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 b2d4c2edb2e4 ("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>

1 files changed, 7 insertions, 1 deletions

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index f0f8e2a8496f..751593ed7c0b 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -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,