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

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Message-ID: <CAFULd4akMtMxk1C-NsfzYFP6rkt5QUuHACq+BFEJV6u_8n+yiA@mail.gmail.com>
Date:   Wed, 18 Oct 2023 19:23:16 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()

On Wed, Oct 18, 2023 at 6:26 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> 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.

In processor.h, we have:

static __always_inline unsigned long current_top_of_stack(void)
{
/*
*  We can't read directly from tss.sp0: sp0 on x86_32 is special in
*  and around vm86 mode and sp0 on x86_64 is special because of the
*  entry trampoline.
*/
return this_cpu_read_stable(pcpu_hot.top_of_stack);
}

But I don't know how much it is used.

Uros.

> 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