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

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Date:	Tue, 15 Jan 2013 03:48:23 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Colin Cross <ccross@...roid.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.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>,
	Tony Lindgren <tony@...mide.com>
Subject: Re: [PATCH v2] hardlockup: detect hard lockups without NMIs using
 secondary cpus

2013/1/15 Colin Cross <ccross@...roid.com>:
> On Mon, Jan 14, 2013 at 4:25 PM, Frederic Weisbecker <fweisbec@...il.com> wrote:
>> 2013/1/15 Colin Cross <ccross@...roid.com>:
>>> On Mon, Jan 14, 2013 at 4:13 PM, Frederic Weisbecker <fweisbec@...il.com> wrote:
>>>> I believe this is pretty much what the RCU stall detector does
>>>> already: checks for other CPUs being responsive. The only difference
>>>> is on how it checks that. For RCU it's about checking for CPUs
>>>> reporting quiescent states when requested to do so. In your case it's
>>>> about ensuring the hrtimer interrupt is well handled.
>>>>
>>>> One thing you can do is to enqueue an RCU callback (cal_rcu()) every
>>>> minute so you can force other CPUs to report quiescent states
>>>> periodically and thus check for lockups.
>>>
>>> That's a good point, I'll take a look at using that.  A minute is too
>>> long, some SoCs have maximum HW watchdog periods of under 30 seconds,
>>> but a call_rcu every 10-20 seconds might be sufficient.
>>
>> Sure. And you can tune CONFIG_RCU_CPU_STALL_TIMEOUT accordingly.
>
> After considering this, I think the hrtimer watchdog is more useful.
> RCU stalls are not usually panic events, and I wouldn't want to add a
> panic on every RCU stall.  The lack of stack traces on the affected
> cpu makes a panic important.  I'm planning to add an ARM DBGPCSR panic
> handler, which will be able to dump the PC of a stuck cpu even if it
> is not responding to interrupts.  kexec or kgdb on panic might also
> allow some inspection of the stack on stuck cpu.
>
> Failing to process interrupts is a much more serious event than an RCU
> stall, and being able to detect them separately may be very valuable
> for debugging.

RCU stalls can happen for different reasons: softlockup (failure to
schedule another task), hardlockup (failure to process interrupts), or
a bug in RCU itself. But if you have a hardlockup, it will report it.

Now why do you need a panic in any case? I don't know DBGPCSR, is this
a breakpoint register? How do you plan to use it remotely from the CPU
that detects the lockup?
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