Received-SPF: pass (google.com: domain of linux-kernel+bounces-46445-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Message-ID: <30718fc8-15cf-41e4-922c-5cdbf00a0840@redhat.com>
Date: Wed, 31 Jan 2024 13:56:27 +0100
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v3 00/15] mm/memory: optimize fork() with PTE-mapped THP
Content-Language: en-US
To: Ryan Roberts <ryan.roberts@arm.com>, linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
 Matthew Wilcox <willy@infradead.org>, Russell King <linux@armlinux.org.uk>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 Dinh Nguyen <dinguyen@kernel.org>, Michael Ellerman <mpe@ellerman.id.au>,
 Nicholas Piggin <npiggin@gmail.com>,
 Christophe Leroy <christophe.leroy@csgroup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
 "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
 Paul Walmsley <paul.walmsley@sifive.com>, Palmer Dabbelt
 <palmer@dabbelt.com>, Albert Ou <aou@eecs.berkeley.edu>,
 Alexander Gordeev <agordeev@linux.ibm.com>,
 Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
 Heiko Carstens <hca@linux.ibm.com>, Vasily Gorbik <gor@linux.ibm.com>,
 Christian Borntraeger <borntraeger@linux.ibm.com>,
 Sven Schnelle <svens@linux.ibm.com>, "David S. Miller"
 <davem@davemloft.net>, linux-arm-kernel@lists.infradead.org,
 linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
 linux-s390@vger.kernel.org, sparclinux@vger.kernel.org
References: <20240129124649.189745-1-david@redhat.com>
 <a335a9d2-9b8f-4eb8-ba22-23a223b59b06@arm.com>
 <a1a0e9b3-dae2-418f-bd63-50e65f471728@redhat.com>
 <57eb82c7-4816-42a2-b5ab-cc221e289b21@arm.com>
 <e6eaba5b-f290-4d1f-990b-a47d89f56ee4@redhat.com>
 <714d0930-2202-48b6-9728-d248f820325e@arm.com>
 <dcaa20c4-bd1f-4f15-bb0a-3a790808937d@arm.com>
From: David Hildenbrand <david@redhat.com>
Autocrypt: addr=david@redhat.com; keydata=
 xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ
 dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL
 QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp
 XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK
 Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9
 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt
 WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc
 UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv
 jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb
 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk
 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW
 AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q
 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp
 rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf
 wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4
 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l
 pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd
 KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE
 BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs
 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF
 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9
 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz
 Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb
 T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A
 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk
 CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G
 NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75
 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx
 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS
 lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv
 AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa
 N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3
 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB
 boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq
 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f
 XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ
 a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq
 Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6
 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8
 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E
 th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr
 jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt
 WNyWQQ==
Organization: Red Hat
In-Reply-To: <dcaa20c4-bd1f-4f15-bb0a-3a790808937d@arm.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit

