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[2003:cb:c73b:eb00:8e25:6953:927:1802]) by smtp.gmail.com with ESMTPSA id a15-20020a5d508f000000b0033330846e76sm13054134wrt.86.2023.12.20.01.54.57 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 20 Dec 2023 01:54:58 -0800 (PST) Message-ID: <28968568-f920-47ac-b6fd-87528ffd8f77@redhat.com> Date: Wed, 20 Dec 2023 10:54:56 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 02/16] mm: Batch-copy PTE ranges during fork() To: Ryan Roberts , Catalin Marinas , Will Deacon , Ard Biesheuvel , Marc Zyngier , Oliver Upton , James Morse , Suzuki K Poulose , Zenghui Yu , Andrey Ryabinin , Alexander Potapenko , Andrey Konovalov , Dmitry Vyukov , Vincenzo Frascino , Andrew Morton , Anshuman Khandual , Matthew Wilcox , Yu Zhao , Mark Rutland , Kefeng Wang , John Hubbard , Zi Yan , Barry Song <21cnbao@gmail.com>, Alistair Popple , Yang Shi Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20231218105100.172635-1-ryan.roberts@arm.com> <20231218105100.172635-3-ryan.roberts@arm.com> <0bef5423-6eea-446b-8854-980e9c23a948@redhat.com> <7c0236ad-01f3-437f-8b04-125d69e90dc0@redhat.com> <9a58b1a2-2c13-4fa0-8ffa-2b3d9655f1b6@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: 3gtcmj4i1y7imnnwkedn8myxyxp3m977 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 9EDFF180029 X-HE-Tag: 1703066102-183721 X-HE-Meta: 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 7IRuizn1 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 20.12.23 10:51, Ryan Roberts wrote: > On 20/12/2023 09:17, David Hildenbrand wrote: >> On 19.12.23 18:42, Ryan Roberts wrote: >>> On 19/12/2023 17:22, David Hildenbrand wrote: >>>> On 19.12.23 09:30, Ryan Roberts wrote: >>>>> On 18/12/2023 17:47, David Hildenbrand wrote: >>>>>> On 18.12.23 11:50, Ryan Roberts wrote: >>>>>>> Convert copy_pte_range() to copy a batch of ptes in one go. A given >>>>>>> batch is determined by the architecture with the new helper, >>>>>>> pte_batch_remaining(), and maps a physically contiguous block of memory, >>>>>>> all belonging to the same folio. A pte batch is then write-protected in >>>>>>> one go in the parent using the new helper, ptep_set_wrprotects() and is >>>>>>> set in one go in the child using the new helper, set_ptes_full(). >>>>>>> >>>>>>> The primary motivation for this change is to reduce the number of tlb >>>>>>> maintenance operations that the arm64 backend has to perform during >>>>>>> fork, as it is about to add transparent support for the "contiguous bit" >>>>>>> in its ptes. By write-protecting the parent using the new >>>>>>> ptep_set_wrprotects() (note the 's' at the end) function, the backend >>>>>>> can avoid having to unfold contig ranges of PTEs, which is expensive, >>>>>>> when all ptes in the range are being write-protected. Similarly, by >>>>>>> using set_ptes_full() rather than set_pte_at() to set up ptes in the >>>>>>> child, the backend does not need to fold a contiguous range once they >>>>>>> are all populated - they can be initially populated as a contiguous >>>>>>> range in the first place. >>>>>>> >>>>>>> This code is very performance sensitive, and a significant amount of >>>>>>> effort has been put into not regressing performance for the order-0 >>>>>>> folio case. By default, pte_batch_remaining() is compile constant 1, >>>>>>> which enables the compiler to simplify the extra loops that are added >>>>>>> for batching and produce code that is equivalent (and equally >>>>>>> performant) as the previous implementation. >>>>>>> >>>>>>> This change addresses the core-mm refactoring only and a separate change >>>>>>> will implement pte_batch_remaining(), ptep_set_wrprotects() and >>>>>>> set_ptes_full() in the arm64 backend to realize the performance >>>>>>> improvement as part of the work to enable contpte mappings. >>>>>>> >>>>>>> To ensure the arm64 is performant once implemented, this change is very >>>>>>> careful to only call ptep_get() once per pte batch. >>>>>>> >>>>>>> The following microbenchmark results demonstate that there is no >>>>>>> significant performance change after this patch. Fork is called in a >>>>>>> tight loop in a process with 1G of populated memory and the time for the >>>>>>> function to execute is measured. 100 iterations per run, 8 runs >>>>>>> performed on both Apple M2 (VM) and Ampere Altra (bare metal). Tests >>>>>>> performed for case where 1G memory is comprised of order-0 folios and >>>>>>> case where comprised of pte-mapped order-9 folios. Negative is faster, >>>>>>> positive is slower, compared to baseline upon which the series is based: >>>>>>> >>>>>>> | Apple M2 VM   | order-0 (pte-map) | order-9 (pte-map) | >>>>>>> | fork          |-------------------|-------------------| >>>>>>> | microbench    |    mean |   stdev |    mean |   stdev | >>>>>>> |---------------|---------|---------|---------|---------| >>>>>>> | baseline      |    0.0% |    1.1% |    0.0% |    1.2% | >>>>>>> | after-change  |   -1.0% |    2.0% |   -0.1% |    1.1% | >>>>>>> >>>>>>> | Ampere Altra  | order-0 (pte-map) | order-9 (pte-map) | >>>>>>> | fork          |-------------------|-------------------| >>>>>>> | microbench    |    mean |   stdev |    mean |   stdev | >>>>>>> |---------------|---------|---------|---------|---------| >>>>>>> | baseline      |    0.0% |    1.0% |    0.0% |    0.1% | >>>>>>> | after-change  |   -0.1% |    1.2% |   -0.1% |    