linux-kernel - Re: [RFC PATCH v1 00/57] Boot-time page size selection for arm64

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Message-ID: <ed43da27-30bf-4f9d-a952-3d1fe80c5302@arm.com>
Date: Wed, 13 Nov 2024 12:56:24 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Petr Tesarik <ptesarik@...e.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Ard Biesheuvel <ardb@...nel.org>, Catalin Marinas <catalin.marinas@....com>,
 David Hildenbrand <david@...hat.com>, Greg Marsden
 <greg.marsden@...cle.com>, Ivan Ivanov <ivan.ivanov@...e.com>,
 Kalesh Singh <kaleshsingh@...gle.com>, Marc Zyngier <maz@...nel.org>,
 Mark Rutland <mark.rutland@....com>, Matthias Brugger <mbrugger@...e.com>,
 Miroslav Benes <mbenes@...e.cz>, Will Deacon <will@...nel.org>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [RFC PATCH v1 00/57] Boot-time page size selection for arm64

On 13/11/2024 12:40, Petr Tesarik wrote:
> On Tue, 12 Nov 2024 11:50:39 +0100
> Petr Tesarik <ptesarik@...e.com> wrote:
> 
>> On Tue, 12 Nov 2024 10:19:34 +0000
>> Ryan Roberts <ryan.roberts@....com> wrote:
>>
>>> On 12/11/2024 09:45, Petr Tesarik wrote:  
>>>> On Mon, 11 Nov 2024 12:25:35 +0000
>>>> Ryan Roberts <ryan.roberts@....com> wrote:
>>>>     
>>>>> Hi Petr,
>>>>>
>>>>> On 11/11/2024 12:14, Petr Tesarik wrote:    
>>>>>> Hi Ryan,
>>>>>>
>>>>>> On Thu, 17 Oct 2024 13:32:43 +0100
>>>>>> Ryan Roberts <ryan.roberts@....com> wrote:    
>>>>> [...]    
>>>>>> Third, a few micro-benchmarks saw a significant regression.
>>>>>>
>>>>>> Most notably, getenv and getenvT2 tests from libMicro were 18% and 20%
>>>>>> slower with variable page size. I don't know why, but I'm looking into
>>>>>> it. The system() library call was also about 18% slower, but that might
>>>>>> be related.      
>>>>>
>>>>> OK, ouch. I think there are some things we can try to optimize the
>>>>> implementation further. But I'll wait for your analysis before digging myself.    
>>>>
>>>> This turned out to be a false positive. The way this microbenchmark was
>>>> invoked did not get enough samples, so it was mostly dependent on
>>>> whether caches were hot or cold, and the timing on this specific system
>>>> with the specific sequence of bencnmarks in the suite happens to favour
>>>> my baseline kernel.
>>>>
>>>> After increasing the batch count, I'm getting pretty much the same
>>>> performance for 6.11 vanilla and patched kernels:
>>>>
>>>>                         prc thr   usecs/call      samples   errors cnt/samp 
>>>> getenv (baseline)         1   1      0.14975           99        0   100000 
>>>> getenv (patched)          1   1      0.14981           92        0   100000     
>>>
>>> Oh that's good news! Does this account for all 3 of the above tests (getenv,
>>> getenvT2 and system())?  
>>
>> It does for getenvT2 (a variant of the test with 2 threads), but not
>> for system. Thanks for asking, I forgot about that one.
>>
>> I'm getting substantial difference there (+29% on average over 100 runs):
>>
>>                         prc thr   usecs/call      samples   errors cnt/samp  command
>> system (baseline)         1   1   6937.18016          102        0      100     A=$$
>> system (patched)          1   1   8959.48032          102        0      100     A=$$
>>
>> So, yeah, this should in fact be my priority #1.
> 
> Further testing reveals the workload is bimodal, that is to say the
> distribution of results has two peaks. The first peak around 3.2 ms
> covers 30% runs, the second peak around 15.7 ms covers 11%. Two per
> cent are faster than the fast peak, 5% are slower than slow peak, the
> rest is distributed almost evenly between them.

FWIW, One source of bimodality I've seen on Ampere systems with 2 NUMA nodes is
placement of the kernel image vs placement of the running thread. If they are
remote from eachother, you'll see a slowdown. I've hacked this source away in
the past by effectively using only a single NUMA node (with the help of
'maxcpus' and 'mem' kernel cmdline options).

> 
> 100 samples were not sufficient to see this distribution, and it was
> mere bad luck that only the patched kernel originally reported bad
> results. I can now see bad results even with the unpatched kernel.
> 
> In short, I don't think there is a difference in system() performance.
> 
> I will still have a look at dup() and VMA performance, but so far it
> all looks good to me. Good job! ;-)

Thanks for digging into all this!

> 
> I will also try running a more complete set of benchmarks during next
> week. That's SUSE Hack Week, and I want to make a PoC for the MM
> changes I proposed at LPC24, so I won't need this Ampere system for
> interactive use.
> 
> Petr T