linux-kernel - Re: [PATCH 2/2] watchdog: Softlockup has regular windows where it is not armed

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Date:	Mon, 28 Nov 2011 16:47:04 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Anton Blanchard <anton@...ba.org>
Cc:	Jeremy Fitzhardinge <jeremy@...source.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, jason.wessel@...driver.com
Subject: Re: [PATCH 2/2] watchdog: Softlockup has regular windows where it is
 not armed

On Thu, Nov 24, 2011 at 02:54:41PM +1100, Anton Blanchard wrote:
> 
> The softlockup watchdog has a two stage sync - touch_softlockup_watchdog
> simply sets the timestamp to 0 and later on the timer routine notices
> this and sets the timestamp.
> 
> The problem is this timer goes off every 4 seconds by default, so
> each time we call touch_softlockup_watchdog there is a period
> of up to 4 seconds where the softlockup watchdog is not armed.
> 
> We call touch_softlockup_watchdog very often in the NO_HZ code and
> end up hitting this issue every time we go in and out of idle.
> 
> I wrote a simple test case:
> 
> http://ozlabs.org/~anton/junkcode/badguy.tar.gz
> 
> That disables interrupts on selected CPUs for a period of time. Don't
> run it on a machine you care about. When I disable interrupts for 30
> seconds on a previously idle CPU I get no warning:
> 
> insmod ./badguy.ko timeout=30 cpus=4
> 
> However if I keep the CPU busy so we don't switch in and out of NO_HZ
> mode I get a warning as expected:
> 
> taskset -c 4 yes > /dev/null &
> insmod ./badguy.ko timeout=30 cpus=4
> 
> With the following patch I get a warning even on a previously idle
> CPU.
> 
> Signed-off-by: Anton Blanchard <anton@...ba.org>
> ---
> 
> There might be a reason for this two stage sync but I haven't been
> able to find it yet. Perhaps the unsynced versions of cpu_clock() and
> sched_clock_tick() are not safe to call from all contexts?

According to commit 8c2238eaaf0f774ca0f8d9daad7a616429bbb7f1 that was the
case, cpu_clock wasn't NMI-safe.  Now it is, thanks to Peter.

I have a couple of concerns about the patch.  I am wondering about the
overhead of getting the timestamp more often now as opposed to just
setting a boolean for later.  It makes sense to stamp it at the time of
the call, don't know what the cost is.

I am also concern about how this affects suspend/resume and kgdb. I cc'd
Jason above for kgdb.  I'll have to run some tests locally to see what
long periods of delay look like.  Oh and virt guests too.  You don't have
any test results from that setup do you?

Cheers,
Don

> 
> Index: linux-build/kernel/watchdog.c
> ===================================================================
> --- linux-build.orig/kernel/watchdog.c	2011-11-16 08:04:56.274478516 +1100
> +++ linux-build/kernel/watchdog.c	2011-11-16 08:04:59.278533261 +1100
> @@ -33,7 +33,6 @@ int __read_mostly watchdog_thresh = 10;
>  static DEFINE_PER_CPU(unsigned long, watchdog_touch_ts);
>  static DEFINE_PER_CPU(struct task_struct *, softlockup_watchdog);
>  static DEFINE_PER_CPU(struct hrtimer, watchdog_hrtimer);
> -static DEFINE_PER_CPU(bool, softlockup_touch_sync);
>  static DEFINE_PER_CPU(bool, soft_watchdog_warn);
>  #ifdef CONFIG_HARDLOCKUP_DETECTOR
>  static DEFINE_PER_CPU(bool, hard_watchdog_warn);
> @@ -134,7 +133,7 @@ static void __touch_watchdog(void)
>  
>  void touch_softlockup_watchdog(void)
>  {
> -	__this_cpu_write(watchdog_touch_ts, 0);
> +	__touch_watchdog();
>  }
>  EXPORT_SYMBOL(touch_softlockup_watchdog);
>  
> @@ -157,8 +156,8 @@ EXPORT_SYMBOL(touch_nmi_watchdog);
>  
>  void touch_softlockup_watchdog_sync(void)
>  {
> -	__raw_get_cpu_var(softlockup_touch_sync) = true;
> -	__raw_get_cpu_var(watchdog_touch_ts) = 0;
> +	sched_clock_tick();
> +	__touch_watchdog();
>  }
>  
>  #ifdef CONFIG_HARDLOCKUP_DETECTOR
> @@ -258,19 +257,6 @@ static enum hrtimer_restart watchdog_tim
>  	/* .. and repeat */
>  	hrtimer_forward_now(hrtimer, ns_to_ktime(get_sample_period()));
>  
> -	if (touch_ts == 0) {
> -		if (unlikely(__this_cpu_read(softlockup_touch_sync))) {
> -			/*
> -			 * If the time stamp was touched atomically
> -			 * make sure the scheduler tick is up to date.
> -			 */
> -			__this_cpu_write(softlockup_touch_sync, false);
> -			sched_clock_tick();
> -		}
> -		__touch_watchdog();
> -		return HRTIMER_RESTART;
> -	}
> -
>  	/* check for a softlockup
>  	 * This is done by making sure a high priority task is
>  	 * being scheduled.  The task touches the watchdog to
> @@ -438,7 +424,7 @@ static int watchdog_enable(int cpu)
>  			goto out;
>  		}
>  		kthread_bind(p, cpu);
> -		per_cpu(watchdog_touch_ts, cpu) = 0;
> +		__touch_watchdog();
>  		per_cpu(softlockup_watchdog, cpu) = p;
>  		wake_up_process(p);
>  	}
--
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