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 8A81429CB4E for ; Mon, 14 Apr 2025 12:11:46 +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=1744632708; cv=none; b=Rn5sI/+OG+02TS6EigBDlPUfMfZWGi4PwmtaJJRsxe2j5/JiWlcbOPotsiw/8kczz91SvXToUyTpIy4D08eUrFWz5U/WFKmDBACq2nVDgwvUrk+3UsjZmBDblYmf85crtVceriyRG+K66hnffw419cVgJLIGzQvQwDdAOAH5mV4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1744632708; c=relaxed/simple; bh=5sHvKU5p6R98G5qUQJWChqm7Hb1WtvUEerciJ7Sg8Wk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=GHQVX3YGBBDksrIynf0abC3j0MiKMvzKYFTET7EXwMjk5o0Sjivg0fQuIGg2Gopa9XpQ5b55n//tGD1nT2ky3rX+ZVerj3MI67AjOG+Rpe1I0Q8Y3xHL4Ntl2pEpB2VN+m1WqrxCRiSYv3BiMAQK/63IGGFhgH9QDWITVHNEjqs= 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 6B6091007; Mon, 14 Apr 2025 05:11:44 -0700 (PDT) Received: from [10.57.86.225] (unknown [10.57.86.225]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E8CA83F694; Mon, 14 Apr 2025 05:11:43 -0700 (PDT) Message-ID: Date: Mon, 14 Apr 2025 13:11:42 +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 v1] mm/contpte: Optimize loop to reduce redundant operations Content-Language: en-GB To: Dev Jain , 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> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 14/04/2025 09:51, Dev Jain wrote: > > > 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. Oh I see. Although you're effectively just saving a tbnz instruction, which will be in the noise. Anyway, come back with numbers and we can talk :) > >> >> ----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 >>>>> >>>> >> >