linux-kernel - Re: [PATCH -tip] x86/locking/atomic: Use asm

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Message-ID: <CAFULd4b=4rHcVAVSg_3yMb8=3ReiSriw_rM4vJL9_HvheXE92w@mail.gmail.com>
Date: Wed, 5 Mar 2025 09:54:11 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...el.com>, x86@...nel.org, linux-kernel@...r.kernel.org, 
	Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...nel.org>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	"H. Peter Anvin" <hpa@...or.com>, Linus Torvalds <torvalds@...uxfoundation.org>
Subject: Re: [PATCH -tip] x86/locking/atomic: Use asm_inline for atomic
 locking insns

On Sat, Mar 1, 2025 at 1:38 PM Borislav Petkov <bp@...en8.de> wrote:
>
> On Sat, Mar 01, 2025 at 10:05:56AM +0100, Uros Bizjak wrote:
> > OTOH, -Os, where different code size/performance heuristics are used, now
> > performs better w.r.t code size.
>
> Did anything change since:
>
> 281dc5c5ec0f ("Give up on pushing CC_OPTIMIZE_FOR_SIZE")
> 3a55fb0d9fe8 ("Tell the world we gave up on pushing CC_OPTIMIZE_FOR_SIZE")
>
> wrt -Os?
>
> Because if not, we still don't love -Os and you can drop the -Os argument.

The -Os argument was to show the effect of the patch when the compiler
is instructed to take care of the overall size. Giving the compiler
-O2 and then looking at the overall size of the produced binary is
just wrong.

> And without any perf data showing any improvement, this patch does nothing but
> enlarge -O2 size...

Even to my surprise, the patch has some noticeable effects on the
performance, please see the attachment in [1] for LMBench data or [2]
for some excerpts from the data. So, I think the patch has potential
to improve the performance.

[1] https://lore.kernel.org/lkml/CAFULd4YBcG45bigHBox2pu+To+Y5BzbRxG+pUr42AVOWSnfKsg@mail.gmail.com/
[2] https://lore.kernel.org/lkml/CAFULd4ZsSKwJ4Dz3cCAgaVsa4ypbb0e2savO-3_Ltbs=1wzgKQ@mail.gmail.com/

Thanks,
Uros.