linux-kernel - Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu

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Date:   Wed, 11 Oct 2023 14:54:45 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     Nadav Amit <namit@...are.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()

On Wed, 11 Oct 2023 at 14:33, Uros Bizjak <ubizjak@...il.com> wrote:
>
> Reading the above, it looks to me that we don't want to play games
> with "const aliased" versions of current_task [1], as proposed by
> Nadav in his patch series.

Well, maybe I'd like it if I saw what the effect of it was, but that
patch mentions "sync_mm_rss()" which doesn't actually exist
(SPLIT_RSS_COUNTING is never defined, the split version is gone and
hasn't existed since commit f1a7941243c1 "mm: convert mm's rss stats
into percpu_counter")

I'm not sure why gcc used to get code generation wrong there, and I
don't see what could be improved in the current implementation of
'current_task', but I guess there could be things that could be done
to make gcc more likely to CSE the cases..

IOW, I don't understand why Navad's patch would improve gcc code gen.
It presumably depends on some gcc internal issue (ie "gcc just happens
to be better at optimization X").

Which is obviously a valid thing in general.

              Linus