linux-kernel - Re: [PATCH v2] noinstr: Use asm_inline() in instrumentation

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Message-ID: <20250415074849.GJ5600@noisy.programming.kicks-ass.net>
Date: Tue, 15 Apr 2025 09:48:49 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>, Uros Bizjak <ubizjak@...il.com>
Subject: Re: [PATCH v2] noinstr: Use asm_inline() in
 instrumentation_{begin,end}()

On Mon, Apr 14, 2025 at 05:27:11PM -0700, Josh Poimboeuf wrote:
> Use asm_inline() in the instrumentation begin/end macros to prevent the
> compiler from making poor inlining decisions based on the length of the
> objtool annotations.
> 
> Without the objtool annotations, each macro resolves to a single NOP.
> Using inline_asm() seems obviously correct here as it accurately
> communicates the actual code size to the compiler.
> 
> These macros are used by WARN() and lockdep, so this change can affect a
> lot of functions.
> 
> For a defconfig kernel built with GCC 14.2.1, bloat-o-meter reports a
> 0.17% increase in text size:
> 
>   add/remove: 74/352 grow/shrink: 914/353 up/down: 80747/-47120 (33627)
>   Total: Before=19460272, After=19493899, chg +0.17%

Hmm, I was surprised that defconfig was affected at all. Why does
defconfig have DEBUG_ENTRY on?