Ingo Molnar <[email protected]> wrote:
>
>
> * Keith Owens <[email protected]> wrote:
>
> > My question has nothing to do with NMI. I am querying inconsistent
> > behaviour amongst normal IPIs, this list :-
> >
> > i386 function irq_enter?
> > smp_apic_timer_interrupt yes
> > smp_call_function_interrupt yes
> > smp_error_interrupt yes
> > smp_invalidate_interrupt no - why
> > smp_reschedule_interrupt no (does not need it)
> > smp_spurious_interrupt yes
> > smp_thermal_interrupt yes
> >
> > x86_64 function irq_enter?
> > mce_threshold_interrupt yes
> > smp_apic_timer_interrupt yes
> > smp_call_function_interrupt yes
> > smp_error_interrupt yes
> > smp_invalidate_interrupt no - why
> > smp_reschedule_interrupt no (does not need it)
> > smp_spurious_interrupt yes
> > smp_thermal_interrupt yes
>
> irq_enter() is mostly just for the purpose of in_interrupt()/in_irq() to
> work as expected, not much else. [also the timer code assumes that
> update_process_times() is called in a HARDIRQ_OFFSET elevated context,
> so the apic timer IRQ needs irq_enter() too.] The
> smp_invalidate_interrupt() and smp_reschedule_interrupt() is
> performance-critical and they dont need irq_enter()/irq_exit().
>
> Since smp_call_function_interrupt() can be called with driver-supplied
> function vectors, it's best to keep the irq_enter()/exit there. [for
> example mm/slab.c has some in_interrupt() sanity checks.] Obviously
> do_IRQ() itself needs irq_enter()/exit() too - plus the APIC timer irq
> as mentioned above.
>
> Otherwise, the rest of the SMP functions technically dont need
> irq_enter()/irq_exit(). [i.e. threshold, error, spurious and thermal] We
> could remove it from them.
>
There's a risk that spin_unlock() in an IPI handler could blow up due to it
trying to reschedule. But preempt_schedule() explicitly checks the CPU's
interupt flag so as long as that doesn't change we're OK.
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