linux-kernel - Re: [PATCH 00/31 v2] PTI support for x86

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Message-ID: <20180211191312.54apu5edk3olsfz3@gmail.com>
Date:   Sun, 11 Feb 2018 20:13:12 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Andy Lutomirski <luto@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH 00/31 v2] PTI support for x86_32


* Joerg Roedel <jroedel@...e.de> wrote:

> Hi Andy,
> 
> On Fri, Feb 09, 2018 at 05:47:43PM +0000, Andy Lutomirski wrote:
> > One thing worth noting is that performance of this whole series is
> > going to be abysmal due to the complete lack of 32-bit PCID.  Maybe
> > any kernel built with this option set that runs on a CPU that has the
> > PCID bit set in CPUID should print a big fat warning like "WARNING:
> > you are using 32-bit PTI on a 64-bit PCID-capable CPU.  Your
> > performance will increase dramatically if you switch to a 64-bit
> > kernel."
> 
> Thanks for your review. I can add this warning, but I just hope that not
> a lot of people will actually see it :)

Could you please measure the PTI kernel vs. vanilla kernel?

Nothing complex, just perf's built-in scheduler and syscall benchmark should be 
enough:

   perf stat --null --sync --repeat 10 perf bench sched messaging -g 20

this should give us a pretty good worst-case overhead figure for process 
workloads.

Add '-t' to test threaded workloads as well:

   perf stat --null --sync --repeat 10 perf bench sched messaging -g 20 -t

The 10 runs used should be enough to reach good stability in practice:

 Performance counter stats for 'perf bench sched messaging -g 20 -t' (10 runs):

       0.380742219 seconds time elapsed                                          ( +-  0.73% )

Maybe do the same on the 64-bit kernel as well, so that we have 4 good data points 
on the same hardware?

Thanks,

	Ingo