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From: Ingo Molnar <mingo@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: "Dave Hansen" <dave.hansen@linux.intel.com>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	"Andrea Arcangeli" <aarcange@redhat.com>,
	"Andrew Lutomirski" <luto@kernel.org>,
	"Kees Cook" <keescook@google.com>,
	"Hugh Dickins" <hughd@google.com>,
	"Jürgen Groß" <jgross@suse.com>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	namit@vmware.com
Subject: Re: [PATCH 00/11] Use global pages with PTI
Date: Tue, 27 Mar 2018 22:07:19 +0200	[thread overview]
Message-ID: <20180327200719.lvdomez6hszpmo4s@gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1803271949250.1618@nanos.tec.linutronix.de>


* Thomas Gleixner <tglx@linutronix.de> wrote:

> > systems.  Atoms are going to be the easiest thing to get my hands on,
> > but I tend to shy away from them for performance work.
> 
> What I have in mind is that I wonder whether the whole circus is worth it
> when there is no performance advantage on PCID systems.

I'd still love to:

 - To see at minimum stddev numbers, to make sure we are not looking at some weird
   statistical artifact. (I also outlined a more robust measurement method.)

 - If the numbers are right, a CPU engineer should have a look if possible, 
   because frankly this effect is not expected and is not intuitive. Where global 
   pages can be used safely they are almost always an unconditional win.
   Maybe we are missing some limitation or some interaction with PCID.

Since we'll be using PCID even on Meltdown-fixed hardware, maybe the same negative 
performance effect already exists on non-PTI kernels as well, we just never 
noticed?

I.e. there are multiple grounds to get to the bottom of this.

Thanks,

	Ingo

  reply	other threads:[~2018-03-27 20:07 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-03-23 17:44 [PATCH 00/11] Use global pages with PTI Dave Hansen
2018-03-23 17:44 ` [PATCH 01/11] x86/mm: factor out pageattr _PAGE_GLOBAL setting Dave Hansen
2018-03-23 17:44 ` [PATCH 02/11] x86/mm: undo double _PAGE_PSE clearing Dave Hansen
2018-03-23 17:44 ` [PATCH 03/11] x86/mm: introduce "default" kernel PTE mask Dave Hansen
2018-03-23 17:44 ` [PATCH 04/11] x86/espfix: document use of _PAGE_GLOBAL Dave Hansen
2018-03-23 17:44 ` [PATCH 05/11] x86/mm: do not auto-massage page protections Dave Hansen
2018-03-23 19:15   ` Nadav Amit
2018-03-23 19:26     ` Dave Hansen
2018-03-23 19:34       ` Nadav Amit
2018-03-23 19:38         ` Dave Hansen
2018-03-24 15:10   ` kbuild test robot
2018-03-24 15:21   ` kbuild test robot
2018-03-23 17:44 ` [PATCH 06/11] x86/mm: remove extra filtering in pageattr code Dave Hansen
2018-03-23 17:44 ` [PATCH 07/11] x86/mm: comment _PAGE_GLOBAL mystery Dave Hansen
2018-03-23 17:44 ` [PATCH 08/11] x86/mm: do not forbid _PAGE_RW before init for __ro_after_init Dave Hansen
2018-03-23 17:45 ` [PATCH 09/11] x86/pti: enable global pages for shared areas Dave Hansen
2018-03-23 19:12   ` Nadav Amit
2018-03-23 19:36     ` Dave Hansen
2018-03-23 17:45 ` [PATCH 10/11] x86/pti: clear _PAGE_GLOBAL for kernel image Dave Hansen
2018-03-23 17:45 ` [PATCH 11/11] x86/pti: leave kernel text global for !PCID Dave Hansen
2018-03-23 18:26 ` [PATCH 00/11] Use global pages with PTI Linus Torvalds
2018-03-24  0:40   ` Dave Hansen
2018-03-24  0:46     ` Linus Torvalds
2018-03-24  0:54       ` Linus Torvalds
2018-03-24 11:05     ` Ingo Molnar
2018-03-27 13:36     ` Thomas Gleixner
2018-03-27 16:32       ` Dave Hansen
2018-03-27 17:51         ` Thomas Gleixner
2018-03-27 20:07           ` Ingo Molnar [this message]
2018-03-27 20:19             ` Dave Hansen
2018-03-29  0:17             ` Dave Hansen
2018-03-30 12:09               ` Ingo Molnar
2018-03-30 12:17                 ` Ingo Molnar
2018-03-30 20:26                   ` Dave Hansen
2018-03-30 20:32                     ` Thomas Gleixner
2018-03-30 21:40                       ` Dave Hansen
2018-03-31  5:39                         ` Ingo Molnar
2018-03-31 18:19                           ` Dave Hansen

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