From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id E09CBC369B2 for ; Mon, 14 Apr 2025 08:08:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=luQB3rGRXIt6X3UGaDK70S81omAKnYZDiL+Tx8r9EZA=; b=QvrWRNwYwS9BOECRoOCTopJ+Yq hog6HGpPMqxjY9PCYzE6QglD+bwyi/5q6CgJ113SZJZtTk2me7CbYOtkDqNanHutv8wUUDIjMBUKE 7F81PZKWeKhbbp/a9jc+9FofVEUf4c+qSdfK/x+3AYuzyhnDzRXN7/cILhgMxI0Thu+6nYxZfvMFC yJKANz3YjKy6qTUfRMR2NjoMj2nTb0EBhXc+QpCmt+hyQlslkRuTmKrXs75C/z1Vt7S7GBiUgGvDb fTT9COck/lr3DlQptFRVxxSJhUXrvsnW4ZsxbesTSGbHJNzbWwpBF3loGDSVDXwbABB4m6mMNlnyZ ESI6JU/Q==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1u4ErJ-000000011t6-1Ird; Mon, 14 Apr 2025 08:08:01 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1u4EpR-000000011dr-0tq0 for linux-arm-kernel@lists.infradead.org; Mon, 14 Apr 2025 08:06:06 +0000 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 C21FE1007; Mon, 14 Apr 2025 01:06:02 -0700 (PDT) Received: from [10.57.88.7] (unknown [10.57.88.7]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 21D1E3F694; Mon, 14 Apr 2025 01:06:01 -0700 (PDT) Message-ID: <1613c84e-1e65-44cd-9f23-1c9870df4655@arm.com> Date: Mon, 14 Apr 2025 09:06:00 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1] mm/contpte: Optimize loop to reduce redundant operations Content-Language: en-GB To: Lance Yang , Dev Jain , Xavier Cc: Barry Song <21cnbao@gmail.com>, catalin.marinas@arm.com, will@kernel.org, akpm@linux-foundation.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, David Hildenbrand , Matthew Wilcox , Zi Yan References: <20250407092243.2207837-1-xavier_qy@163.com> <7dccb3a2-f5e2-4f9e-8f5c-465a1d3ffdb6@arm.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250414_010605_359160_8796E243 X-CRM114-Status: GOOD ( 22.12 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 12/04/2025 06:05, Lance Yang wrote: > On Sat, Apr 12, 2025 at 1:30 AM Dev Jain wrote: >> >> +others >> >> On 11/04/25 2:55 am, Barry Song wrote: >>> On Mon, Apr 7, 2025 at 9:23 PM Xavier wrote: >>>> >>>> This commit optimizes the contpte_ptep_get 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. >>>> >>>> Signed-off-by: Xavier >>>> --- >>>> arch/arm64/mm/contpte.c | 13 +++++++++++-- >>>> 1 file changed, 11 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c >>>> index bcac4f55f9c1..ca15d8f52d14 100644 >>>> --- a/arch/arm64/mm/contpte.c >>>> +++ b/arch/arm64/mm/contpte.c >>>> @@ -163,17 +163,26 @@ pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte) >>>> >>>> pte_t pte; >>>> int i; >>>> + bool dirty = false; >>>> + bool young = false; >>>> >>>> ptep = contpte_align_down(ptep); >>>> >>>> for (i = 0; i < CONT_PTES; i++, ptep++) { >>>> pte = __ptep_get(ptep); >>>> >>>> - if (pte_dirty(pte)) >>>> + if (!dirty && pte_dirty(pte)) { >>>> + dirty = true; >>>> orig_pte = pte_mkdirty(orig_pte); >>>> + } >>>> >>>> - if (pte_young(pte)) >>>> + if (!young && pte_young(pte)) { >>>> + young = true; >>>> orig_pte = pte_mkyoung(orig_pte); >>>> + } >>>> + >>>> + if (dirty && young) >>>> + break; >>> >>> This kind of optimization is always tricky. Dev previously tried a similar >>> approach to reduce the loop count, but it ended up causing performance >>> degradation: >>> https://lore.kernel.org/linux-mm/20240913091902.1160520-1-dev.jain@arm.com/ >>> >>> So we may need actual data to validate this idea. >> >> The original v2 patch does not work, I changed it to the following: >> >> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c >> index bcac4f55f9c1..db0ad38601db 100644 >> --- a/arch/arm64/mm/contpte.c >> +++ b/arch/arm64/mm/contpte.c >> @@ -152,6 +152,16 @@ void __contpte_try_unfold(struct mm_struct *mm, >> unsigned long addr, >> } >> EXPORT_SYMBOL_GPL(__contpte_try_unfold); >> >> +#define CHECK_CONTPTE_FLAG(start, ptep, orig_pte, flag) \ >> + int _start; \ >> + pte_t *_ptep = ptep; \ >> + for (_start = start; _start < CONT_PTES; _start++, ptep++) { \ >> + if (pte_##flag(__ptep_get(_ptep))) { \ >> + orig_pte = pte_mk##flag(orig_pte); \ >> + break; \ >> + } \ >> + } >> + >> pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte) >> { >> /* >> @@ -169,11 +179,17 @@ 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); >> + CHECK_CONTPTE_FLAG(i, ptep, orig_pte, young); >> + break; >> + } >> >> - if (pte_young(pte)) >> + if (pte_young(pte)) { >> orig_pte = pte_mkyoung(orig_pte); >> + CHECK_CONTPTE_FLAG(i, ptep, orig_pte, dirty); >> + break; >> + } >> } >> >> return orig_pte; >> >> Some rudimentary testing with micromm reveals that this may be >> *slightly* faster. I cannot say for sure yet. > > Yep, this change works as expected, IIUC. > > However, I'm still wondering if the added complexity is worth it for > such a slight/negligible performance gain. That said, if we have > solid numbers/data to back it up, all doubts would disappear ;) I agree with Barry; we need clear performance improvement numbers to consider this type of optimization. I doubt there will be measurable improvement for 4K and 64K base pages (because the the number of PTEs in a contpte block are 16 and 32 respectively). But 16K base pages may benefit given there are 128 PTEs in a contpte block for that case. Also FWIW, I'm struggling to understand CHECK_CONTPTE_FLAG(); Perhaps something like this would suffice? ----8<---- diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c index 55107d27d3f8..7787b116b339 100644 --- a/arch/arm64/mm/contpte.c +++ b/arch/arm64/mm/contpte.c @@ -169,11 +169,17 @@ 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(orig_pte)) + break; + } - if (pte_young(pte)) + if (pte_young(pte)) { orig_pte = pte_mkyoung(orig_pte); + if (pte_dirty(orig_pte)) + break; + } } return orig_pte; ----8<---- Thanks, Ryan > > Thanks, > Lance > >> >>> >>>> } >>>> >>>> return orig_pte; >>>> -- >>>> 2.34.1 >>>> >>> >>> Thanks >>> Barry >>> >>