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 [off-list ref] wrote:

quoted

On Mon, Mar 27, 2017 at 09:58:17AM -0700, Matthew Wilcox wrote:

quoted

On Mon, Mar 27, 2017 at 05:15:00PM +0200, Jesper Dangaard Brouer wrote:  

quoted

And I also verified it worked:

  0.63 │       mov    __preempt_count,%eax
       │     free_hot_cold_page():
  1.25 │       test   $0x1f0000,%eax
       │     ↓ 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.

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