linux-kernel - Re: [RFC 0/6] x86: prefetch

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Date:   Thu, 25 Feb 2021 10:55:00 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org
Subject: Re: [RFC 0/6] x86: prefetch_page() vDSO call

On Thu, Feb 25, 2021 at 01:32:56AM -0800, Nadav Amit wrote:
> > On Feb 25, 2021, at 12:52 AM, Nadav Amit <nadav.amit@...il.com> wrote:

> > Can you give me a reference to the “removing explicit prefetch from some
> > parts of the kernel”?

75d65a425c01 ("hlist: remove software prefetching in hlist iterators")
e66eed651fd1 ("list: remove prefetching from regular list iterators")

> Oh. I get it - you mean we remove we remove the use of explicit memory
> prefetch from the kernel code. Well, I don’t think it is really related,
> but yes, performance numbers are needed.

Right, so my main worry was that use of the prefetch instruction
actually hurt performance once the hardware prefetchers got smart
enough, this being a very similar construct (just on a different level
of the stack) should be careful not to suffer the same fate.