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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-39eae9640e3sm16841706f8f.12.2025.04.16.01.57.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 16 Apr 2025 01:57:07 -0700 (PDT) Date: Wed, 16 Apr 2025 09:57:06 +0100 From: David Laight To: Xavier Cc: ryan.roberts@arm.com, dev.jain@arm.com, ioworker0@gmail.com, 21cnbao@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 Subject: Re: [mm/contpte v3 1/1] mm/contpte: Optimize loop to reduce redundant operations Message-ID: <20250416095706.47a8afa2@pumpkin> In-Reply-To: <20250415082205.2249918-2-xavier_qy@163.com> References: <20250415082205.2249918-1-xavier_qy@163.com> <20250415082205.2249918-2-xavier_qy@163.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Tue, 15 Apr 2025 16:22:05 +0800 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. Benchmarks? As has been pointed out before CONT_PTES is small and IIRC dirty+young is unusual. > > Signed-off-by: Xavier > --- > arch/arm64/mm/contpte.c | 20 ++++++++++++++++++-- > 1 file changed, 18 insertions(+), 2 deletions(-) > > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c > index bcac4f55f9c1..0acfee604947 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); > > +/* Note: in order to improve efficiency, using this macro will modify the > + * passed-in parameters.*/ ... this macro modifies ... But you can make it obvious my passing by reference. The compiler will generate the same code. David > +#define CHECK_CONTPTE_FLAG(start, ptep, orig_pte, flag) \ > + for (; (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;