Re: in_irq_or_nmi()

From: Peter Zijlstra
Date: Wed Mar 29 2017 - 05:20:03 EST


On Wed, Mar 29, 2017 at 10:59:28AM +0200, Jesper Dangaard Brouer wrote:
> On Wed, 29 Mar 2017 10:12:19 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> > On Mon, Mar 27, 2017 at 09:58:17AM -0700, Matthew Wilcox wrote:
> > > On Mon, Mar 27, 2017 at 05:15:00PM +0200, Jesper Dangaard Brouer wrote:
> > > > And I also verified it worked:
> > > >
> > > > 0.63 â mov __preempt_count,%eax
> > > > â free_hot_cold_page():
> > > > 1.25 â test $0x1f0000,%eax
> > > > â â jne 1e4
> > > >
> > > > And this simplification also made the compiler change this into a
> > > > unlikely branch, which is a micro-optimization (that I will leave up to
> > > > the compiler).
> > >
> > > Excellent! That said, I think we should define in_irq_or_nmi() in
> > > preempt.h, rather than hiding it in the memory allocator. And since we're
> > > doing that, we might as well make it look like the other definitions:
> > >
> > > diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> > > index 7eeceac52dea..af98c29abd9d 100644
> > > --- a/include/linux/preempt.h
> > > +++ b/include/linux/preempt.h
> > > @@ -81,6 +81,7 @@
> > > #define in_interrupt() (irq_count())
> > > #define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET)
> > > #define in_nmi() (preempt_count() & NMI_MASK)
> > > +#define in_irq_or_nmi() (preempt_count() & (HARDIRQ_MASK | NMI_MASK))
> > > #define in_task() (!(preempt_count() & \
> > > (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
> > >
> >
> > No, that's horrible. Also, wth is this about? A memory allocator that
> > needs in_nmi()? That sounds beyond broken.
>
> It is the other way around. We want to exclude NMI and HARDIRQ from
> using the per-cpu-pages (pcp) lists "order-0 cache" (they will
> fall-through using the normal buddy allocator path).

Any in_nmi() code arriving at the allocator is broken. No need to fix
the allocator.