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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        "Paul E. McKenney" <paulmck@linux.ibm.com>,
        Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 4.20 283/352] kernel/hung_task.c: break RCU locks based on jiffies
Date:   Mon, 11 Feb 2019 15:18:30 +0100
Message-Id: <20190211141904.940859034@linuxfoundation.org>
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4.20-stable review patch.  If anyone has any objections, please let me know.

------------------

[ Upstream commit 304ae42739b108305f8d7b3eb3c1aec7c2b643a9 ]

check_hung_uninterruptible_tasks() is currently calling rcu_lock_break()
for every 1024 threads.  But check_hung_task() is very slow if printk()
was called, and is very fast otherwise.

If many threads within some 1024 threads called printk(), the RCU grace
period might be extended enough to trigger RCU stall warnings.
Therefore, calling rcu_lock_break() for every some fixed jiffies will be
safer.

Link: http://lkml.kernel.org/r/1544800658-11423-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Paul E. McKenney <paulmck@linux.ibm.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/hung_task.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index cb8e3e8ac7b9..444b8b5dd69f 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -34,7 +34,7 @@ int __read_mostly sysctl_hung_task_check_count = PID_MAX_LIMIT;
  * is disabled during the critical section. It also controls the size of
  * the RCU grace period. So it needs to be upper-bound.
  */
-#define HUNG_TASK_BATCHING 1024
+#define HUNG_TASK_LOCK_BREAK (HZ / 10)
 
 /*
  * Zero means infinite timeout - no checking done:
@@ -173,7 +173,7 @@ static bool rcu_lock_break(struct task_struct *g, struct task_struct *t)
 static void check_hung_uninterruptible_tasks(unsigned long timeout)
 {
 	int max_count = sysctl_hung_task_check_count;
-	int batch_count = HUNG_TASK_BATCHING;
+	unsigned long last_break = jiffies;
 	struct task_struct *g, *t;
 
 	/*
@@ -188,10 +188,10 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
 	for_each_process_thread(g, t) {
 		if (!max_count--)
 			goto unlock;
-		if (!--batch_count) {
-			batch_count = HUNG_TASK_BATCHING;
+		if (time_after(jiffies, last_break + HUNG_TASK_LOCK_BREAK)) {
 			if (!rcu_lock_break(g, t))
 				goto unlock;
+			last_break = jiffies;
 		}
 		/* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
 		if (t->state == TASK_UNINTERRUPTIBLE)
-- 
2.19.1