linux-kernel - Re: [PATCH v4 02/16] mm: Batch-copy PTE ranges during fork()

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Message-ID: <cfd88172-1624-4966-92c6-5197b26a820b@redhat.com>
Date: Wed, 20 Dec 2023 16:35:30 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Ard Biesheuvel <ardb@...nel.org>, Marc Zyngier <maz@...nel.org>,
 Oliver Upton <oliver.upton@...ux.dev>, James Morse <james.morse@....com>,
 Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu
 <yuzenghui@...wei.com>, Andrey Ryabinin <ryabinin.a.a@...il.com>,
 Alexander Potapenko <glider@...gle.com>,
 Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>,
 Vincenzo Frascino <vincenzo.frascino@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Matthew Wilcox <willy@...radead.org>, Yu Zhao <yuzhao@...gle.com>,
 Mark Rutland <mark.rutland@....com>, Kefeng Wang
 <wangkefeng.wang@...wei.com>, John Hubbard <jhubbard@...dia.com>,
 Zi Yan <ziy@...dia.com>, Barry Song <21cnbao@...il.com>,
 Alistair Popple <apopple@...dia.com>, Yang Shi <shy828301@...il.com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 02/16] mm: Batch-copy PTE ranges during fork()

On 20.12.23 16:05, Ryan Roberts wrote:
> On 20/12/2023 14:00, David Hildenbrand wrote:
>> [...]
>>
>>>>>
>>>>
>>>> gcc version 13.2.1 20231011 (Red Hat 13.2.1-4) (GCC)
>>>>
>>>>   From Fedora 38. So "a bit" newer :P
>>>>
>>>
>>> I'll retry with newer toolchain.
>>>
>>> FWIW, with the code fix and the original compiler:
>>>
>>> Fork, order-0, Apple M2:
>>> | kernel                |   mean_rel |   std_rel |
>>> |:----------------------|-----------:|----------:|
>>> | mm-unstable           |       0.0% |      0.8% |
>>> | hugetlb-rmap-cleanups |       1.3% |      2.0% |
>>> | fork-batching         |       4.3% |      1.0% |
>>>
>>> Fork, order-9, Apple M2:
>>> | kernel                |   mean_rel |   std_rel |
>>> |:----------------------|-----------:|----------:|
>>> | mm-unstable           |       0.0% |      0.8% |
>>> | hugetlb-rmap-cleanups |       0.9% |      0.9% |
>>> | fork-batching         |     -37.3% |      1.0% |
>>>
>>> Fork, order-0, Ampere Altra:
>>> | kernel                |   mean_rel |   std_rel |
>>> |:----------------------|-----------:|----------:|
>>> | mm-unstable           |       0.0% |      0.7% |
>>> | hugetlb-rmap-cleanups |       3.2% |      0.7% |
>>> | fork-batching         |       5.5% |      1.1% |
>>>
>>> Fork, order-9, Ampere Altra:
>>> | kernel                |   mean_rel |   std_rel |
>>> |:----------------------|-----------:|----------:|
>>> | mm-unstable           |       0.0% |      0.1% |
>>> | hugetlb-rmap-cleanups |       0.5% |      0.1% |
>>> | fork-batching         |     -10.4% |      0.1% |
>>>
>>
>> I just gave it another quick benchmark run on that Intel system.
>>
>> hugetlb-rmap-cleanups -> fork-batching
>>
>> order-0: 0.014114 -> 0.013848
>>
>> -1.9%
>>
>> order-9: 0.014262 -> 0.009410
>>
>> -34%
>>
>> Note that I disable SMT and turbo, and pin the test to one CPU, to make the
>> results as stable as possible. My kernel config has anything related to
>> debugging disabled.
>>
> 
> And with gcc 13.2 on arm64:
> 
> Fork, order-0, Apple M2 VM:
> | kernel                |   mean_rel |   std_rel |
> |:----------------------|-----------:|----------:|
> | mm-unstable           |       0.0% |      1.5% |
> | hugetlb-rmap-cleanups |      -3.3% |      1.1% |
> | fork-batching         |      -3.6% |      1.4% |
> 
> Fork, order-9, Apple M2 VM:
> | kernel                |   mean_rel |   std_rel |
> |:----------------------|-----------:|----------:|
> | mm-unstable           |       0.0% |      1.8% |
> | hugetlb-rmap-cleanups |      -5.8% |      1.3% |
> | fork-batching         |     -38.1% |      2.3% |
> 
> Fork, order-0, Ampere Altra:
> | kernel                |   mean_rel |   std_rel |
> |:----------------------|-----------:|----------:|
> | mm-unstable           |       0.0% |      1.3% |
> | hugetlb-rmap-cleanups |      -0.1% |      0.4% |
> | fork-batching         |      -0.4% |      0.5% |
> 
> Fork, order-9, Ampere Altra:
> | kernel                |   mean_rel |   std_rel |
> |:----------------------|-----------:|----------:|
> | mm-unstable           |       0.0% |      0.1% |
> | hugetlb-rmap-cleanups |      -0.1% |      0.1% |
> | fork-batching         |     -13.9% |      0.1% |
> 
> 
> So all looking good. Compiler was the issue. Sorry for the noise.

No need to be sorry, good that we figured out what's going wrong here.

Weird that the compiler makes such a difference here.

> 
> So please go ahead with you rmap v2 stuff, and I'll wait for you to post the
> fork and zap batching patches properly, then rebase my arm64 contpte stuff on
> top and remeasure everything.

Yes, will get rmap v2 out soon, then start working on fork, and then try 
tackling zap. I have some holiday coming up, so it might take some time 
-- but there is plenty of time left.

-- 
Cheers,

David / dhildenb