linux-kernel - Re: [PATCH] bitops: remove "optimizations"

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Date:	Tue, 29 Apr 2008 05:34:12 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	tglx@...utronix.de
Cc:	torvalds@...ux-foundation.org, harvey.harrison@...il.com,
	mingo@...e.hu, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bitops: remove "optimizations"

From: David Miller <davem@...emloft.net>
Date: Tue, 29 Apr 2008 03:03:32 -0700 (PDT)

> From: Thomas Gleixner <tglx@...utronix.de>
> Date: Tue, 29 Apr 2008 12:01:02 +0200 (CEST)
> 
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> 
> Acked-by: David S. Miller <davem@...emloft.net>

Ironically, I just bisected sparc64 bootup failures to the following
changeset.  It's very late here, and I haven't looked into the
details, but it seems to wedge in free_area_init_nodes() on a non-NUMA
system with CONFIG_NUMA disabled.

Isn't it funny that this optimization not only was useless, but also
broke things. :-/

64970b68d2b3ed32b964b0b30b1b98518fde388e is first bad commit
commit 64970b68d2b3ed32b964b0b30b1b98518fde388e
Author: Alexander van Heukelum <heukelum@...lshack.com>
Date:   Tue Mar 11 16:17:19 2008 +0100

    x86, generic: optimize find_next_(zero_)bit for small constant-size bitmaps
    
    This moves an optimization for searching constant-sized small
    bitmaps form x86_64-specific to generic code.
    
    On an i386 defconfig (the x86#testing one), the size of vmlinux hardly
    changes with this applied. I have observed only four places where this
    optimization avoids a call into find_next_bit:
    
    In the functions return_unused_surplus_pages, alloc_fresh_huge_page,
    and adjust_pool_surplus, this patch avoids a call for a 1-bit bitmap.
    In __next_cpu a call is avoided for a 32-bit bitmap. That's it.
    
    On x86_64, 52 locations are optimized with a minimal increase in
    code size:
    
    Current #testing defconfig:
        146 x bsf, 27 x find_next_*bit
       text    data     bss     dec     hex filename
       5392637  846592  724424 6963653  6a41c5 vmlinux
    
    After removing the x86_64 specific optimization for find_next_*bit:
        94 x bsf, 79 x find_next_*bit
       text    data     bss     dec     hex filename
       5392358  846592  724424 6963374  6a40ae vmlinux
    
    After this patch (making the optimization generic):
        146 x bsf, 27 x find_next_*bit
       text    data     bss     dec     hex filename
       5392396  846592  724424 6963412  6a40d4 vmlinux
    
    [ tglx@...utronix.de: build fixes ]
    
    Signed-off-by: Ingo Molnar <mingo@...e.hu>

:040000 040000 c0407f7df7dc5333c78c300780931a269ae0dedd 2f6ef634a35b6dd46e40ea70128c39a742e60501 M      include
:040000 040000 556ee6ccb15cdbb1aa5a690c732a5492d58dfe6f 937d05977f46056c12b5da64ca62959c06a912c0 M      lib
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