linux-kernel - Re: [PATCH 0/5] x86/percpu semantics and fixes

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Message-ID: <64526663-1F10-42A6-A005-92657264D587@vmware.com>
Date:   Fri, 8 Mar 2019 19:35:17 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "luto@...nel.org" <luto@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/5] x86/percpu semantics and fixes

> On Mar 8, 2019, at 6:50 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Wed, Feb 27, 2019 at 11:12:52AM +0100, Peter Zijlstra wrote:
> 
>> This is a collection of x86/percpu changes that I had pending and got reminded
>> of by Linus' comment yesterday about __this_cpu_xchg().
>> 
>> This tidies up the x86/percpu primitives and fixes a bunch of 'fallout'.
> 
> (Sorry; this is going to have _wide_ output)
> 
> OK, so what I did is I build 4 kernels (O=defconfig-build{,1,2,3}) with
> resp that many patches of this series applied.
> 
> When I look at just the vmlinux size output:
> 
> $ size defconfig-build*/vmlinux
> text    data     bss     dec     hex filename
> 19540631        5040164 1871944 26452739        193a303 defconfig-build/vmlinux
> 19540635        5040164 1871944 26452743        193a307 defconfig-build1/vmlinux
> 19540685        5040164 1871944 26452793        193a339 defconfig-build2/vmlinux
> 19540685        5040164 1871944 26452793        193a339 defconfig-build3/vmlinux
> 
> Things appear to get slightly larger; however when I look in more
> detail using my (newly written compare script, find attached), I get
> things like:

Nice script! I keep asking myself how comparing two binaries can provide
some “number” to indicate how “good” the binary is (at least relatively to
another one) - either during compilation or after. Code size, as you show,
is the wrong metric.

Anyhow, I am a little disappointed (and surprised) that in most cases that I
played with, this kind of optimizations have marginal impact on performance
at best, even when the binary changes “a lot” and when microbenchmarks are
used.