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

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Date:   Wed, 11 Oct 2023 20:42:15 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Nadav Amit <namit@...are.com>,
        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 Tue, Oct 10, 2023 at 8:52 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Tue, 10 Oct 2023 at 11:41, Uros Bizjak <ubizjak@...il.com> wrote:
> >
> > Yes, but does it CSE the load from multiple addresses?
>
> Yes, it should do that just right, because the *asm* itself is
> identical, just the offsets (that gcc then adds separately) would be
> different.
>
> This is not unlike how we depend on gcc CSE'ing the "current" part
> when doing multiple accesses of different members off that:
>
>     static __always_inline struct task_struct *get_current(void)
>     {
>         return this_cpu_read_stable(pcpu_hot.current_task);
>     }
>
> with this_cpu_read_stable() being an inline asm that lacks the memory
> component (the same way the fallback hides it by just using
> "%%gs:this_cpu_off" directly inside the asm, instead of exposing it as
> a memory access to gcc).
>
> Of course, I think that with the "__seg_gs" patches, we *could* expose
> the "%%gs:this_cpu_off" part to gcc, since gcc hopefully then can do
> the alias analysis on that side and see that it can CSE the thing
> anyway.
>
> That might be a better choice than __FORCE_ORDER, in fact.
>
> IOW, something like
>
>     static __always_inline unsigned long new_cpu_offset(void)
>     {
>         unsigned long res;
>         asm(ALTERNATIVE(
>                         "movq " __percpu_arg(1) ",%0",
>                         "rdgsbase %0",
>                         X86_FEATURE_FSGSBASE)
>                 : "=r" (res)
>                 : "m" (this_cpu_off));
>         return res;
>     }
>
> would presumably work together with your __seg_gs stuff.
>
> UNTESTED!!

The attached patch was tested on a target with fsgsbase CPUID and
without it. It works!

The patch improves amd_pmu_enable_virt() in the same way as reported
in the original patch submission and also reduces the number of percpu
offset reads (either from this_cpu_off or with rdgsbase) from 1663 to
1571.

The only drawback is a larger binary size:

  text    data     bss     dec     hex filename
25546594        4387686  808452 30742732        1d518cc vmlinux-new.o
25515256        4387814  808452 30711522        1d49ee2 vmlinux-old.o

that increases by 31k (0.123%), probably due to 1578 rdgsbase alternatives.

I'll prepare and submit a patch for tip/percpu branch.

Uros.


>
>             Linus

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