linux-kernel - Re: [PATCH v2] hardlockup: detect hard lockups without NMIs using secondary cpus

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Date:	Mon, 14 Jan 2013 16:25:04 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Colin Cross <ccross@...roid.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Don Zickus <dzickus@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	liu chuansheng <chuansheng.liu@...el.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Tony Lindgren <tony@...mide.com>
Subject: Re: [PATCH v2] hardlockup: detect hard lockups without NMIs using
 secondary cpus

On Mon, 14 Jan 2013 16:19:23 -0800
Colin Cross <ccross@...roid.com> wrote:

> >> +static void watchdog_check_hardlockup_other_cpu(void)
> >> +{
> >> +     unsigned int next_cpu;
> >> +
> >> +     /*
> >> +      * Test for hardlockups every 3 samples.  The sample period is
> >> +      *  watchdog_thresh * 2 / 5, so 3 samples gets us back to slightly over
> >> +      *  watchdog_thresh (over by 20%).
> >> +      */
> >> +     if (__this_cpu_read(hrtimer_interrupts) % 3 != 0)
> >> +             return;
> >
> > The hardwired interval Seems Wrong.  watchdog_thresh is tunable at runtime.
> >
> > The comment could do with some fleshing out.  *why* do we want to test
> > at an interval "slightly over watchdog_thresh"?  What's going on here?
> 
> I'll reword it.  We don't want to be slightly over watchdog_thresh,
> ideally we would be exactly at watchdog_thresh.  However, since this
> relies on the hrtimer interrupts that are scheduled at watchdog_thresh
> * 2 / 5, there is no multiple of hrtimer_interrupts that will result
> in watchdog_thresh.  watchdog_thresh * 2 / 5 * 3 (watchdog_thresh *
> 1.2) is the closest I can get to testing for a hardlockup once every
> watchdog_thresh seconds.

It needs more than rewording, doesn't it?  What happens if watchdog_thresh is
altered at runtime?
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