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SubjectRe: NMI watchdog + NOHZ question
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From: David Miller <davem@davemloft.net>
Date: Mon, 22 Jun 2009 02:27:52 -0700 (PDT)

> I think something else is afoot, because while using "nohz=off" makes
> the problem go away, simply adding a NMI watchdog touch after the
> schedule() call in cpu_idle() does not make the problem go away.
>
> Also, the cpu that gets the NMI watchdog is different from the cpu
> running the qla2xxx driver init. That basically destroys the bulk
> of my theory :-)

Ok, I think I know what's happening now.

CPU 0 is in the driver init and looping submitting mailbox
commands to load the firmware, then waiting for completion.

CPU 1 is receiving the device interrupts. CPU 1 is where the
NMI watchdog triggers.

CPU 0 is submitting mailbox commands fast enough that by the
time CPU 1 returns from the device interrupt handler, a new
one is pending. This sequence runs for more than 5 seconds.

The problematic case is CPU 1's timer interrupt running when
the barrage of device interrupts begin. Then we have:

timer interrupt
return for softirq checking
pending, thus enable interrupts

qla2xxx interrupt
return
qla2xxx interrupt
return
... 5+ seconds pass
final qla2xxx interrupt for fw load
return

run timer softirq
return

At some point in the multi-second qla2xxx interrupt storm we trigger
the NMI watchdog on CPU 1 from the NMI interrupt handler.

The timer softirq, once we get back to running it, is smart enough to
run the timer work enough times to make up for the missed timer
interrupts.

However, the NMI watchdogs (both x86 and sparc) use the timer
interrupt count to notice the cpu is wedged. But in the above
scenerio we'll receive only one such timer interrupt even if we
last all the way back to running the timer softirq.

I'm not exactly sure what to do about this.


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