Messages in this thread | | | Date | Mon, 14 Jan 2013 19:26:07 -0800 | Subject | Re: [PATCH v2] hardlockup: detect hard lockups without NMIs using secondary cpus | From | Colin Cross <> |
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On Mon, Jan 14, 2013 at 6:48 PM, Frederic Weisbecker <fweisbec@gmail.com> wrote: > 2013/1/15 Colin Cross <ccross@android.com>: >> On Mon, Jan 14, 2013 at 4:25 PM, Frederic Weisbecker <fweisbec@gmail.com> wrote: >>> 2013/1/15 Colin Cross <ccross@android.com>: >>>> On Mon, Jan 14, 2013 at 4:13 PM, Frederic Weisbecker <fweisbec@gmail.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.
It will report it, but it will report it in the same way that it reports a less serious issue, and in this case with zero debugging information since the affected cpu won't dump its backtrace. Better than nothing, but not as useful as a panic can be.
> 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?
Panics can trigger extra debugging tools, like my previous examples kexec and kgdb.
DBGPCSR is the "DeBuG Program Counter Sampling Register". It is a memory mapped register available on many (all?) ARM Cortex cpus that returns a recent PC value for the cpu. I have used it along with this patch, and it produces very useful information.
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