linux-kernel - Re: PCID and TLB flushes (was: [GIT PULL] kdbus for 4.1-rc1)

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Message-ID: <CA+55aFyG8dsL_dwZ=t+4ZwcUGjo1rq6gZteuwFdPmpUe6niX5w@mail.gmail.com>
Date:	Tue, 28 Apr 2015 15:56:29 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Dave Hansen <dave.hansen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	"the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: PCID and TLB flushes (was: [GIT PULL] kdbus for 4.1-rc1)

On Tue, Apr 28, 2015 at 3:15 PM, Kirill A. Shutemov
<kirill@...temov.name> wrote:
>
> I talked with Dave about implementing PCID and he thinks that it will be
> net loss.

So I'm told that Suresh Siddha actually had a patch inside Intel to
use PCID (back when he worked for Intel, I think he left), and that it
was a wash in their testing.

I never saw the patch, and it might be interesting to try it again,
but there is some reason to believe that it doesn't make much of a
difference. Unlike most of the traditional RISC machines that got big
speedups, Intel TLB walking is so good that it likely isn't nearly as
noticeable, and it likely *does* result in more IPI's etc. Possibly
not a lot more, but if the win isn't big...

So I don't want to discourage you, because I'd love to see what the
patch looks like and if we can find cases where it matters, but I do
want to set expectations right. It's unlikely to be a big issue.

                           Linus
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