linux-kernel - Re: [PATCH v3 6/6] x86: bug.h: use asm_inline in _BUG

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Date:   Fri, 13 Sep 2019 07:42:15 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        ndesaulniers@...gle.com,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Nadav Amit <namit@...are.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v3 6/6] x86: bug.h: use asm_inline in _BUG_FLAGS
 definitions


* Rasmus Villemoes <linux@...musvillemoes.dk> wrote:

> This helps preventing a BUG* or WARN* in some static inline from
> preventing that (or one of its callers) being inlined, so should allow
> gcc to make better informed inlining decisions.
> 
> For example, with gcc 9.2, tcp_fastopen_no_cookie() vanishes from
> net/ipv4/tcp_fastopen.o. It does not itself have any BUG or WARN, but
> it calls dst_metric() which has a WARN_ON_ONCE - and despite that
> WARN_ON_ONCE vanishing since the condition is compile-time false,
> dst_metric() is apparently sufficiently "large" that when it gets
> inlined into tcp_fastopen_no_cookie(), the latter becomes too large
> for inlining.
> 
> Overall, if one asks size(1), .text decreases a little and .data
> increases by about the same amount (x86-64 defconfig)
> 
> $ size vmlinux.{before,after}
>    text    data     bss     dec     hex filename
> 19709726        5202600 1630280 26542606        195020e vmlinux.before
> 19709330        5203068 1630280 26542678        1950256 vmlinux.after
> 
> while bloat-o-meter says
> 
> add/remove: 10/28 grow/shrink: 103/51 up/down: 3669/-2854 (815)
> ...
> Total: Before=14783683, After=14784498, chg +0.01%
> 
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>

Acked-by: Ingo Molnar <mingo@...nel.org>

Thanks,

	Ingo