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

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Message-ID: <20260118225802.5e658c2a@pumpkin>
Date: Sun, 18 Jan 2026 22:58:02 +0000
From: David Laight <david.laight.linux@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Eric Dumazet <edumazet@...gle.com>, 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, 18 Jan 2026 11:47:24 -0800
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?
> 
> 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

The next questions are does anything actually run faster (either way),
and should anything at all be marked 'inline' rather than 'always_inline'.

After all, if you call a function twice (not in a loop) you may
want a real function in order to avoid I-cache misses.

I've had to mark things that are called once 'always_inline', and
also 'big looking' functions that are called with constants and optimise
to almost nothing.

But I'm sure there is a lot of code that is 'inline_for_bloat' :-)
(Don't talk to me about C++ class definitions....)

On 32bit you probably don't want to inline __arch_xprod_64(), but you do
want to pass (bias ? m : 0) and may want separate functions for the
'no overflow' case (if it is common enough to worry about).

	David