linux-kernel - Re: [PATCH 00/11] Use global pages with PTI

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Date:   Sat, 31 Mar 2018 07:39:56 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jürgen Groß <jgross@...e.com>,
        the arch/x86 maintainers <x86@...nel.org>, namit@...are.com
Subject: Re: [PATCH 00/11] Use global pages with PTI


* Dave Hansen <dave.hansen@...ux.intel.com> wrote:

> On 03/30/2018 01:32 PM, Thomas Gleixner wrote:
> > On Fri, 30 Mar 2018, Dave Hansen wrote:
> > 
> >> On 03/30/2018 05:17 AM, Ingo Molnar wrote:
> >>> BTW., the expectation on !PCID Intel hardware would be for global pages to help 
> >>> even more than the 0.6% and 1.7% you measured on PCID hardware: PCID already 
> >>> _reduces_ the cost of TLB flushes - so if there's not even PCID then global pages 
> >>> should help even more.
> >>>
> >>> In theory at least. Would still be nice to measure it.
> >>
> >> I did the lseek test on a modern, non-PCID system:
> >>
> >> No Global pages (baseline): 6077741 lseeks/sec
> >> 94 Global pages (this set): 8433111 lseeks/sec
> >> 			   +2355370 lseeks/sec (+38.8%)
> > 
> > That's all kernel text, right? What's the result for the case where global
> > is only set for all user/kernel shared pages?
> 
> Yes, that's all kernel text (94 global entries).  Here's the number with
> just the entry data/text set global (88 global entries on this system):
> 
> No Global pages (baseline): 6077741 lseeks/sec
> 88 Global Pages (kentry  ): 7528609 lseeks/sec (+23.9%)
> 94 Global pages (this set): 8433111 lseeks/sec (+38.8%)

Very impressive!

Please incorporate the performance numbers in patches #9 and #11.

There were a couple of valid review comments which need to be addressed as well, 
but other than that it all looks good to me and I plan to apply the next 
iteration.

In fact I think I'll try to put it into the backporting tree: as PGE was really 
the pre PTI status quo and thus we should expect few quirks/bugs in this area, 
plus we still want to share as much core PTI logic with the -stable kernels as 
possible. The performance plus doesn't hurt either ... after so much lost 
performance.

Thanks,

	Ingo