NMI for ARC

[Vineet Gupta]

On 09/28/2016 12:16 AM, Peter Zijlstra wrote:
>> 1. Is it OK in general to short-circuit preemption off irq checks for NMI style
>> > interrupts. 
> Yes. If the NMI returns to kernel space you must not attempt preemption
> for reasons you found :-), if the NMI returns to userspace you should do
> the normal return to user bits, I think.

Just to add, eliding preemption check in return path might not be sufficient as we
could still have timer intr leading to scheduler_tick() - whcih is what i found as
well ;-) So scheduler nevertheless needs to be told to not reschedule in this code
path.

So eliding preempt-off-irq just becomes an optimization for NMI code paths IMHO.

Now I was a stupid fool, fudging preemption counts in low level code, to achieve
this, whereas we have nice generic nmi_{enter,exit}() and in_nmi() helpers which
can be used.

Thx,
-Vineet