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 54766230D0D for ; Fri, 11 Apr 2025 17:30:51 +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=1744392654; cv=none; b=GF4gDNyA3VxThMrN3UW+wgr4QhNPpZFbokIdioaJzE6vhY2fGs49CRQS1VFGdLM33v7mIqYBNc7+zB2n4R7aJVE89THGLqcKN9rgPcx4YtSRwBZmarQa32bAY+51Dfw9qWHbo3fD+TveBg3FiK8BVQ4P8QxPds3OLwSq5pE9Q98= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1744392654; c=relaxed/simple; bh=gXCUSf3XDzPifGKQZ5fcXPrPfIH4zdU2bln8aCuqOvA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=qSUYxmabsDG25v00Mku7cWJ0WMOK+GH4SbRyo6uA/X43l74IJNCYoK5RgZnAeWXFdLt5uzEmHROK2CW1+m+fa2GQvc11cj7ciFLhgZdtWKmR4/r7eF35mIaGY7e7xyNx8qBq2Cq1F1f+wPglHQG2BGtQkV8kgU79lLqS8xXvjC0= 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 1D8BC106F; Fri, 11 Apr 2025 10:30:50 -0700 (PDT) Received: from [10.163.75.48] (unknown [10.163.75.48]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 91C733F59E; Fri, 11 Apr 2025 10:30:45 -0700 (PDT) Message-ID: <7dccb3a2-f5e2-4f9e-8f5c-465a1d3ffdb6@arm.com> Date: Fri, 11 Apr 2025 23:00:41 +0530 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 v1] mm/contpte: Optimize loop to reduce redundant operations To: Barry Song <21cnbao@gmail.com>, Xavier Cc: catalin.marinas@arm.com, will@kernel.org, akpm@linux-foundation.org, ryan.roberts@arm.com, ioworker0@gmail.com, 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> Content-Language: en-US From: Dev Jain In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit +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. > >> } >> >> return orig_pte; >> -- >> 2.34.1 >> > > Thanks > Barry >