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

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Message-ID: <20260119093339.024f8d57@pumpkin>
Date: Mon, 19 Jan 2026 09:33:39 +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 16:01:25 -0800
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Sun, 18 Jan 2026 22:58:02 +0000 David Laight <david.laight.linux@...il.com> wrote:
> 
> > > 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.  
> 
> yup

I had two adjacent strlen() calls in a bit of code, the first was an
array (in a structure) and gcc inlined the 'word at a time' code, the
second was a pointer and it called the library function.
That had to be sub-optimal...

> > But I'm sure there is a lot of code that is 'inline_for_bloat' :-)  
> 
> ooh, can we please have that?

Or 'inline_to_speed_up_benchmark' and the associated 'unroll this loop
because that must make it faster'.

> I do think that every always_inline should be justified and commented,
> but I haven't been energetic about asking for that.

Apart from the 4-line functions where it is clearly obvious.
Especially since the compiler can still decide to not-inline them
if they are only 'inline'.

> A fun little project would be go through each one, figure out whether
> were good reasons and if not, just remove them and see if anyone
> explains why that was incorrect.

It's not just always_inline, a lot of the inline are dubious.
Probably why the networking code doesn't like it.

Maybe persuade Linus to do some of that.
He can use his 'god' bit to just change them.

	David