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Date: Wed, 29 Mar 2017 10:12:19 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>,
        Pankaj Gupta <pagupta@redhat.com>,
        Tariq Toukan <ttoukan.linux@gmail.com>,
        Mel Gorman <mgorman@techsingularity.net>,
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        akpm@linux-foundation.org, linux-mm <linux-mm@kvack.org>,
        Saeed Mahameed <saeedm@mellanox.com>, linux-kernel@vger.kernel.org
Subject: Re: in_irq_or_nmi()
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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 │       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.