linux-kernel - Re: [RFC] de-asmify the x86-64 system call slowpath

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Message-ID: <CA+55aFwfGVCVouh5uap3UPy92vc4N9OrRRa-NpYT9ZMicU6w8w@mail.gmail.com>
Date:	Wed, 5 Feb 2014 20:33:27 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Peter Anvin <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ning Qu <quning@...gle.com>
Subject: Re: [RFC] de-asmify the x86-64 system call slowpath

On Wed, Feb 5, 2014 at 6:32 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> No, I was thinking "try to optimistically map 8 adjacent aligned pages
> at a time" - that would be the same cacheline in the page tables, so
> it would be fairly cheap if we couple it with a gang-lookup of the
> pages in the page cache

Doing the gang-lookup is hard, since it's all abstracted away, but the
attached patch kind of tries to do what I described.

This patch probably doesn't work, but something *like* this might be
worth playing with.

Except I suspect the page-backed faults are actually the minority,
judging by how high clear_page_c_e is in the profile it's probably
mostly anonymous memory. I have no idea why I started with the (more
complex) case of file-backed prefaulting. Oh well.

            Linus

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