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 8A2EE25A349 for ; Mon, 14 Apr 2025 08:51:35 +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=1744620697; cv=none; b=X9L9yyYsmhZvn9j/q+flAadwRDe7mhcAHXo7qP8Tk+p0IVfwn5I+VSSiSnSadShnRI+eAZ/ry9Pv6XcQrZ5xLnMR1ewM5GkBhWrTpgq3zIoG4i1myTZLIOH0Ov4H0dZ6uGy+tR57NahHiMsbMHWCDaCMu2kMI/ffdw/sv2IM8kQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1744620697; c=relaxed/simple; bh=qTBEdEvrXe4rP0P8azJ8A59y0FcmHycSAHTGl1pLiGA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=BOBEIYX3/UPEZxHI/mv/iCeFSYihw3Vb+ULI1DqKWmjDQ4KqhWEu70HzptXHN87bznt5c0OACB76aVvB/1/jiUlmmhtqkE8mKcC5ziYvFhJc+NKZR/E/A6EDRb1V8K20wTJ/qOF9IDhvOrEpVB++X5onm/lnRdFLbUGqdYCSlvg= 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 8693A1007; Mon, 14 Apr 2025 01:51:33 -0700 (PDT) Received: from [10.163.75.110] (unknown [10.163.75.110]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D2F5E3F66E; Mon, 14 Apr 2025 01:51:30 -0700 (PDT) Message-ID: Date: Mon, 14 Apr 2025 14:21:27 +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: Ryan Roberts , Lance Yang , 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> <1613c84e-1e65-44cd-9f23-1c9870df4655@arm.com> Content-Language: en-US From: Dev Jain In-Reply-To: <1613c84e-1e65-44cd-9f23-1c9870df4655@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 14/04/25 1:36 pm, Ryan Roberts wrote: > 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? The idea of CHECK_CONTPTE_FLAG() is to check only for the other flag until it's found; the diff below will still keep checking for both until both are found. > > ----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 >>>> >>> >