linux-kernel - Re: [PATCH v3 00/20] Speculative page faults

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Message-Id: <177195b3-2d99-94e6-f334-843bd19042a1@linux.vnet.ibm.com>
Date:   Tue, 3 Oct 2017 10:03:18 +0200
From:   Laurent Dufour <ldufour@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>, kirill@...temov.name,
        Andi Kleen <ak@...ux.intel.com>,
        Michal Hocko <mhocko@...nel.org>, dave@...olabs.net,
        Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Will Deacon <will.deacon@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, haren@...ux.vnet.ibm.com,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        npiggin@...il.com, bsingharora@...il.com,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v3 00/20] Speculative page faults

On 03/10/2017 03:27, Michael Ellerman wrote:
> Laurent Dufour <ldufour@...ux.vnet.ibm.com> writes:
> 
>> Hi Andrew,
>>
>> On 28/09/2017 22:38, Andrew Morton wrote:
>>> On Thu, 28 Sep 2017 14:29:02 +0200 Laurent Dufour <ldufour@...ux.vnet.ibm.com> wrote:
>>>
>>>>> Laurent's [0/n] provides some nice-looking performance benefits for
>>>>> workloads which are chosen to show performance benefits(!) but, alas,
>>>>> no quantitative testing results for workloads which we may suspect will
>>>>> be harmed by the changes(?).  Even things as simple as impact upon
>>>>> single-threaded pagefault-intensive workloads and its effect upon
>>>>> CONFIG_SMP=n .text size?
>>>>
>>>> I forgot to mention in my previous email the impact on the .text section.
>>>>
>>>> Here are the metrics I got :
>>>>
>>>> .text size	UP		SMP		Delta
>>>> 4.13-mmotm	8444201		8964137		6.16%
>>>> '' +spf		8452041		8971929		6.15%
>>>> 	Delta	0.09%		0.09%	
>>>>
>>>> No major impact as you could see.
>>>
>>> 8k text increase seems rather a lot actually.  That's a lot more
>>> userspace cacheclines that get evicted during a fault...
>>>
>>> Is the feature actually beneficial on uniprocessor?
>>
>> This is useless on uniprocessor, and I will disable it on x86 when !SMP 
>> by not defining __HAVE_ARCH_CALL_SPF.
>> So the speculative page fault handler will not be built but the vm 
>> sequence counter and the SCRU stuff will still be there. I may also make 
>> it disabled through macro when __HAVE_ARCH_CALL_SPF is not defined, but 
>> this may obfuscated the code a bit...
>>
>> On ppc64, as this feature requires book3s, it can't be built without SMP 
>> support.
> 
> Book3S doesn't force SMP, eg. PMAC is Book3S but can be built with SMP=n.
> 
> It's true that POWERNV and PSERIES both force SMP, and those are the
> platforms used on modern Book3S CPUs.

Thanks Michael,

I'll add a check on CONFIG_SMP on ppc too.

Laurent.