linux-kernel - Re: [PATCH v3 1/5] m68k/bitops: force inlining of all bitops functions

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Date: Sun, 7 Jan 2024 21:01:34 +0900
From: Vincent MAILHOL <mailhol.vincent@...adoo.fr>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	Yury Norov <yury.norov@...il.com>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Douglas Anderson <dianders@...omium.org>, Kees Cook <keescook@...omium.org>, 
	Petr Mladek <pmladek@...e.com>, Randy Dunlap <rdunlap@...radead.org>, 
	Zhaoyang Huang <zhaoyang.huang@...soc.com>, Geert Uytterhoeven <geert+renesas@...der.be>, 
	Marco Elver <elver@...gle.com>, Brian Cain <bcain@...cinc.com>, 
	Matthew Wilcox <willy@...radead.org>, "Paul E . McKenney" <paulmck@...nel.org>, linux-hexagon@...r.kernel.org, 
	linux-m68k@...ts.linux-m68k.org
Subject: Re: [PATCH v3 1/5] m68k/bitops: force inlining of all bitops functions

On Tue. 2 janv. 2024 at 19:28, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>
> Hi Vincent,
>
> Thanks for your patch!

Thanks for the review and for running the benchmark.

> On Sun, Dec 17, 2023 at 8:13 AM Vincent Mailhol
> <mailhol.vincent@...adoo.fr> wrote:
> > The inline keyword actually does not guarantee that the compiler will
> > inline a functions. Whenever the goal is to actually inline a
> > function, __always_inline should always be preferred instead.
> >
> > On an allyesconfig, with GCC 13.2.1, it saves roughly 5 KB.
> >
> >   $ size --format=GNU vmlinux.before vmlinux.after
> >         text       data        bss      total filename
> >     60449738   70975612    2288988  133714338 vmlinux.before
> >     60446534   70972412    2289596  133708542 vmlinux.after
>
> With gcc 9.5.0-1ubuntu1~22.04, the figures are completely different
> (i.e. a size increase):

Those results are not normal, there should not be such a big
discrepancy between two versions of the same compiler. I double
checked everything and found out that I made a mistake when computing
the figures: not sure what exactly, but at some point, the ASLR seeds
(or other similar randomization feature) got reset and so, the
decrease I witnessed was just a "lucky roll".

After rerunning the benchmark (making sure to keep every seeds), I got
similar results as you:

        text       data        bss      total filename
    60449738   70975356    2288988  133714082
vmlinux_allyesconfig.before_this_series
    60446534   70979068    2289596  133715198
vmlinux_allyesconfig.after_first_patch
    60429746   70979132    2291676  133700554
vmlinux_allyesconfig.final_second_patch

Note that there are still some kind of randomness on the data segment
as shown in those other benchmarks I run:

        text       data        bss      total filename
    60449738   70976124    2288988  133714850
vmlinux_allyesconfig.before_this_series
    60446534   70980092    2289596  133716222
vmlinux_allyesconfig.after_first_patch
    60429746   70979388    2291676  133700810
vmlinux_allyesconfig.after_second_patch

        text       data        bss      total filename
    60449738   70975612    2288988  133714338
vmlinux_allyesconfig.before_this_series
    60446534   70980348    2289596  133716478
vmlinux_allyesconfig.after_first_patch
    60429746   70979900    2291676  133701322
vmlinux_allyesconfig.after_second_patch

But the error margin is within 1K.

So, in short, I inlined some functions which I shouldn't have. I am
preparing a v4 in which I will only inline the bit-find functions
(namely: __ffs(), ffs(), ffz(), __fls(), fls() and fls64()). Here are
the new figures:

        text       data        bss      total filename
    60453552   70955485    2288620  133697657
vmlinux_allyesconfig.before_this_series
    60450304   70953085    2289260  133692649
vmlinux_allyesconfig.after_first_patch
    60433536   70952637    2291340  133677513
vmlinux_allyesconfig.after_second_patch

N.B. The new figures were after a rebase, so do not try to compare
with the previous benchmarks. I will send the v4 soon, after I finish
to update the patch comments and double check things.

Concerning the other functions in bitops.h, there may be some other
ones worth a __always_inline. But I will narrow the scope of this
series only to the bit-find function. If a good samaritan wants to
investigate the other functions, go ahead!

Yours sincerely,
Vincent Mailhol




> allyesconfig:
>
>       text       data        bss      total filename
>   58878600   72415994    2283652  133578246 vmlinux.before
>   58882250   72419706    2284004  133585960 vmlinux.after
>
> atari_defconfig:
>
>       text       data        bss      total filename
>    4112060    1579862     151680    5843602 vmlinux-v6.7-rc8
>    4117008    1579350     151680    5848038
> vmlinux-v6.7-rc8-1-m68k-bitops-force-inlining
>
> The next patch offsets that for allyesconfig, but not for atari_defconfig.
>
> > Reference: commit 8dd5032d9c54 ("x86/asm/bitops: Force inlining of
> > test_and_set_bit and friends")
>
> Please don't split lines containing tags.
>
> > Link: https://git.kernel.org/torvalds/c/8dd5032d9c54
> >
> > Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
>
> Reviewed-by: Geert Uytterhoeven <geert@...ux-m68k.org>
>
> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds