hung_task_show_lock = false;
 	rcu_read_lock();
 	for_each_process_thread(g, t) {
-		unsigned int state;
 
 		if (!max_count--)
 			goto unlock;
@@ -308,15 +319,8 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
 				goto unlock;
 			last_break = jiffies;
 		}
-		/*
-		 * skip the TASK_KILLABLE tasks -- these can be killed
-		 * skip the TASK_IDLE tasks -- those are genuinely idle
-		 */
-		state = READ_ONCE(t->__state);
-		if ((state & TASK_UNINTERRUPTIBLE) &&
-		    !(state & TASK_WAKEKILL) &&
-		    !(state & TASK_NOLOAD))
-			check_hung_task(t, timeout);
+
+		check_hung_task(t, timeout);
 	}
  unlock:
 	rcu_read_unlock();