linux-kernel - [PATCH 3/3] hung_task.c: change the "max_count" code to use for_each_process_thread

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Date:	Mon, 25 Jul 2016 18:23:53 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dave Anderson <anderson@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Wang Shu <shuwang@...hat.com>, linux-kernel@...r.kernel.org
Subject: [PATCH 3/3] hung_task.c: change the "max_count" code to use
 for_each_process_thread_break/continue

If sysctl_hung_task_check_count is low and the first "max_count" threads do
not exit, check_hung_uninterruptible_tasks() will never check other tasks,
this is just ugly.

Now that we have for_each_process_thread_continue(), we can save g/t before
return and restart on the next call.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 kernel/hung_task.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 517f52e..e6e516d 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -148,6 +148,9 @@ static void rcu_lock_break(void)
  */
 static void check_hung_uninterruptible_tasks(unsigned long timeout)
 {
+	/* we have only one watchdog() thread */
+	static struct task_struct *g_saved, *t_saved;
+
 	int max_count = sysctl_hung_task_check_count;
 	int batch_count = HUNG_TASK_BATCHING;
 	struct task_struct *g, *t;
@@ -160,18 +163,29 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
 		return;
 
 	rcu_read_lock();
+	if (g_saved) {
+		g = g_saved;
+		t = t_saved;
+		g_saved = NULL;
+		goto resume;
+	}
+
 	for_each_process_thread(g, t) {
 		/* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
 		if (t->state == TASK_UNINTERRUPTIBLE)
 			check_hung_task(t, timeout);
 
-		if (!--max_count)
-			goto unlock;
-		if (!--batch_count) {
-			batch_count = HUNG_TASK_BATCHING;
+		if (!--max_count || !--batch_count) {
 			for_each_process_thread_break(g, t);
+			if (!max_count) {
+				g_saved = g;
+				t_saved = t;
+				goto unlock;
+			}
 			rcu_lock_break();
+ resume:
 			for_each_process_thread_continue(&g, &t);
+			batch_count = HUNG_TASK_BATCHING;
 		}
 	}
  unlock:
-- 
2.5.0