From: Jesper Dangaard Brouer <brouer@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Pankaj Gupta <pagupta@redhat.com>,
Tariq Toukan <ttoukan.linux@gmail.com>,
Mel Gorman <mgorman@techsingularity.net>,
Tariq Toukan <tariqt@mellanox.com>,
netdev@vger.kernel.org, akpm@linux-foundation.org,
linux-mm <linux-mm@kvack.org>,
Saeed Mahameed <saeedm@mellanox.com>,
brouer@redhat.com
Subject: Re: Page allocator order-0 optimizations merged
Date: Mon, 27 Mar 2017 17:15:00 +0200 [thread overview]
Message-ID: <20170327171500.4beef762@redhat.com> (raw)
In-Reply-To: <20170327141518.GB27285@bombadil.infradead.org>
On Mon, 27 Mar 2017 07:15:18 -0700
Matthew Wilcox <willy@infradead.org> wrote:
> On Mon, Mar 27, 2017 at 02:39:47PM +0200, Jesper Dangaard Brouer wrote:
> >
> > +static __always_inline int in_irq_or_nmi(void)
> > +{
> > + return in_irq() || in_nmi();
> > +// XXX: hoping compiler will optimize this (todo verify) into:
> > +// #define in_irq_or_nmi() (preempt_count() & (HARDIRQ_MASK | NMI_MASK))
> > +
> > + /* compiler was smart enough to only read __preempt_count once
> > + * but added two branches
> > +asm code:
> > + │ mov __preempt_count,%eax
> > + │ test $0xf0000,%eax // HARDIRQ_MASK: 0x000f0000
> > + │ ┌──jne 2a
> > + │ │ test $0x100000,%eax // NMI_MASK: 0x00100000
> > + │ │↓ je 3f
> > + │ 2a:└─→mov %rbx,%rdi
> > +
> > + */
> > +}
>
> To be fair, you told the compiler to do that with your use of fancy-pants ||
> instead of optimisable |. Try this instead:
Thanks you! -- good point! :-)
> static __always_inline int in_irq_or_nmi(void)
> {
> return in_irq() | in_nmi();
> }
>
> 0000000000001770 <test_fn>:
> 1770: 65 8b 05 00 00 00 00 mov %gs:0x0(%rip),%eax # 1777 <test_fn+0x7>
> 1773: R_X86_64_PC32 __preempt_count-0x4
> #define in_nmi() (preempt_count() & NMI_MASK)
> #define in_task() (!(preempt_count() & \
> (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
> static __always_inline int in_irq_or_nmi(void)
> {
> return in_irq() | in_nmi();
> 1777: 25 00 00 1f 00 and $0x1f0000,%eax
> }
> 177c: c3 retq
> 177d: 0f 1f 00 nopl (%rax)
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).
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
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next prev parent reply other threads:[~2017-03-27 15:15 UTC|newest]
Thread overview: 63+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-02-27 20:25 [merged] mm-page_alloc-only-use-per-cpu-allocator-for-irq-safe-requests.patch removed from -mm tree akpm
2017-03-01 13:48 ` Page allocator order-0 optimizations merged Jesper Dangaard Brouer
2017-03-01 17:36 ` Tariq Toukan
2017-03-01 17:36 ` Tariq Toukan
2017-03-22 17:39 ` Tariq Toukan
2017-03-22 17:39 ` Tariq Toukan
2017-03-22 23:40 ` Mel Gorman
2017-03-23 13:43 ` Jesper Dangaard Brouer
2017-03-23 14:51 ` Mel Gorman
2017-03-26 8:21 ` Tariq Toukan
2017-03-26 10:17 ` Tariq Toukan
2017-03-27 7:32 ` Pankaj Gupta
2017-03-27 8:55 ` Jesper Dangaard Brouer
2017-03-27 12:28 ` Mel Gorman
2017-03-27 12:39 ` Jesper Dangaard Brouer
2017-03-27 13:32 ` Mel Gorman
2017-03-28 7:32 ` Tariq Toukan
2017-03-28 8:29 ` Jesper Dangaard Brouer
2017-03-28 16:05 ` Tariq Toukan
2017-03-28 18:24 ` Jesper Dangaard Brouer
2017-03-29 7:13 ` Tariq Toukan
2017-03-28 8:28 ` Pankaj Gupta
2017-03-27 14:15 ` Matthew Wilcox
2017-03-27 14:15 ` Matthew Wilcox
2017-03-27 15:15 ` Jesper Dangaard Brouer [this message]
2017-03-27 16:58 ` in_irq_or_nmi() Matthew Wilcox
2017-03-27 16:58 ` in_irq_or_nmi() Matthew Wilcox
2017-03-27 16:58 ` in_irq_or_nmi() Matthew Wilcox
2017-03-29 8:12 ` in_irq_or_nmi() Peter Zijlstra
2017-03-29 8:12 ` in_irq_or_nmi() Peter Zijlstra
2017-03-29 8:12 ` in_irq_or_nmi() Peter Zijlstra
2017-03-29 8:59 ` in_irq_or_nmi() Jesper Dangaard Brouer
2017-03-29 8:59 ` in_irq_or_nmi() Jesper Dangaard Brouer
2017-03-29 9:19 ` in_irq_or_nmi() Peter Zijlstra
2017-03-29 9:19 ` in_irq_or_nmi() Peter Zijlstra
2017-03-29 9:19 ` in_irq_or_nmi() Peter Zijlstra
2017-03-29 18:12 ` in_irq_or_nmi() Matthew Wilcox
2017-03-29 18:12 ` in_irq_or_nmi() Matthew Wilcox
2017-03-29 19:11 ` in_irq_or_nmi() Jesper Dangaard Brouer
2017-03-29 19:11 ` in_irq_or_nmi() Jesper Dangaard Brouer
2017-03-29 19:44 ` in_irq_or_nmi() and RFC patch Jesper Dangaard Brouer
2017-03-29 19:44 ` Jesper Dangaard Brouer
2017-03-30 6:49 ` Peter Zijlstra
2017-03-30 6:49 ` Peter Zijlstra
2017-03-30 7:12 ` Jesper Dangaard Brouer
2017-03-30 7:12 ` Jesper Dangaard Brouer
2017-03-30 7:35 ` Peter Zijlstra
2017-03-30 7:35 ` Peter Zijlstra
2017-03-30 9:46 ` Jesper Dangaard Brouer
2017-03-30 9:46 ` Jesper Dangaard Brouer
2017-03-30 13:04 ` Mel Gorman
2017-03-30 13:04 ` Mel Gorman
2017-03-30 15:07 ` Jesper Dangaard Brouer
2017-03-30 15:07 ` Jesper Dangaard Brouer
2017-04-03 12:05 ` Mel Gorman
2017-04-03 12:05 ` Mel Gorman
2017-04-05 8:53 ` Mel Gorman
2017-04-05 8:53 ` Mel Gorman
2017-04-10 14:31 ` Page allocator order-0 optimizations merged zhong jiang
2017-04-10 14:31 ` zhong jiang
2017-04-10 15:10 ` Mel Gorman
2017-04-11 1:54 ` zhong jiang
2017-04-11 1:54 ` zhong jiang
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