linux-kernel - Re: [PATCH V2] kernel/watchdog: fix spurious hard lockups

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Date:   Fri, 23 Jun 2017 10:01:55 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Don Zickus <dzickus@...hat.com>
cc:     Kan Liang <kan.liang@...el.com>, linux-kernel@...r.kernel.org,
        mingo@...nel.org, akpm@...ux-foundation.org, babu.moger@...cle.com,
        atomlin@...hat.com, prarit@...hat.com,
        torvalds@...ux-foundation.org, peterz@...radead.org,
        eranian@...gle.com, acme@...hat.com, ak@...ux.intel.com,
        stable@...r.kernel.org
Subject: Re: [PATCH V2] kernel/watchdog: fix spurious hard lockups

On Thu, 22 Jun 2017, Don Zickus wrote:
> On Wed, Jun 21, 2017 at 11:53:57PM +0200, Thomas Gleixner wrote:
> > On Wed, 21 Jun 2017, kan.liang@...el.com wrote:
> > > We now have more and more systems where the Turbo range is wide enough
> > > that the NMI watchdog expires faster than the soft watchdog timer that
> > > updates the interrupt tick the NMI watchdog relies on.
> > > 
> > > This problem was originally added by commit 58687acba592
> > > ("lockup_detector: Combine nmi_watchdog and softlockup detector").
> > > Previously the NMI watchdog would always check jiffies, which were
> > > ticking fast enough. But now the backing is quite slow so the expire
> > > time becomes more sensitive.
> > 
> > And slapping a factor 3 on the NMI period is the wrong answer to the
> > problem. The simple solution would be to increase the hrtimer frequency,
> > but that's not really desired either.
> > 
> > Find an untested patch below, which should cure the issue.
> 
> A simple low pass filter.  It compiles. :-) I don't think I have knowledge
> to test it.  Kan?

Yes, and it has an interesting twist. It's only working once we have
switched to TSC as clocksource.

As long as jiffies are the clocksource, this will miserably fail because
when the hrtimer interrupt is not delivered jiffies wont be incremented
either and the NMI will say: Oh. not enough time elapsed. Lather, rinse and
repeat.

One simple way to fix this is with the delta patch below.

Thanks,

	tglx

8<--------------------------
--- a/kernel/watchdog_hld.c
+++ b/kernel/watchdog_hld.c
@@ -72,6 +72,7 @@ EXPORT_SYMBOL(touch_nmi_watchdog);
 
 #ifdef CONFIG_HARDLOCKUP_CHECK_TIMESTAMP
 static DEFINE_PER_CPU(ktime_t, last_timestamp);
+static DEFINE_PER_CPU(unsigned int, nmi_rearmed);
 static ktime_t watchdog_hrtimer_sample_threshold __read_mostly;
 
 void watchdog_update_hrtimer_threshold(u64 period)
@@ -105,8 +106,11 @@ static bool watchdog_check_timestamp(voi
 	ktime_t delta, now = ktime_get_mono_fast_ns();
 
 	delta = now - __this_cpu_read(last_timestamp);
-	if (delta < watchdog_hrtimer_sample_threshold)
-		return false;
+	if (delta < watchdog_hrtimer_sample_threshold) {
+		if (__this_cpu_inc_return(nmi_rearmed) < 10)
+			return false;
+	}
+	__this_cpu_write(nmi_rearmed, 0);
 	__this_cpu_write(last_timestamp, now);
 	return true;
 }