linux-kernel - Re: [PATCH 1/5] x86/percpu: Differentiate this_cpu_{}() and __this_cpu

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Message-ID: <CAHk-=wj2DPz0GVP3zYU_4vrH8iH_AasH3xu4TfMgfh_3n71qVQ@mail.gmail.com>
Date:   Wed, 27 Feb 2019 09:38:04 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Lutomirski <luto@...nel.org>,
        Nadav Amit <namit@...are.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH 1/5] x86/percpu: Differentiate this_cpu_{}() and __this_cpu_{}()

On Wed, Feb 27, 2019 at 9:34 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> But the ONCE thing defeats CSE (on purpose!) so for code-gen that
> likes/wants that we then have to use the __this_cpu crud.

Right. But I do think that if you want CSE on a percpu access, you
might want to write it as such (ie load the value once and then re-use
the value).

But I'm not sure exactly which accesses Nadav noticed to be a problem,
so.. I guess I should just look at the other patches, but I found them
rather nasty and ad-hoc too so I just skimmed them with an "eww" ;)

                  Linus