linux-kernel - Re: [PATCH] compiler_types: Introduce inline_for

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Message-ID: <CANn89i+RBRNuftz5HfsEVW39VvnQWiUdins4CTRzGXoeJ3jAMQ@mail.gmail.com>
Date: Sun, 18 Jan 2026 21:38:59 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel <linux-kernel@...r.kernel.org>, netdev@...r.kernel.org, 
	Jakub Kicinski <kuba@...nel.org>, Eric Dumazet <eric.dumazet@...il.com>, 
	Paolo Abeni <pabeni@...hat.com>, Nicolas Pitre <npitre@...libre.com>
Subject: Re: [PATCH] compiler_types: Introduce inline_for_performance

On Sun, Jan 18, 2026 at 8:47 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Sun, 18 Jan 2026 15:24:48 +0000 Eric Dumazet <edumazet@...gle.com> wrote:
>
> > inline keyword is often ignored by compilers.
> >
> > We need something slightly stronger in networking fast paths
> > but __always_inline is too strong.
> >
> > Instead, generalize idea Nicolas used in commit d533cb2d2af4
> > ("__arch_xprod64(): make __always_inline when optimizing for performance")
> >
> > This will help CONFIG_CC_OPTIMIZE_FOR_SIZE=y users keeping
> > their kernels small.
>
> This is good.  __always_inline is ambiguous and the name lacks
> commentary value.
>
> If we take away __always_inline's for-performance role then what
> remains?  __always_inline is for tricky things where the compiler needs
> to be coerced into doing what we want?

Some functions should  not be out-of-line, even if
CONFIG_CC_OPTIMIZE_FOR_SIZE=y

A case-by case study would be needed.

>
> IOW, I wonder if we should take your concept further, create more
> fine-grained controls over this which have self-explanatory names.
>
>
>
> mm/ alone has 74 __always_inlines, none are documented, I don't know
> why they're present, many are probably wrong.
>
> Shit, uninlining only __get_user_pages_locked does this:
>
>    text    data     bss     dec     hex filename
>  115703   14018      64  129785   1faf9 mm/gup.o
>  103866   13058      64  116988   1c8fc mm/gup.o-after

mm/slub.c has __fastpath_inline, depending on CONFIG_SLUB_TINY

This probably could also depend on CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE

->

diff --git a/mm/slub.c b/mm/slub.c
index 861592ac54257b9d148ff921e6d8f62aced607b3..a8ca150a90355dd7a812f390c068ff9a7ccc2562
100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -224,7 +224,7 @@ do {                                        \
 #endif

 #ifndef CONFIG_SLUB_TINY
-#define __fastpath_inline __always_inline
+#define __fastpath_inline inline_for_performance
 #else
 #define __fastpath_inline
 #endif