linux-kernel - Re: [PATCH] x86: Change x86 to use generic find_next

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Date:	Thu, 13 Mar 2008 18:14:24 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Alexander van Heukelum <heukelum@...lshack.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>, heukelum@...tmail.fm
Subject: Re: [PATCH] x86: Change x86 to use generic find_next_bit

On Sun, Mar 09, 2008 at 09:01:04PM +0100, Alexander van Heukelum wrote:
> x86: Change x86 to use the generic find_next_bit implementation
> 
> The versions with inline assembly are in fact slower on the machines I
> tested them on (in userspace) (Athlon XP 2800+, p4-like Xeon 2.8GHz, AMD
> Opteron 270). The i386-version needed a fix similar to 06024f21 to avoid
> crashing the benchmark.
> 
> Benchmark using: gcc -fomit-frame-pointer -Os. For each bitmap size 
> 1...512, for each possible bitmap with one bit set, for each possible 
> offset: find the position of the first bit starting at offset. If you 
> follow ;). Times include setup of the bitmap and checking of the 
> results.
> 
> 		Athlon		Xeon		Opteron 32/64bit
> x86-specific:	0m3.692s	0m2.820s	0m3.196s / 0m2.480s
> generic:	0m2.622s	0m1.662s	0m2.100s / 0m1.572s
> 
> If the bitmap size is not a multiple of BITS_PER_LONG, and no set 
> (cleared) bit is found, find_next_bit (find_next_zero_bit) returns a 
> value outside of the range [0,size]. The generic version always returns 
> exactly size. The generic version also uses unsigned long everywhere, 
> while the x86 versions use a mishmash of int, unsigned (int), long and 
> unsigned long.
> 

This problem is observed on x86_64 and powerpc also. We need a long
aligned address for test_bit, set_bit find_bit etc.  In ext4 we have
to make sure we align the address passed to

ext4_test_bit
ext4_set_bit
ext4_find_next_zero_bit
ext4_find_next_bit

fs/ext4/mballoc.c have some examples.

-aneesh
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