Commit 780cbcf2 authored May 08, 2017 by Tetsuo Handa Committed by Linus Torvalds May 08, 2017

kernel/hung_task.c: defer showing held locks

When I was running my testcase which may block hundreds of threads on fs
locks, I got lockup due to output from debug_show_all_locks() added by
commit b2d4c2ed ("locking/hung_task: Show all locks").

For example, if 1000 threads were blocked in TASK_UNINTERRUPTIBLE state
and 500 out of 1000 threads hold some lock, debug_show_all_locks() from
for_each_process_thread() loop will report locks held by 500 threads for
1000 times. This is a too much noise.

In order to make sure rcu_lock_break() is called frequently, we should
avoid calling debug_show_all_locks() from for_each_process_thread() loop
because debug_show_all_locks() effectively calls for_each_process_thread()
loop. Let's defer calling debug_show_all_locks() till before panic() or
leaving for_each_process_thread() loop.

Link: http://lkml.kernel.org/r/1489296834-60436-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Vegard Nossum <vegard.nossum@oracle.com>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 31b8cc80

kernel/hung_task.c

+7 −1

Original line number	Diff line number	Diff line
		@@ -43,6 +43,7 @@ unsigned long __read_mostly sysctl_hung_task_timeout_secs = CONFIG_DEFAULT_HUNG_
		int __read_mostly sysctl_hung_task_warnings = 10;

		static int __read_mostly did_panic;
		static bool hung_task_show_lock;

		static struct task_struct *watchdog_task;

		@@ -120,12 +121,14 @@ 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_all_locks();
		hung_task_show_lock = true;
		}

		touch_nmi_watchdog();

		if (sysctl_hung_task_panic) {
		if (hung_task_show_lock)
		debug_show_all_locks();
		trigger_all_cpu_backtrace();
		panic("hung_task: blocked tasks");
		}
		@@ -172,6 +175,7 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
		if (test_taint(TAINT_DIE) \|\| did_panic)
		return;

		hung_task_show_lock = false;
		rcu_read_lock();
		for_each_process_thread(g, t) {
		if (!max_count--)
		@@ -187,6 +191,8 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
		}
		unlock:
		rcu_read_unlock();
		if (hung_task_show_lock)
		debug_show_all_locks();
		}

		static long hung_timeout_jiffies(unsigned long last_checked,

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