linux-kernel - Re: [PATCH RFC 3/5] noinstr: Use asm_inline() in instrumentation

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Message-ID: <Z_TeoLQfZA858jk-@gmail.com>
Date: Tue, 8 Apr 2025 10:30:24 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Uros Bizjak <ubizjak@...il.com>
Subject: Re: [PATCH RFC 3/5] noinstr: Use asm_inline() in
 instrumentation_{begin,end}()

* Josh Poimboeuf <jpoimboe@...nel.org> wrote:

> Use asm_inline() to prevent the compiler from making poor inlining
> decisions based on the length of the objtool annotations.
> 
> For a defconfig kernel built with GCC 14.2.1, bloat-o-meter reports a
> 0.18% text size increase:
> 
>   add/remove: 88/433 grow/shrink: 967/487 up/down: 87579/-52630 (34949)
>   Total: Before=19448407, After=19483356, chg +0.18%
> 
> Presumably the text growth is due to increased inlining.  A net total of
> 345 functions were removed.

Since +0.18% puts this into the 'significant' category of .text size 
increases, it would be nice to see a bit more details about the nature 
of these function calls removed: were they really explicit calls to 
__instrumentation_begin()/end(), or somehow tail-call optimized out, or 
something else?

Also, I'm wondering where the 34,949 bytes bloat comes from: with 345 
functions removed that's 100 bytes per function? Doesn't sound right.

Also, is the bloat-o-meter output limited to the .text section, or does 
it include growth in out-of-line sections too?

Thanks,

	Ingo