[PATCH v2] hardlockup: detect hard lockups without NMIs using secondary cpus

[Colin Cross]

On Mon, Jan 14, 2013 at 4:13 PM, Frederic Weisbecker <fweisbec at gmail.com> wrote:
> 2013/1/11 Colin Cross <ccross at android.com>:
>> Emulate NMIs on systems where they are not available by using timer
>> interrupts on other cpus.  Each cpu will use its softlockup hrtimer
>> to check that the next cpu is processing hrtimer interrupts by
>> verifying that a counter is increasing.
>>
>> This patch is useful on systems where the hardlockup detector is not
>> available due to a lack of NMIs, for example most ARM SoCs.
>> Without this patch any cpu stuck with interrupts disabled can
>> cause a hardware watchdog reset with no debugging information,
>> but with this patch the kernel can detect the lockup and panic,
>> which can result in useful debugging info.
>>
>> Signed-off-by: Colin Cross <ccross at android.com>
>
> 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.

> Now you'll face the same problem in the end: if you don't have NMIs,
> you won't have a very useful report.

Yes, but its still better than a silent reset.