linux-kernel - Re: [RESEND PATCH v2 0/6] lib/find_bit: fast path for small bitmaps

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Date:   Mon, 15 Feb 2021 13:30:44 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     linux-m68k@...ts.linux-m68k.org, linux-kernel@...r.kernel.org,
        linux-sh@...r.kernel.org, linux-arch@...r.kernel.org
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, Arnd Bergmann <arnd@...db.de>,
        Dennis Zhou <dennis@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        David Sterba <dsterba@...e.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Stefano Brivio <sbrivio@...hat.com>,
        "Ma, Jianpeng" <jianpeng.ma@...el.com>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Joe Perches <joe@...ches.com>
Subject: Re: [RESEND PATCH v2 0/6] lib/find_bit: fast path for small bitmaps

[add David Laight <David.Laight@...LAB.COM> ]

On Sat, Jan 30, 2021 at 11:17:11AM -0800, Yury Norov wrote:
> Bitmap operations are much simpler and faster in case of small bitmaps
> which fit into a single word. In linux/bitmap.h we have a machinery that
> allows compiler to replace actual function call with a few instructions
> if bitmaps passed into the function are small and their size is known at
> compile time.
> 
> find_*_bit() API lacks this functionality; despite users will benefit from
> it a lot. One important example is cpumask subsystem when
> NR_CPUS <= BITS_PER_LONG. In the very best case, the compiler may replace
> a find_*_bit() call for such a bitmap with a single ffs or ffz instruction.
> 
> Tools is synchronized with new implementation where needed.
> 
> v1: https://www.spinics.net/lists/kernel/msg3804727.html
> v2: - employ GENMASK() for bitmaps;
>     - unify find_bit inliners in;
>     - address comments to v1;

Comments so far:
 - increased image size (patch #8) - addressed by introducing
   CONFIG_FAST_PATH;
 - split tools and kernel parts - not clear why it's better.

 Anything else?