linux-kernel - Re: [PATCH 0/3] Use global pages with PTI

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Date:   Thu, 15 Feb 2018 15:55:45 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        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>
Subject: Re: [PATCH 0/3] Use global pages with PTI

On 02/15/2018 09:47 AM, Linus Torvalds wrote:
> On Thu, Feb 15, 2018 at 5:20 AM, Dave Hansen
> <dave.hansen@...ux.intel.com> wrote:
>> During the switch over to PTI, we seem to have lost our ability to have
>> GLOBAL mappings.
...
> Did you perhaps re-run any benchmark numbers just to verify? Because
> it's always good to back up patches that should improve performance
> with actual numbers..

Same test as last time except I'm using all 4 cores on a Skylake desktop
instead of just 1.  The test is this:

> https://github.com/antonblanchard/will-it-scale/blob/master/tests/lseek1.c

With PCIDs, lseek()s/second go up around 2% to 3% with the these patches
enabling the global bit (it's noisy).  I measured it at 3% before, so
definitely the same ballpark.  That was also before all of Andy's
trampoline stuff and the syscall fast path removal.