Date:	Mon, 17 Mar 2014 14:32:07 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Don Zickus <dzickus@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Aaron Tomlin <atomlin@...hat.com>
Subject: Re: [PATCH] watchdog: Printing traces for all cpus on lockup
 detection

On Mon, 17 Mar 2014 15:10:05 -0400 Don Zickus <dzickus@...hat.com> wrote:

> From: Aaron Tomlin <atomlin@...hat.com>
> 
> A 'softlockup' is defined as a bug that causes the kernel to
> loop in kernel mode for more than a predefined period to
> time, without giving other tasks a chance to run.
> 
> Currently, upon detection of this condition by the per-cpu
> watchdog task, debug information (including a stack trace)
> is sent to the system log.
> 
> On some occasions, we have observed that the "victim" rather
> than the actual "culprit" (i.e. the owner/holder of the
> contended resource) is reported to the user.
> Often this information has proven to be insufficient to
> assist debugging efforts.
> 
> To avoid loss of useful debug information, for architectures
> which support NMI, this patch makes it possible to improve
> soft lockup reporting. This is accomplished by issuing an
> NMI to each cpu to obtain a stack trace.
> 
> If NMI is not supported we just revert back to the old method.
> A sysctl and boot-time parameter is available to toggle this
> feature.
> 
> [updated kernel-parameters.txt too]

Is a bit like
http://ozlabs.org/~akpm/mmots/broken-out/watchdog-trigger-all-cpu-backtrace-when-locked-up-and-going-to-panic.patch.
Please compare and comment.  I've been sitting on that patch for over
a year because I thought it would cause double-traces on uniprocessor
and nothing happened about that.

>  Documentation/kernel-parameters.txt |    5 +++++
>  Documentation/sysctl/kernel.txt     |   17 +++++++++++++++++
>  include/linux/nmi.h                 |    1 +
>  kernel/sysctl.c                     |    9 +++++++++
>  kernel/watchdog.c                   |   35 +++++++++++++++++++++++++++++++++++

Patch adds a whole bunch of useless stuff to uniprocessor builds.

> +int __read_mostly softlockup_all_cpu_backtrace = 0;

"= 0" is implicit.

There's a poorly-followed convention of prefixing things like this with
"sysctl_", to make it clear that this is a userspace-controlled option.
It's quite a useful thing.

>  static int __read_mostly watchdog_running;
>  static u64 __read_mostly sample_period;
>  
> @@ -47,6 +48,7 @@ static DEFINE_PER_CPU(bool, watchdog_nmi_touch);
>  static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts_saved);
>  static DEFINE_PER_CPU(struct perf_event *, watchdog_ev);
>  #endif
> +static unsigned long soft_lockup_nmi_warn;

This could be local to watchdog_timer_fn()

What does it do, anyway?  The name is unilluminating.  A comment would
be nice.

>  /* boot commands */
>  /*
> @@ -95,6 +97,20 @@ static int __init nosoftlockup_setup(char *str)
>  }
>  __setup("nosoftlockup", nosoftlockup_setup);
>  /*  */
> +static int __init softlockup_all_cpu_backtrace_setup(char *str)
> +{
> +	if (!str)
> +		return -EINVAL;
> +
> +	softlockup_all_cpu_backtrace = simple_strtoul(str, NULL, 0);
> +
> +	if (softlockup_all_cpu_backtrace < 0 ||
> +		softlockup_all_cpu_backtrace > 1)
> +		return -EINVAL;

We've just successfully modified `softlockup_all_cpu_backtrace'!  It's
too late to check for errors.

> +	return 1;
> +}
> +__setup("softlockup_all_cpu_backtrace=", softlockup_all_cpu_backtrace_setup);
>  
>  /*
>   * Hard-lockup warnings should be triggered after just a few seconds. Soft-
> @@ -313,6 +329,14 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
>  		if (__this_cpu_read(soft_watchdog_warn) == true)
>  			return HRTIMER_RESTART;
>  
> +		if (softlockup_all_cpu_backtrace) {
> +			if (test_and_set_bit(0, &soft_lockup_nmi_warn)) {
> +				/* Someone else will report us. Let's give up */
> +				__this_cpu_write(soft_watchdog_warn, true);
> +				return HRTIMER_RESTART;
> +			}
> +		}
> +
>  		printk(KERN_EMERG "BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",
>  			smp_processor_id(), duration,
>  			current->comm, task_pid_nr(current));
> @@ -323,6 +347,17 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
>  		else
>  			dump_stack();
>  
> +		if (softlockup_all_cpu_backtrace) {
> +			/* Inject an NMI to gather a stack trace for
> +			 * each cpu in the hope to obtain further
> +			 * debug information
> +			 */

That's pretty poor comment, sorry.  It tells us "what", not "why".  And
what it's saying was very obvious anyway.

> +			trigger_all_cpu_backtrace();
> +
> +			clear_bit(0, &soft_lockup_nmi_warn);
> +			smp_mb__after_clear_bit();
> +		}

I'm surely having a stupid day, but it's taking far too long for me to
work out what this code does.  What's with this flag in
soft_lockup_nmi_warn?  What's it doing?

And why is that barrier there?  What's it barriering against?  Barriers
should always be commented, please.

>  		if (softlockup_panic)
>  			panic("softlockup: hung tasks");
>  		__this_cpu_write(soft_watchdog_warn, true);

We forgot to document arch_trigger_all_cpu_backtrace().  It's
actually arch_trigger_all_other_cpus_backtrace, yes?  So we don't
generate two backtraces for the current cpu?

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