0.1% | >>>>>>> >>>>>>> Tested-by: John Hubbard >>>>>>> Reviewed-by: Alistair Popple >>>>>>> Signed-off-by: Ryan Roberts >>>>>>> --- >>>>>>>     include/linux/pgtable.h | 80 +++++++++++++++++++++++++++++++++++ >>>>>>>     mm/memory.c             | 92 ++++++++++++++++++++++++++--------------- >>>>>>>     2 files changed, 139 insertions(+), 33 deletions(-) >>>>>>> >>>>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h >>>>>>> index af7639c3b0a3..db93fb81465a 100644 >>>>>>> --- a/include/linux/pgtable.h >>>>>>> +++ b/include/linux/pgtable.h >>>>>>> @@ -205,6 +205,27 @@ static inline int pmd_young(pmd_t pmd) >>>>>>>     #define arch_flush_lazy_mmu_mode()    do {} while (0) >>>>>>>     #endif >>>>>>>     +#ifndef pte_batch_remaining >>>>>>> +/** >>>>>>> + * pte_batch_remaining - Number of pages from addr to next batch boundary. >>>>>>> + * @pte: Page table entry for the first page. >>>>>>> + * @addr: Address of the first page. >>>>>>> + * @end: Batch ceiling (e.g. end of vma). >>>>>>> + * >>>>>>> + * Some architectures (arm64) can efficiently modify a contiguous batch of >>>>>>> ptes. >>>>>>> + * In such cases, this function returns the remaining number of pages to >>>>>>> the end >>>>>>> + * of the current batch, as defined by addr. This can be useful when >>>>>>> iterating >>>>>>> + * over ptes. >>>>>>> + * >>>>>>> + * May be overridden by the architecture, else batch size is always 1. >>>>>>> + */ >>>>>>> +static inline unsigned int pte_batch_remaining(pte_t pte, unsigned long >>>>>>> addr, >>>>>>> +                        unsigned long end) >>>>>>> +{ >>>>>>> +    return 1; >>>>>>> +} >>>>>>> +#endif >>>>>> >>>>>> It's a shame we now lose the optimization for all other archtiectures. >>>>>> >>>>>> Was there no way to have some basic batching mechanism that doesn't require >>>>>> arch >>>>>> specifics? >>>>> >>>>> I tried a bunch of things but ultimately the way I've done it was the only way >>>>> to reduce the order-0 fork regression to 0. >>>>> >>>>> My original v3 posting was costing 5% extra and even my first attempt at an >>>>> arch-specific version that didn't resolve to a compile-time constant 1 still >>>>> cost an extra 3%. >>>>> >>>>> >>>>>> >>>>>> I'd have thought that something very basic would have worked like: >>>>>> >>>>>> * Check if PTE is the same when setting the PFN to 0. >>>>>> * Check that PFN is consecutive >>>>>> * Check that all PFNs belong to the same folio >>>>> >>>>> I haven't tried this exact approach, but I'd be surprised if I can get the >>>>> regression under 4% with this. Further along the series I spent a lot of time >>>>> having to fiddle with the arm64 implementation; every conditional and every >>>>> memory read (even when in cache) was a problem. There is just so little in the >>>>> inner loop that every instruction matters. (At least on Ampere Altra and Apple >>>>> M2). >>>>> >>>>> Of course if you're willing to pay that 4-5% for order-0 then the benefit to >>>>> order-9 is around 10% in my measurements. Personally though, I'd prefer to play >>>>> safe and ensure the common order-0 case doesn't regress, as you previously >>>>> suggested. >>>>> >>>> >>>> I just hacked something up, on top of my beloved rmap cleanup/batching series. I >>>> implemented very generic and simple batching for large folios (all PTE bits >>>> except the PFN have to match). >>>> >>>> Some very quick testing (don't trust each last % ) on Intel(R) Xeon(R) Silver >>>> 4210R CPU. >>>> >>>> order-0: 0.014210 -> 0.013969 >>>> >>>> -> Around 1.7 % faster >>>> >>>> order-9: 0.014373 -> 0.009149 >>>> >>>> -> Around 36.3 % faster >>> >>> Well I guess that shows me :) >>> >>> I'll do a review and run the tests on my HW to see if it concurs. >> >> >> I pushed a simple compile fixup (we need pte_next_pfn()). > > I've just been trying to compile and noticed this. Will take a look at your update. > > But upon review, I've noticed the part that I think makes this difficult for > arm64 with the contpte optimization; You are calling ptep_get() for every pte in > the batch. While this is functionally correct, once arm64 has the contpte > changes, its ptep_get() has to read every pte in the contpte block in order to > gather the access and dirty bits. So if your batching function ends up wealking > a 16 entry contpte block, that will cause 16 x 16 reads, which kills > performance. That's why I added the arch-specific pte_batch_remaining() > function; this allows the core-mm to skip to the end of the contpte block and > avoid ptep_get() for the 15 tail ptes. So we end up with 16 READ_ONCE()s instead > of 256. > > I considered making a ptep_get_noyoungdirty() variant, which would avoid the bit > gathering. But we have a similar problem in zap_pte_range() and that function > needs the dirty bit to update the folio. So it doesn't work there. (see patch 3 > in my series). > > I guess you are going to say that we should combine both approaches, so that > your batching loop can skip forward an arch-provided number of ptes? That would > certainly work, but feels like an orthogonal change to what I'm trying to > achieve :). Anyway, I'll spend some time playing with it today. You can overwrite the function or add special-casing internally, yes. Right now, your patch is called "mm: Batch-copy PTE ranges during fork()" and it doesn't do any of that besides preparing for some arm64 work. -- Cheers, David / dhildenb