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Tue, 13 Aug 2024 19:01:37 +0000 (UTC) Message-ID: <183ee6fa-1d42-4a01-8446-4f20942680d2@redhat.com> Date: Tue, 13 Aug 2024 15:01:36 -0400 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC] Avoid memory barrier in read_seqcount() through load acquire To: cl@gentwo.org, Catalin Marinas , Will Deacon , Peter Zijlstra , Ingo Molnar , Boqun Feng Cc: Linus Torvalds , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org References: <20240813-seq_optimize-v1-1-84d57182e6a7@gentwo.org> Content-Language: en-US From: Waiman Long In-Reply-To: <20240813-seq_optimize-v1-1-84d57182e6a7@gentwo.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 X-Rspam-User: X-Rspamd-Queue-Id: 287714002B X-Rspamd-Server: rspam01 X-Stat-Signature: ww55qnw5q9dofb7xxxp7ti8kc4igtzfd X-HE-Tag: 1723575705-826166 X-HE-Meta: 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 siKt+T13 B5r/Y4KHSBCj+SFvFJ+53f9dUD2Jj/FNTUg7WFFjTgy7bkh//k9wjKVwy1c0ULLc8EfZO6D3ELXQppVDCOFeIpFaBmU6xJqiJy5ROnPiiSNcV/PZndDSWzZcfbtWjqAS8jjLXxljiQ98ZfJb5j9zwIVYisrftxGYyWSCacL4pqxAKgmiEFze3NW7ZLN/bmufxlooJPFhDO+aMwBo3pLeuIlv+fxIi8p5cCs7ZiEoD/bGNz7jarrtQVj5dnZwYByTPCzWgQ1SwkLptYgYiwfZY2wdEp0On3vsuckoXVVHWskE2JEfXqPvbPqG3dm2g//2ZDL6ngIbAyaqqXtGDTlPhFNB7JgtBbL6qs6fsuSYMybeFkXn202Yic8NnvJr9/cQHo09GPyThaqHmu/12lRmvExYB+b8RouMtPZMvAeTW3Aormyu1ayzMpYPzhemU88poQS/3wSUxGE2NzcyifzUlg+iX5g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/13/24 14:26, Christoph Lameter via B4 Relay wrote: > From: "Christoph Lameter (Ampere)" > > Some architectures support load acquire which can save us a memory > barrier and save some cycles. > > A typical sequence > > do { > seq = read_seqcount_begin(&s); > > } while (read_seqcount_retry(&s, seq); > > requires 13 cycles on ARM64 for an empty loop. Two read memory barriers are > needed. One for each of the seqcount_* functions. > > We can replace the first read barrier with a load acquire of > the seqcount which saves us one barrier. > > On ARM64 doing so reduces the cycle count from 13 to 8. > > Signed-off-by: Christoph Lameter (Ampere) > --- > arch/Kconfig | 5 +++++ > arch/arm64/Kconfig | 1 + > include/linux/seqlock.h | 41 +++++++++++++++++++++++++++++++++++++++++ > 3 files changed, 47 insertions(+) > > diff --git a/arch/Kconfig b/arch/Kconfig > index 975dd22a2dbd..3f8867110a57 100644 > --- a/arch/Kconfig > +++ b/arch/Kconfig > @@ -1600,6 +1600,11 @@ config ARCH_HAS_KERNEL_FPU_SUPPORT > Architectures that select this option can run floating-point code in > the kernel, as described in Documentation/core-api/floating-point.rst. > > +config ARCH_HAS_ACQUIRE_RELEASE > + bool > + help > + Architectures that support acquire / release can avoid memory fences > + > source "kernel/gcov/Kconfig" > > source "scripts/gcc-plugins/Kconfig" > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > index a2f8ff354ca6..19e34fff145f 100644 > --- a/arch/arm64/Kconfig > +++ b/arch/arm64/Kconfig > @@ -39,6 +39,7 @@ config ARM64 > select ARCH_HAS_PTE_DEVMAP > select ARCH_HAS_PTE_SPECIAL > select ARCH_HAS_HW_PTE_YOUNG > + select ARCH_HAS_ACQUIRE_RELEASE > select ARCH_HAS_SETUP_DMA_OPS > select ARCH_HAS_SET_DIRECT_MAP > select ARCH_HAS_SET_MEMORY Do we need a new ARCH flag? I believe barrier APIs like smp_load_acquire() will use the full barrier for those arch'es that don't define their own smp_load_acquire(). BTW, acquire/release can be considered memory barriers too. Maybe you are talking about preferring acquire/release barriers over read/write barriers. Right? Cheers, Longman