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

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Message-ID: <20190308205637.GC2482@worktop.programming.kicks-ass.net>
Date:   Fri, 8 Mar 2019 21:56:37 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <namit@...are.com>
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 Fri, Mar 08, 2019 at 07:35:17PM +0000, Nadav Amit wrote:

> 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.

Right; I'm still pondering other metrics, like:

  readelf -WS | grep AX | grep -v -e init -e exit -e altinstr -e unlikely -e fixup

which is only 'fast' path text.

> 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.

Right, but if we don't care, it'll be death by 1000 cuts.

Anyway, can anybody explain percpu_stable_op() vs percpu_from_op() ?

I'm thinking of a variant of Linus' patch, but I'm confused about the
above.