From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id DB69D1C27 for ; Thu, 5 Jun 2025 07:16:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749107763; cv=none; b=RMr9SSFDfD1BvOfFjeyXYk71s4rDTvDueL6NvdNOOo5Ro6KqG+d+XjR13c+KAPLXklh0iV0xpdxpmUsRAGXaWPzYQT+vDVba1n8su3MuF4fvoFqBI+6ySW8I20z/LUTjV9e3kkWzkZloUSnifNoXJzKkTX8CRpL2GoPDR9+71vc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749107763; c=relaxed/simple; bh=RluTDPZZAAHd0mhkD4PNFzc1ZKqmL9z2OLi9qJgQQok=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=RSMonbjlteu9ss9NvkcMjXC36wD9NGWLkuougzAk8vGBKILiJwBZcSx6Sxnttbs2AB+oDpU1CQKCNi4NLDuAz2uQA4G+dwpzPoYyZn1CnvAN6VoiBYcjVqXQfIVdi9C+dwXzbxR5k26LJ9M5eFk89LUk/pDgSXcHxeCB8xJFNzk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 7E8B61692; Thu, 5 Jun 2025 00:15:42 -0700 (PDT) Received: from [10.57.94.236] (unknown [10.57.94.236]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D93C33F59E; Thu, 5 Jun 2025 00:15:57 -0700 (PDT) Message-ID: <225fd9dd-2b97-4ec6-a9a6-fe148c4b901e@arm.com> Date: Thu, 5 Jun 2025 08:15:56 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6] arm64/mm: Optimize loop to reduce redundant operations of contpte_ptep_get Content-Language: en-GB To: Xavier Xia Cc: Xavier Xia , 21cnbao@gmail.com, dev.jain@arm.com, ioworker0@gmail.com, akpm@linux-foundation.org, catalin.marinas@arm.com, david@redhat.com, gshan@redhat.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, will@kernel.org, willy@infradead.org, ziy@nvidia.com, Barry Song , linux-mm@kvack.org References: <20250510125948.2383778-1-xavier_qy@163.com> <99a0a2c8-d98e-4c81-9207-c55c72c00872@arm.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 05/06/2025 06:54, Xavier Xia wrote: > Hi Ryan, > > Thank you for your review, and for reproducing and verifying the test cases. > I am using a Gmail email to reply to your message, hoping you can receive it. > Please check the details below. Ahh yes, this arrived in my inbox without issue! Thanks, Ryan > > > > On Thu, Jun 5, 2025 at 11:20 AM Ryan Roberts wrote: >> >> On 10/05/2025 13:59, Xavier Xia wrote: >>> This commit optimizes the contpte_ptep_get and contpte_ptep_get_lockless >>> function by adding early termination logic. It checks if the dirty and >>> young bits of orig_pte are already set and skips redundant bit-setting >>> operations during the loop. This reduces unnecessary iterations and >>> improves performance. >>> >>> In order to verify the optimization performance, a test function has been >>> designed. The function's execution time and instruction statistics have >>> been traced using perf, and the following are the operation results on a >>> certain Qualcomm mobile phone chip: >>> >>> Test Code: >> >> nit: It would have been good to include the source for the whole program, >> including #includes and the main() function to make it quicker for others to get >> up and running. > > OK, I will pay attention to it in the future. This test case is quite > simple, so I didn't add it. > >> >>> >>> #define PAGE_SIZE 4096 >>> #define CONT_PTES 16 >>> #define TEST_SIZE (4096* CONT_PTES * PAGE_SIZE) >>> #define YOUNG_BIT 8 >>> void rwdata(char *buf) >>> { >>> for (size_t i = 0; i < TEST_SIZE; i += PAGE_SIZE) { >>> buf[i] = 'a'; >>> volatile char c = buf[i]; >>> } >>> } >>> void clear_young_dirty(char *buf) >>> { >>> if (madvise(buf, TEST_SIZE, MADV_FREE) == -1) { >>> perror("madvise free failed"); >>> free(buf); >>> exit(EXIT_FAILURE); >>> } >>> if (madvise(buf, TEST_SIZE, MADV_COLD) == -1) { >>> perror("madvise free failed"); >>> free(buf); >>> exit(EXIT_FAILURE); >>> } >> >> nit: MADV_FREE clears both young and dirty so I don't think MADV_COLD is >> required? (MADV_COLD only clears young I think?) > > You're right, MADV_COLD here can probably be removed. > >> >>> } >>> void set_one_young(char *buf) >>> { >>> for (size_t i = 0; i < TEST_SIZE; i += CONT_PTES * PAGE_SIZE) { >>> volatile char c = buf[i + YOUNG_BIT * PAGE_SIZE]; >>> } >>> } >>> >>> void test_contpte_perf() { >>> char *buf; >>> int ret = posix_memalign((void **)&buf, CONT_PTES * PAGE_SIZE, >>> TEST_SIZE); >>> if ((ret != 0) || ((unsigned long)buf % CONT_PTES * PAGE_SIZE)) { >>> perror("posix_memalign failed"); >>> exit(EXIT_FAILURE); >>> } >>> >>> rwdata(buf); >>> #if TEST_CASE2 || TEST_CASE3 >>> clear_young_dirty(buf); >>> #endif >>> #if TEST_CASE2 >>> set_one_young(buf); >>> #endif >>> >>> for (int j = 0; j < 500; j++) { >>> mlock(buf, TEST_SIZE); >>> >>> munlock(buf, TEST_SIZE); >>> } >>> free(buf); >>> } >>> >>> Descriptions of three test scenarios >>> >>> Scenario 1 >>> The data of all 16 PTEs are both dirty and young. >>> #define TEST_CASE2 0 >>> #define TEST_CASE3 0 >>> >>> Scenario 2 >>> Among the 16 PTEs, only the 8th one is young, and there are no dirty ones. >>> #define TEST_CASE2 1 >>> #define TEST_CASE3 0 >>> >>> Scenario 3 >>> Among the 16 PTEs, there are neither young nor dirty ones. >>> #define TEST_CASE2 0 >>> #define TEST_CASE3 1 >>> >>> Test results >>> >>> |Scenario 1 | Original| Optimized| >>> |-------------------|---------------|----------------| >>> |instructions | 37912436160| 18731580031| >>> |test time | 4.2797| 2.2949| >>> |overhead of | | | >>> |contpte_ptep_get() | 21.31%| 4.80%| >>> >>> |Scenario 2 | Original| Optimized| >>> |-------------------|---------------|----------------| >>> |instructions | 36701270862| 36115790086| >>> |test time | 3.2335| 3.0874| >>> |Overhead of | | | >>> |contpte_ptep_get() | 32.26%| 33.57%| >>> >>> |Scenario 3 | Original| Optimized| >>> |-------------------|---------------|----------------| >>> |instructions | 36706279735| 36750881878| >>> |test time | 3.2008| 3.1249| >>> |Overhead of | | | >>> |contpte_ptep_get() | 31.94%| 34.59%| >>> >>> For Scenario 1, optimized code can achieve an instruction benefit of 50.59% >>> and a time benefit of 46.38%. >>> For Scenario 2, optimized code can achieve an instruction count benefit of >>> 1.6% and a time benefit of 4.5%. >>> For Scenario 3, since all the PTEs have neither the young nor the dirty >>> flag, the branches taken by optimized code should be the same as those of >>> the original code. In fact, the test results of optimized code seem to be >>> closer to those of the original code. >> >> I re-ran these tests on Apple M2 with 4K base pages + 64K mTHP. >> >> Scenario 1: reduced to 56% of baseline execution time >> Scenario 2: reduced to 89% of baseline execution time >> Scenario 3: reduced to 91% of baseline execution time >> >> I'm pretty amazed that scenario 3 got faster given it is doing the same number >> of loops. > > It seems that the data you obtained is similar to my test data. For > scenario 3, it's > faster even when running the same code, which I can't quite figure out either. > >>> >>> It can be proven through test function that the optimization for >>> contpte_ptep_get is effective. Since the logic of contpte_ptep_get_lockless >>> is similar to that of contpte_ptep_get, the same optimization scheme is >>> also adopted for it. >>> >>> Reviewed-by: Barry Song >>> Signed-off-by: Xavier Xia >> >> I don't love the extra complexity, but this version is much tidier. While the >> micro-benchmark is clearly contrived, it shows that there will be cases where it >> will be faster and there are no cases where it is slower. This will probably be >> more valuable for 16K kernels because the number of PTEs in a contpte block is >> 128 there: > > Okay, this version has been revised multiple times based on your > previous feedback > and Barry's comments, and it seems much less complicated to understand now. :) > >> >> Reviewed-by: Ryan Roberts >> Tested-by: Ryan Roberts >> >>> --- >>> Changes in v6: >>> - Move prot = pte_pgprot(pte_mkold(pte_mkclean(pte))) into the contpte_is_consistent(), >>> as suggested by Barry. >>> - Link to v5: https://lore.kernel.org/all/20250509122728.2379466-1-xavier_qy@163.com/ >>> >>> Changes in v5: >>> - Replace macro CHECK_CONTPTE_CONSISTENCY with inline function contpte_is_consistent >>> for improved readability and clarity, as suggested by Barry. >>> - Link to v4: https://lore.kernel.org/all/20250508070353.2370826-1-xavier_qy@163.com/ >>> >>> Changes in v4: >>> - Convert macro CHECK_CONTPTE_FLAG to an internal loop for better readability. >>> - Refactor contpte_ptep_get_lockless using the same optimization logic, as suggested by Ryan. >>> - Link to v3: https://lore.kernel.org/all/3d338f91.8c71.1965cd8b1b8.Coremail.xavier_qy@163.com/ >>> --- >>> arch/arm64/mm/contpte.c | 74 +++++++++++++++++++++++++++++++++++------ >>> 1 file changed, 64 insertions(+), 10 deletions(-) >>> >>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c >>> index bcac4f55f9c1..71efe7dff0ad 100644 >>> --- a/arch/arm64/mm/contpte.c >>> +++ b/arch/arm64/mm/contpte.c >>> @@ -169,17 +169,46 @@ pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte) >>> for (i = 0; i < CONT_PTES; i++, ptep++) { >>> pte = __ptep_get(ptep); >>> >>> - if (pte_dirty(pte)) >>> + if (pte_dirty(pte)) { >>> orig_pte = pte_mkdirty(orig_pte); >>> - >>> - if (pte_young(pte)) >>> + for (; i < CONT_PTES; i++, ptep++) { >>> + pte = __ptep_get(ptep); >>> + if (pte_young(pte)) { >>> + orig_pte = pte_mkyoung(orig_pte); >>> + break; >>> + } >>> + } >>> + break; >>> + } >>> + >>> + if (pte_young(pte)) { >>> orig_pte = pte_mkyoung(orig_pte); >>> + i++; >>> + ptep++; >>> + for (; i < CONT_PTES; i++, ptep++) { >>> + pte = __ptep_get(ptep); >>> + if (pte_dirty(pte)) { >>> + orig_pte = pte_mkdirty(orig_pte); >>> + break; >>> + } >>> + } >>> + break; >>> + } >>> } >>> >>> return orig_pte; >>> } >>> EXPORT_SYMBOL_GPL(contpte_ptep_get); >>> >>> +static inline bool contpte_is_consistent(pte_t pte, unsigned long pfn, >>> + pgprot_t orig_prot) >>> +{ >>> + pgprot_t prot = pte_pgprot(pte_mkold(pte_mkclean(pte))); >>> + >>> + return pte_valid_cont(pte) && pte_pfn(pte) == pfn && >>> + pgprot_val(prot) == pgprot_val(orig_prot); >>> +} >>> + >>> pte_t contpte_ptep_get_lockless(pte_t *orig_ptep) >>> { >>> /* >>> @@ -202,7 +231,6 @@ pte_t contpte_ptep_get_lockless(pte_t *orig_ptep) >>> pgprot_t orig_prot; >>> unsigned long pfn; >>> pte_t orig_pte; >>> - pgprot_t prot; >>> pte_t *ptep; >>> pte_t pte; >>> int i; >>> @@ -219,18 +247,44 @@ pte_t contpte_ptep_get_lockless(pte_t *orig_ptep) >>> >>> for (i = 0; i < CONT_PTES; i++, ptep++, pfn++) { >>> pte = __ptep_get(ptep); >>> - prot = pte_pgprot(pte_mkold(pte_mkclean(pte))); >>> >>> - if (!pte_valid_cont(pte) || >>> - pte_pfn(pte) != pfn || >>> - pgprot_val(prot) != pgprot_val(orig_prot)) >>> + if (!contpte_is_consistent(pte, pfn, orig_prot)) >>> goto retry; >>> >>> - if (pte_dirty(pte)) >>> + if (pte_dirty(pte)) { >>> orig_pte = pte_mkdirty(orig_pte); >>> + for (; i < CONT_PTES; i++, ptep++, pfn++) { >>> + pte = __ptep_get(ptep); >>> + >>> + if (!contpte_is_consistent(pte, pfn, orig_prot)) >>> + goto retry; >>> + >>> + if (pte_young(pte)) { >>> + orig_pte = pte_mkyoung(orig_pte); >>> + break; >>> + } >>> + } >>> + break; >> >> I considered for a while whether it is safe for contpte_ptep_get_lockless() to >> exit early having not seen every PTE in the contpte block and confirmed that >> they are all consistent. I eventually concluded that it is, as long as all the >> PTEs that it does check are consistent I believe this is fine. > > So, it looks like my changes here will be okay. > >> >>> + } >>> >>> - if (pte_young(pte)) >>> + if (pte_young(pte)) { >>> orig_pte = pte_mkyoung(orig_pte); >>> + i++; >>> + ptep++; >>> + pfn++; >>> + for (; i < CONT_PTES; i++, ptep++, pfn++) { >>> + pte = __ptep_get(ptep); >>> + >>> + if (!contpte_is_consistent(pte, pfn, orig_prot)) >>> + goto retry; >>> + >>> + if (pte_dirty(pte)) { >>> + orig_pte = pte_mkdirty(orig_pte); >>> + break; >>> + } >>> + } >>> + break; >>> + } >>> } >>> >>> return orig_pte; >>