linux-kernel - Re: [PATCH 08/14] lib/Kconfig: introduce FAST

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Date:   Thu, 18 Feb 2021 17:15:43 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
        linux-arch@...r.kernel.org, linux-sh@...r.kernel.org,
        Alexey Klimov <aklimov@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>, David Sterba <dsterba@...e.com>,
        Dennis Zhou <dennis@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Jianpeng Ma <jianpeng.ma@...el.com>,
        Joe Perches <joe@...ches.com>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Rich Felker <dalias@...c.org>,
        Stefano Brivio <sbrivio@...hat.com>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH 08/14] lib/Kconfig: introduce FAST_PATH option

On Wed, Feb 17, 2021 at 08:05:06PM -0800, Yury Norov wrote:
> This series introduces fast paths for find_bit() routines. It is
> beneficial for typical systems, but those who limited in I-cache
> may be concerned about increasing the .text size of the Image.
> 
> To address this concern, one can disable FAST_PATH option in the config
> and some save memory.
> 
> The effect of this option on my arm64 next-20210217 build is:

(Maybe bloat-o-meter will give better view on this, i.e. more human-readable)

> Before:
> 	Sections:
> 	Idx Name          Size      VMA               LMA               File off  Algn
> 	  0 .head.text    00010000  ffff800010000000  ffff800010000000  00010000  2**16
> 			  CONTENTS, ALLOC, LOAD, READONLY, CODE
> 	  1 .text         0115e3a8  ffff800010010000  ffff800010010000  00020000  2**16
> 			  CONTENTS, ALLOC, LOAD, READONLY, CODE
> 	  2 .got.plt      00000018  ffff80001116e3a8  ffff80001116e3a8  0117e3a8  2**3
> 			  CONTENTS, ALLOC, LOAD, DATA
> 	  3 .rodata       007a72ca  ffff800011170000  ffff800011170000  01180000  2**12
> 			  CONTENTS, ALLOC, LOAD, DATA
> 	  ...
> 
> After:
> 	Sections:
> 	Idx Name          Size      VMA               LMA               File off  Algn
> 	  0 .head.text    00010000  ffff800010000000  ffff800010000000  00010000  2**16
> 			  CONTENTS, ALLOC, LOAD, READONLY, CODE
> 	  1 .text         011623a8  ffff800010010000  ffff800010010000  00020000  2**16
> 			  CONTENTS, ALLOC, LOAD, READONLY, CODE
> 	  2 .got.plt      00000018  ffff8000111723a8  ffff8000111723a8  011823a8  2**3
> 			  CONTENTS, ALLOC, LOAD, DATA
> 	  3 .rodata       007a772a  ffff800011180000  ffff800011180000  01190000  2**12
> 			  CONTENTS, ALLOC, LOAD, DATA
> 	  ...
> 
> Notice that this is the cumulive effect on already existing fast paths
> controlled by SMALL_CONST() together with ones added by this series.

...

> +config FAST_PATH

I think the name is to broad for this cases, perhaps BITS_FAST_PATH? or BITMAP?

> +	bool "Enable fast path code generation"
> +	default y
> +	help
> +	  This option enables fast path optimization with the cost of increasing
> +	  the text section.

-- 
With Best Regards,
Andy Shevchenko