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

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Message-ID: <CAHk-=whS8-Lk_=mFp=mr-JrbRYtScgz-4s_GLAOQGafa_3zP9g@mail.gmail.com>
Date:   Wed, 18 Oct 2023 09:26:38 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Uros Bizjak <ubizjak@...il.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>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()

On Wed, 18 Oct 2023 at 09:03, Nadav Amit <namit@...are.com> wrote:
>
> Having said that, I am not sure what other usages you have in mind.
> “current” is a pretty obvious straight forward case with considerable
> impact on code generation. There may be additional variables, but it is
> likely that there would be more functions/TU in which they would not be
> constant and would require more refined techniques to avoid mistakes
> such as the use of stale cached values.

Yeah, I don't think there really are other cases.

We do have things that could be considered stable (like
"smp_processor_id()" which is stable as long as preemption or
migration is disabled (or it's in an irq-off section).

And it might be lovely to optimize those too, *BUT* that would require
that there be a barrier against that optimization that works.

And if there is anything that this thread has made clear, it's that
the whole 'load from a constant section' doesn't seem to have any sane
barriers.

So while the CSE for inline asm statements is a bit too weak with that
whole "only CSE within a basic block" thing, the CSE of "load a
constant value from memory" is too *strong*, in that we don't seem to
have _any_ sane way to say "now you need to reload".

The traditional way we've done that is with our "barrier()" macro,
which does the whole inline asm with a memory clobber, but even that
doesn't act as a barrier for gcc optimizing the constant load.

Which means that while we'd probably love for the compiere to optimize
smp_processor_id() a bit more, we can't use the 'stable memory
location' trick for it.

Because I can't think of anything but 'current' that would be _that_
stable as far as C code is concerned.

                Linus