linux-kernel - Re: [PATCH v2 0/3] Improve bitmap_empty and bitmap

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Message-ID: <87bnaqtlb6.fsf@rasmusvillemoes.dk>
Date:	Thu, 19 Nov 2015 10:01:17 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Jia He <hejianet@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Yury Norov <yury.norov@...il.com>, Tejun Heo <tj@...nel.org>,
	Martin Kepplinger <martink@...teo.de>,
	George Spelvin <linux@...izon.com>,
	Ingo Molnar <mingo@...nel.org>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v2 0/3] Improve bitmap_empty and bitmap_full

On Thu, Nov 19 2015, Jia He <hejianet@...il.com> wrote:

> find_fisrt_{zero_}bit are too heavy for bitmap_{full,empty}. We don't 
> need to calculate and compare the position of bitmap. This set of patch
> instroduces lightweight api and replaces the heavy one.
>

Please check the history of the code you're modifying. git blame
include/linux/bitmap.h would immediately point you to 2afe27c718b
"lib/bitmap.c: bitmap_[empty,full]: remove code duplication". While it's
obviously true that find_first_bit does slightly more work than strictly
necessary to establish whether the bitmap is empty, it does the same
number of memory accesses, so I wouldn't consider it particularly
heavy-weight. Getting rid of .text as 2afe27c718b did is a good thing,
so you'd have to explain why we should reintroduce specialized functions
for this.

Your code is also buggy :-(

Rasmus
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