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

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Date:   Fri, 19 Feb 2021 12:52:31 +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 Thu, Feb 18, 2021 at 11:24:19AM -0800, Yury Norov wrote:
> On Thu, Feb 18, 2021 at 05:15:43PM +0200, Andy Shevchenko wrote:
> > 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)
> 
> Never heard about this tool, thanks for the hint.
> 
> scripts/bloat-o-meter vmlinux vmlinux.new
> add/remove: 16/13 grow/shrink: 111/439 up/down: 3616/-19352 (-15736)
> Function                                     old     new   delta
> find_next_bit.constprop                        -     220    +220
> apply_wqattrs_cleanup                          -     176    +176
> memcg_free_shrinker_maps                       -     172    +172
> ...
> cpuset_hotplug_workfn                       2584    2288    -296
> task_numa_fault                             3640    3320    -320
> kmem_cache_free_bulk                        1684    1280    -404
> Total: Before=26085140, After=26069404, chg -0.06%
> 
> The complete output is here:
> https://pastebin.com/kBSdVJcK
> 
> So if I understand the output correctly, the size of .text is decreased...
> Looks weird, but if it's true, we don't need the FAST_BIT config at all
> because there's no tradeoff, and I should drop the patch.

I actually expected the text size decrease when it's about constants.
I remember that in PCI case we discussed with Bjorn the use of
for_each_set_bit() that brought entire function into the object file that
increased it by ~300 bytes (or so). But the code is something like

	for_each_set_bit(i, &addr, 32)

...

> > I think the name is to broad for this cases, perhaps BITS_FAST_PATH? or BITMAP?
> 
> My logic was that since SMALL_CONST() is global, and FAST_PATH
> controls the SMALL_CONST, it should also be global. I believe,
> Linux should have a global switch to control the behaviour in
> such cases, similarly to -Os compiler option. And I was surprized
> when I found nothing like FAST_PATH in the config.
> 
> What about having FAST_PATH as a global option, and later if someone
> will request for granularity, we'll introduce nested configs?

I think it is too far from now. Let's do one step at a time.

-- 
With Best Regards,
Andy Shevchenko