linux-kernel - Re: [PATCH 5/8] x86/clear_page: add clear_page

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Date:   Wed, 14 Oct 2020 23:12:14 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Ankur Arora <ankur.a.arora@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Michal Hocko <mhocko@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ira Weiny <ira.weiny@...el.com>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 5/8] x86/clear_page: add clear_page_uncached()

On Wed, Oct 14, 2020 at 02:07:30PM -0700, Andy Lutomirski wrote:
> I assume it’s for a little optimization of clearing more than one
> page per SFENCE.
>
> In any event, based on the benchmark data upthread, we only want to do
> NT clears when they’re rather large, so this shouldn’t be just an
> alternative. I assume this is because a page or two will fit in cache
> and, for most uses that allocate zeroed pages, we prefer cache-hot
> pages. When clearing 1G, on the other hand, cache-hot is impossible
> and we prefer the improved bandwidth and less cache trashing of NT
> clears.

Yeah, use case makes sense but people won't know what to use. At the
time I was experimenting with this crap, I remember Linus saying that
that selection should be made based on the size of the area cleared, so
users should not have to know the difference.

Which perhaps is the only sane use case I see for this.

> Perhaps SFENCE is so fast that this is a silly optimization, though,
> and we don’t lose anything measurable by SFENCEing once per page.

Yes, I'd like to see real use cases showing improvement from this, not
just microbenchmarks.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette