Hi,
We have a feature request, the customer needs something more hookable
than just printk's from check_hung_task() to implement the user-space
watchdog which can potentially resolve the problems which caused the
hang.
The patch simply adds a tracepoint into check_hung_task(), do you
think we can (ab)use tp this way?
And I am just curious, perhaps another patch (below) makes sense too?
Oleg.
------------------------------------------------------------------------------
[PATCH] hung_task debugging: Do not report the same task twice
If a task hangs, check_hung_task() will blame it again and again
until it wakes up or sysctl_hung_task_warnings becomes zero. IMO,
this just adds the unnecessary noise and if another task hangs
after sysctl_hung_task_timeout_secs * sysctl_hung_task_warnings
it won't be reported.
With this patch check_hung_task() simply sets the most significant
bit in ->last_switch_count to mark this task as "already reported".
This bit is cleared if we notice the change in nvcsw/nivcsw, so we
do not skip this task if it hangs again later.
Note that we also ignore the MSB in switch_count, we need this to
avoid the false-positive "already reported". This means that
->last_switch_count is not necessarily equal to ->nvcsw + ->nivcsw
but we do not care, we have enough bits to notice the change.
And this allows to remove the special switch_count == 0 case, just
we need to initialize ->last_switch_count = LONG_MIN.
Signed-off-by: Oleg Nesterov <o...@redhat.com>
---
kernel/fork.c | 2 +-
kernel/hung_task.c | 19 +++++++++----------
2 files changed, 10 insertions(+), 11 deletions(-)
diff --git a/kernel/fork.c b/kernel/fork.c
index 8531609..aea397b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -866,7 +866,7 @@ static int copy_mm(unsigned long clone_flags, struct
task_struct *tsk)
tsk->min_flt = tsk->maj_flt = 0;
tsk->nvcsw = tsk->nivcsw = 0;
#ifdef CONFIG_DETECT_HUNG_TASK
- tsk->last_switch_count = tsk->nvcsw + tsk->nivcsw;
+ tsk->last_switch_count = LONG_MIN; /* see check_hung_task() */
#endif
tsk->mm = NULL;
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 3952ab1..0f6233c 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -72,7 +72,7 @@ static struct notifier_block panic_block = {
static void check_hung_task(struct task_struct *t, unsigned long timeout)
{
- unsigned long switch_count = t->nvcsw + t->nivcsw;
+ unsigned long switch_count;
/*
* Ensure the task is not frozen.
@@ -81,19 +81,18 @@ static void check_hung_task(struct task_struct *t, unsigned
long timeout)
if (unlikely(t->flags & (PF_FROZEN | PF_FREEZER_SKIP)))
return;
- /*
- * When a freshly created task is scheduled once, changes its state to
- * TASK_UNINTERRUPTIBLE without having ever been switched out once, it
- * musn't be checked.
- */
- if (unlikely(!switch_count))
- return;
-
- if (switch_count != t->last_switch_count) {
+ /* Ignore MSB, see below. LONG_MAX = ~LONG_MIN. */
+ switch_count = (t->nvcsw + t->nivcsw) & LONG_MAX;
+ if (switch_count != (LONG_MAX & t->last_switch_count)) {
t->last_switch_count = switch_count;
return;
}
+ /* We use MSB to mark this task as already reported. */
+ if (t->last_switch_count & LONG_MIN)
+ return;
+ t->last_switch_count |= LONG_MIN;
+
trace_sched_process_hang(t);
if (!sysctl_hung_task_warnings)
--
1.5.5.1
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