On 31.01.24 13:37, Ryan Roberts wrote:
> On 31/01/2024 11:49, Ryan Roberts wrote:
>> On 31/01/2024 11:28, David Hildenbrand wrote:
>>> On 31.01.24 12:16, Ryan Roberts wrote:
>>>> On 31/01/2024 11:06, David Hildenbrand wrote:
>>>>> On 31.01.24 11:43, Ryan Roberts wrote:
>>>>>> On 29/01/2024 12:46, David Hildenbrand wrote:
>>>>>>> Now that the rmap overhaul[1] is upstream that provides a clean interface
>>>>>>> for rmap batching, let's implement PTE batching during fork when processing
>>>>>>> PTE-mapped THPs.
>>>>>>>
>>>>>>> This series is partially based on Ryan's previous work[2] to implement
>>>>>>> cont-pte support on arm64, but its a complete rewrite based on [1] to
>>>>>>> optimize all architectures independent of any such PTE bits, and to
>>>>>>> use the new rmap batching functions that simplify the code and prepare
>>>>>>> for further rmap accounting changes.
>>>>>>>
>>>>>>> We collect consecutive PTEs that map consecutive pages of the same large
>>>>>>> folio, making sure that the other PTE bits are compatible, and (a) adjust
>>>>>>> the refcount only once per batch, (b) call rmap handling functions only
>>>>>>> once per batch and (c) perform batch PTE setting/updates.
>>>>>>>
>>>>>>> While this series should be beneficial for adding cont-pte support on
>>>>>>> ARM64[2], it's one of the requirements for maintaining a total mapcount[3]
>>>>>>> for large folios with minimal added overhead and further changes[4] that
>>>>>>> build up on top of the total mapcount.
>>>>>>>
>>>>>>> Independent of all that, this series results in a speedup during fork with
>>>>>>> PTE-mapped THP, which is the default with THPs that are smaller than a PMD
>>>>>>> (for example, 16KiB to 1024KiB mTHPs for anonymous memory[5]).
>>>>>>>
>>>>>>> On an Intel Xeon Silver 4210R CPU, fork'ing with 1GiB of PTE-mapped folios
>>>>>>> of the same size (stddev < 1%) results in the following runtimes
>>>>>>> for fork() (shorter is better):
>>>>>>>
>>>>>>> Folio Size | v6.8-rc1 |      New | Change
>>>>>>> ------------------------------------------
>>>>>>>          4KiB | 0.014328 | 0.014035 |   - 2%
>>>>>>>         16KiB | 0.014263 | 0.01196  |   -16%
>>>>>>>         32KiB | 0.014334 | 0.01094  |   -24%
>>>>>>>         64KiB | 0.014046 | 0.010444 |   -26%
>>>>>>>        128KiB | 0.014011 | 0.010063 |   -28%
>>>>>>>        256KiB | 0.013993 | 0.009938 |   -29%
>>>>>>>        512KiB | 0.013983 | 0.00985  |   -30%
>>>>>>>       1024KiB | 0.013986 | 0.00982  |   -30%
>>>>>>>       2048KiB | 0.014305 | 0.010076 |   -30%
>>>>>>
>>>>>> Just a heads up that I'm seeing some strange results on Apple M2. Fork for
>>>>>> order-0 is seemingly costing ~17% more. I'm using GCC 13.2 and was pretty
>>>>>> sure I
>>>>>> didn't see this problem with version 1; although that was on a different
>>>>>> baseline and I've thrown the numbers away so will rerun and try to debug this.
>>
>> Numbers for v1 of the series, both on top of 6.8-rc1 and rebased to the same
>> mm-unstable base as v3 of the series (first 2 rows are from what I just posted
>> for context):
>>
>> | kernel             |   mean_rel |   std_rel |
>> |:-------------------|-----------:|----------:|
>> | mm-unstabe (base)  |       0.0% |      1.1% |
>> | mm-unstable + v3   |      16.7% |      0.8% |
>> | mm-unstable + v1   |      -2.5% |      1.7% |
>> | v6.8-rc1 + v1      |      -6.6% |      1.1% |
>>
>> So all looks good with v1. And seems to suggest mm-unstable has regressed by ~4%
>> vs v6.8-rc1. Is this really a useful benchmark? Does the raw performance of
>> fork() syscall really matter? Evidence suggests its moving all over the place -
>> breath on the code and it changes - not a great place to be when using the test
>> for gating purposes!
>>
>> Still with the old tests - I'll move to the new ones now.
>>
>>
>>>>>>
>>>>>
>>>>> So far, on my x86 tests (Intel, AMD EPYC), I was not able to observe this.
>>>>> fork() for order-0 was consistently effectively unchanged. Do you observe that
>>>>> on other ARM systems as well?
>>>>
>>>> Nope; running the exact same kernel binary and user space on Altra, I see
>>>> sensible numbers;
>>>>
>>>> fork order-0: -1.3%
>>>> fork order-9: -7.6%
>>>> dontneed order-0: -0.5%
>>>> dontneed order-9: 0.1%
>>>> munmap order-0: 0.0%
>>>> munmap order-9: -67.9%
>>>>
>>>> So I guess some pipelining issue that causes the M2 to stall more?
>>>
>>> With one effective added folio_test_large(), it could only be a code layout
>>> problem? Or the compiler does something stupid, but you say that you run the
>>> exact same kernel binary, so that doesn't make sense.
>>
>> Yup, same binary. We know this code is very sensitive - 1 cycle makes a big
>> difference. So could easily be code layout, branch prediction, etc...
>>
>>>
>>> I'm also surprised about the dontneed vs. munmap numbers.
>>
>> You mean the ones for Altra that I posted? (I didn't post any for M2). The altra
>> numbers look ok to me; dontneed has no change, and munmap has no change for
>> order-0 and is massively improved for order-9.
>>
>>   Doesn't make any sense
>>> (again, there was this VMA merging problem but it would still allow for batching
>>> within a single VMA that spans exactly one large folio).
>>>
>>> What are you using as baseline? Really just mm-unstable vs. mm-unstable+patches?
>>
>> yes. except for "v6.8-rc1 + v1" above.
>>
>>>
>>> Let's see if the new test changes the numbers you measure.
> 
> Nope: looks the same. I've taken my test harness out of the picture and done
> everything manually from the ground up, with the old tests and the new. Headline
> is that I see similar numbers from both.

I took me a while to get really reproducible numbers on Intel. Most 
importantly:
* Set a fixed CPU frequency, disabling any boost and avoiding any
   thermal throttling.
* Pin the test to CPUs and set a nice level.

Another thing is, to avoid systems where you can have NUMA effects 
within a single socket. Otherwise, memory access latency is just random 
and depends on what the buddy enjoys giving you.

But you seem to get the same +17 even after reboots, so that indicates 
that the CPU is not happy about the code for some reason. And the weird 
thing is, that nothing significantly changed for order-0 folios between 
v1 and v3 that could explain any of this.

I'm not worried about 5% or so, nobody cares. But it would be good to 
have at least an explanation why only that system shows +17%.

> 
> Some details:
>   - I'm running for 10 seconds then averaging the output

Same here.

>   - test is bimodal; first run (of 10 seconds) after boot is a bit faster on
>     average (up to 10%) than the rest; I could guess this is due to the memory
>     being allocated more contiguously the first few times through, so struct
>     pages have better locality, but that's a guess.

I think it also has to do with the PCP lists, and the high-pcp auto 
tuning (I played with disabling that). Running on a freshly booted 
system gave me reproducible results.

But yes: I was observing something similar on AMD EPYC, where you get 
consecutive pages from the buddy, but once you allocate from the PCP it 
might no longer be consecutive.

>   - test is 5-10% slower when output is printed to terminal vs when redirected to
>     file. I've always effectively been redirecting. Not sure if this overhead
>     could start to dominate the regression and that's why you don't see it?

That's weird, because we don't print while measuring? Anyhow, 5/10% 
variance on some system is not the end of the world.

> 
> I'm inclined to run this test for the last N kernel releases and if the number
> moves around significantly, conclude that these tests don't really matter.
> Otherwise its an exercise in randomly refactoring code until it works well, but
> that's just overfitting to the compiler and hw. What do you think?

Personally, I wouldn't lose sleep if you see weird, unexplainable 
behavior on some system (not even architecture!). Trying to optimize for 
that would indeed be random refactorings.

But I would not be so fast to say that "these tests don't really matter" 
and then go wild and degrade them as much as you want. There are use 
cases that care about fork performance especially with order-0 pages -- 
such as Redis.

-- 
Cheers,

David / dhildenb