linux-kernel - Re: [PATCH] mm, page_alloc: convert page_group_by_mobility

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Date:   Thu, 29 Dec 2016 11:08:15 -0500
From:   Rik van Riel <riel@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] mm, page_alloc: convert page_group_by_mobility_disable
 to static key

On Tue, 2016-12-20 at 14:43 +0100, Vlastimil Babka wrote:
> The flag is rarely enabled or even changed, so it's an ideal static
> key
> candidate. Since it's being checked in the page allocator fastpath
> via
> gfpflags_to_migratetype(), it may actually save some valuable cycles.
> 
> Here's a diff excerpt from __alloc_pages_nodemask() assembly:
> 
>         -movl    page_group_by_mobility_disabled(%rip), %ecx
> 	+.byte 0x0f,0x1f,0x44,0x00,0
>          movl    %r9d, %eax
>          shrl    $3, %eax
>          andl    $3, %eax
>         -testl   %ecx, %ecx
>         -movl    $0, %ecx
>         -cmovne  %ecx, %eax
> 
> I.e. a NOP instead of test, conditional move and some assisting
> moves.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Acked-by: Rik van Riel <riel@...hat.com>

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