netdev - Re: Page allocator order-0 optimizations merged

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Date:   Mon, 27 Mar 2017 07:15:18 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Pankaj Gupta <pagupta@...hat.com>,
        Tariq Toukan <ttoukan.linux@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Tariq Toukan <tariqt@...lanox.com>, netdev@...r.kernel.org,
        akpm@...ux-foundation.org, linux-mm <linux-mm@...ck.org>,
        Saeed Mahameed <saeedm@...lanox.com>
Subject: Re: Page allocator order-0 optimizations merged

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:

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)