[poll / rfc] kdb_stop_cpus

Robert N. M. Watson rwatson at FreeBSD.org
Fri Jun 3 17:57:25 UTC 2011

On 3 Jun 2011, at 16:13, Andriy Gapon wrote:

> I wonder if anybody uses kdb_stop_cpus with non-default value.
> If, yes, I am very interested to learn about your usecase for it.

The issue that prompted the sysctl was non-NMI IPIs being used to enter the debugger or reboot following a core hanging with interrupts disabled. With the switch to NMI IPIs in some of those circumstances, life is better -- at least, on hardware that supports non-maskable IPIs. I seem to recall sparc64 doesn't, however? Not sure about MIPS, etc. Attilio has since significantly improved our shutdown behaviour -- initially, the switch to NMI IPIs broke other things (because certain IPIs then improperly preempted threads holding spinlocks), but that pretty much all seems worked out now.


> I think that the default kdb behavior is the correct one, so it doesn't make sense
> to have a knob to turn on incorrect behavior.
> But I may be missing something obvious.
> The comment in the code doesn't really satisfy me:
> /*
> * Flag indicating whether or not to IPI the other CPUs to stop them on
> * entering the debugger.  Sometimes, this will result in a deadlock as
> * stop_cpus() waits for the other cpus to stop, so we allow it to be
> * disabled.  In order to maximize the chances of success, use a hard
> * stop for that.
> */
> The hard stop should be sufficiently mighty.
> Yes, I am aware of supposedly extremely rare situations where a deadlock could
> happen even when using hard stop.  But I'd rather fix that than have this switch.
> Oh, the commit message (from 2004) explains it:
>> Add a new sysctl, debug.kdb.stop_cpus, which controls whether or not we
>> attempt to IPI other cpus when entering the debugger in order to stop
>> them while in the debugger.  The default remains to issue the stop;
>> however, that can result in a hang if another cpu has interrupts disabled
>> and is spinning, since the IPI won't be received and the KDB will wait
>> indefinitely.  We probably need to add a timeout, but this is a useful
>> stopgap in the mean time.
> But that was before we started using hard stop in this context (in 2009).
> -- 
> Andriy Gapon

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