hung_task: dump blocker task if it is not hung

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).

Link: https://lkml.kernel.org/r/175391351423.688839.11917911323784986774.stgit@devnote2
Signed-off-by: Masami Hiramatsu (Google) <[e-mail]>
Suggested-by: Sergey Senozhatsky <[e-mail]>
Tested-by: Sergey Senozhatsky <[e-mail]>
Acked-by: Lance Yang <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

kernel/hung_task.c

···
 	.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;
···
 			       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;
 
 	/*
···
 		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)
···
 	hung_task_show_lock = false;
 	rcu_read_lock();
 	for_each_process_thread(g, t) {
-		unsigned int state;
 
 		if (!max_count--)
 			goto unlock;
···
 				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();