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

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Date:	Mon, 14 Jan 2013 16:30:39 -0800
From:	Colin Cross <ccross@...roid.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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, Jan 14, 2013 at 4:25 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> 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?

I'm not sure what you mean.  If watchdog_thresh changes, the next
hrtimer interrupt on each cpu will move the following hrtimer
interrupt forward by the new watchdog_thresh * 2 / 5.  There may be a
single cycle of watchdog checks at an intermediate period, but nothing
bad should happen.

This code doesn't ever deal with watchdog_thresh directly, it is only
counting hrtimer interrupts.  3 hrtimer interrupts is always a
reasonable approximation of watchdog_thresh.
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