linux-kernel - Re: [PATCHv14 08/17] x86/mm: Reduce untagged_addr() overhead until the first LAM user

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Date:   Tue, 17 Jan 2023 10:26:22 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Taras Madan <tarasmadan@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Bharata B Rao <bharata@....com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Sami Tolvanen <samitolvanen@...gle.com>,
        joao@...rdrivepizza.com
Subject: Re: [PATCHv14 08/17] x86/mm: Reduce untagged_addr() overhead until
 the first LAM user

On Tue, Jan 17, 2023 at 9:29 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Tue, Jan 17, 2023 at 9:18 AM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > The reason clang seems to generate saner code is that clang seems to
> > largely ignore the whole "__builtin_expect()", at least not to the
> > point where it tries to make the unlikely case be out-of-line.
>
> Side note: that's not something new or unusual. It's been the case
> since I started testing clang - we have several code-paths where we
> use "unlikely()" to try to get very unlikely cases to be out-of-line,
> and clang just mostly ignores it, or treats it as a very weak hint. I
> think the only way to get clang to treat it as a *strong* hint is to
> use PGO.

I'd be surprised if that were intentional or by design.

Do you guys have a bug report we could look at?

> So clang ignoring the likely() hint is probably the right thing here,
> and then the wrong thing in some other places.

-- 
Thanks,
~Nick Desaulniers