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

[Nadav Amit]

> On Mar 8, 2019, at 6:50 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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 ânumberâ to indicate how âgoodâ 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 âa lotâ and when microbenchmarks are
used.