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

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Message-ID: <5596B338-C90B-47CA-BDAD-2B0091EFCD9B@vmware.com>
Date:   Wed, 27 Feb 2019 23:16:46 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "bp@...en8.de" <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 Feb 27, 2019, at 2:12 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> Hi all,
> 
> 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'.
> 
> Built and boot tested with CONFIG_DEBUG_PREEMPT=y.

Overall this series affects 70 functions and shortens the code by 326 bytes.
_local_bh_enable() for example is shorten by 14 bytes (26%).

I must admit that I although I pointed some of these issues before, I am not
sure whether they are really important...

Recently, I tried to see how to make the compiler to generate “better code”
from Linux. I sprinkled “pure” attribute on many common function (e.g.,
page_rmapping()), sg_next()); sprinkled const-attribute on some others
(e.g., jiffies_to_msecs()); created a const-alias variable so the compiler
would consider kaslr variables and sme_me_mask as constant after
initialization, and so on.

I was then looking at the changed code, and while some functions were
shorter and some longer, many common functions did look “better”. The only
problem was that any benchmark that I did not show any measurable impact.

So perhaps it is a matter of measurement, but eventually right now there is
no clean win.