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

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Message-ID: <Z8isNxBxC9pcG4KL@gmail.com>
Date: Wed, 5 Mar 2025 20:55:35 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Uros Bizjak <ubizjak@...il.com>
Cc: Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...el.com>,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	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


* Uros Bizjak <ubizjak@...il.com> wrote:

> 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/

If you are measuring micro-costs, please make sure you pin the workload 
to a single CPU (via 'taskset' for example) and run 'perf stat --null 
--repeat 5' or so to measure the run-over-run noise of the benchmark.

Thanks,

	Ingo