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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-39eae97b249sm1834556f8f.58.2025.04.11.05.03.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 11 Apr 2025 05:03:15 -0700 (PDT) Date: Fri, 11 Apr 2025 13:03:09 +0100 From: David Laight To: Barry Song <21cnbao@gmail.com> Cc: Xavier , 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 Subject: Re: [PATCH v1] mm/contpte: Optimize loop to reduce redundant operations Message-ID: <20250411130309.7894f204@pumpkin> In-Reply-To: References: <20250407092243.2207837-1-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=UTF-8 Content-Transfer-Encoding: quoted-printable On Fri, 11 Apr 2025 09:25:39 +1200 Barry Song <21cnbao@gmail.com> wrote: > On Mon, Apr 7, 2025 at 9:23=E2=80=AFPM 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_pt= e) > > > > pte_t pte; > > int i; > > + bool dirty =3D false; > > + bool young =3D false; > > > > ptep =3D contpte_align_down(ptep); > > > > for (i =3D 0; i < CONT_PTES; i++, ptep++) { > > pte =3D __ptep_get(ptep); > > > > - if (pte_dirty(pte)) > > + if (!dirty && pte_dirty(pte)) { > > + dirty =3D true; > > orig_pte =3D pte_mkdirty(orig_pte); > > + } > > > > - if (pte_young(pte)) > > + if (!young && pte_young(pte)) { > > + young =3D true; > > orig_pte =3D pte_mkyoung(orig_pte); > > + } > > + > > + if (dirty && young) > > + break; =20 >=20 > 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.co= m/ >=20 > So we may need actual data to validate this idea. You might win with 3 loops. The first looks for both 'dirty' and 'young'. If it finds only one it jumps to a different loop that continues the search but only looks for the other flag. David >=20 > > } > > > > return orig_pte; > > -- > > 2.34.1 > > =20 >=20 > Thanks > Barry >=20