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

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Date:   Fri, 30 Mar 2018 14:40:11 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...nel.org>,
        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

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%)