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 b2d4c2edb2e4f89a ("locking/hung_task: Show all locks").
I think we don't need to call debug_show_all_locks() on each blocked
thread. Let's defer calling debug_show_all_locks() till before panic()
or leaving for_each_process_thread() loop.
Signed-off-by: Tetsuo Handa <[email protected]>
Cc: Vegard Nossum <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mandeep Singh Baines <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
---
kernel/hung_task.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 2b59c82..a9ada5d 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -40,6 +40,7 @@
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;
@@ -116,12 +117,14 @@ 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");
}
@@ -168,6 +171,7 @@ 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--)
@@ -183,6 +187,8 @@ 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,
--
1.8.3.1
On 13 December 2016 at 15:45, Tetsuo Handa
<[email protected]> wrote:
> 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 b2d4c2edb2e4f89a ("locking/hung_task: Show all locks").
>
> I think we don't need to call debug_show_all_locks() on each blocked
> thread. Let's defer calling debug_show_all_locks() till before panic()
> or leaving for_each_process_thread() loop.
First of all, sorry for not answering earlier.
I'm not sure I fully understand the problem, you say the "output from
debug_show_all_locks()" caused a lockup, but was the problem simply
that the amount of output caused it to stall for a long time?
Could we instead
1) move the debug_show_all_locks() into the if
(sysctl_hung_task_panic) bit unconditionally
2) call something (touch_nmi_watchdog()?) inside debug_show_all_locks()
3) in another way make debug_show_all_locks() more robust so it doesn't "lockup"
?
Vegard
Vegard Nossum wrote:
> On 13 December 2016 at 15:45, Tetsuo Handa
> <[email protected]> wrote:
> > 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 b2d4c2edb2e4f89a ("locking/hung_task: Show all locks").
> >
> > I think we don't need to call debug_show_all_locks() on each blocked
> > thread. Let's defer calling debug_show_all_locks() till before panic()
> > or leaving for_each_process_thread() loop.
>
> First of all, sorry for not answering earlier.
No problem.
>
> I'm not sure I fully understand the problem, you say the "output from
> debug_show_all_locks()" caused a lockup, but was the problem simply
> that the amount of output caused it to stall for a long time?
In Linux 4.9, in order to tell administrator that something might be wrong
with memory allocation, warn_alloc() which calls printk() periodically
when memory allocation is stalling for too long was added. However, since
printk() waits until all pending data is sent to console using cond_resched(),
printk() continues waiting as long as somebody else calls printk() when
cond_resched() is called. This is problematic under OOM situation.
Since the OOM killer calls printk() with oom_lock held, it happened that
printk() called from the OOM killer is forever unable to return because
warn_alloc() periodically calls printk() since the OOM killer is holding
oom_lock.
And it happened that khungtaskd is another source which calls printk()
periodically when threads are blocked on fs locks waiting for memory
allocation. debug_show_all_locks() generates far more amount of output
compared to warn_alloc() if debug_show_all_locks() is called on each
thread blocked on fs locks waiting for memory allocation. Therefore,
we should avoid calling debug_show_all_locks() on each blocked thread.
Full story starts at http://lkml.kernel.org/r/1481020439-5867-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp but
I appreciate if you can join on http://lkml.kernel.org/r/1478416501-10104-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp .
>
> Could we instead
>
> 1) move the debug_show_all_locks() into the if
> (sysctl_hung_task_panic) bit unconditionally
>
> 2) call something (touch_nmi_watchdog()?) inside debug_show_all_locks()
>
> 3) in another way make debug_show_all_locks() more robust so it doesn't "lockup"
>
> ?
Yes, that might be an improvement. But not needed for this patch.