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

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Date:   Fri, 30 Mar 2018 14:17:25 +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


* Ingo Molnar <mingo@...nel.org> wrote:

> > No Global pages (baseline): 186.951 seconds time elapsed  ( +-  0.35% )
> > 28 Global pages (this set): 185.756 seconds time elapsed  ( +-  0.09% )
> >                              -1.195 seconds (-0.64%)
> > 
> > Lower is better here, obviously.
> > 
> > I also re-checked everything using will-it-scale's llseek1 test[2] which
> > is basically a microbenchmark of a halfway reasonable syscall.  Higher
> > here is better.
> > 
> > No Global pages (baseline): 15783951 lseeks/sec
> > 28 Global pages (this set): 16054688 lseeks/sec
> > 			     +270737 lseeks/sec (+1.71%)
> > 
> > So, both the kernel compile and the microbenchmark got measurably faster.
> 
> Ok, cool, this is much better!
> 
> Mind re-sending the patch-set against latest -tip so it can be merged?
> 
> At this point !PCID Intel hardware is not a primary concern, if something bad 
> happens on them with global pages we can quirk global pages off on them in some 
> way, or so.

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.

Thanks,

	Ingo