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t=1785317286; bh=salgQx0zg68IGEBw4oYDljPNqxYepuzSJBxYIsFQ7as=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=X3tXrVQwuuxI3PfEgO8BALfQdgGKLhyvvicqZgQVGYlA2y6wCjR6m3JrM2mYPYW0b 6O+5/VzLaIMg56oDPvd0YtZ6hZTtV8+CoIMjOei5Xur1IrWfg6FMSQLsqWflDXPwlU aAlFjOTy1+y/JzMBibjA0eHPHGYZLy2a+ujtSdVeOqU2cWpo08+fNGcFWehaJk43I9 ODb/WRJWjMiwNA1q/2rUa0z45XSkHlZzZLcO/YZOi+xyeji+m3Rn/WQWHVGEN1kEWm lT8GjTw6ybgCRjqKYA5eF2hqwmtMqot3mq1ymA20whFGpZrAVqvOLyQbRZcX3aJhl2 n2F/MpYxPeidg== Message-ID: <4887267b-dc26-4c33-96ca-8dff054a0d1f@kernel.org> Date: Wed, 29 Jul 2026 11:28:02 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC v2 PATCH 0/16] Optimize this_cpu_*() ops for non-x86 (ARM64 for this series) To: "Christoph Lameter (Ampere)" , Mark Rutland Cc: Yang Shi , Ryan Roberts , dennis@kernel.org, tj@kernel.org, urezki@gmail.com, catalin.marinas@arm.com, will@kernel.org, akpm@linux-foundation.org, hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20260715180455.515692-1-yang@os.amperecomputing.com> <0344c559-1959-4531-9265-d5a5180eb7cd@arm.com> <25d1e09b-53e4-7cd5-87db-b58437e4e690@gentwo.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: qdgqdc6w9cff8eiw9wshqu1sybj6sz8d X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 237EF40005 X-Rspam-User: X-HE-Tag: 1785317287-915975 X-HE-Meta: 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 8vYB+kNZ kcXEnai+FMGly2ks8aigWleiEjXhN6CAOog6nqb7+tLpD87D7PCUj685yp9Q1hlW9DEDeMEQW8qrqNvLpeTTy0L0WQ2kraxPyhm0O9EL7kF/HMSm3D7Fmt+hAvzqilonpsoqIKPAmmhKESBRUz88TTsF3epVHvGhMv2NEdZtxlXZCvwoKLFhciO5LxhZntRF309wKU4OlnJCOlFgLMK6tOd6WHFGa32GTQvBimSpXNIz1FDw63qQGI4E21tJqTQKXab4SohUYMo6vvVs2BmZ1lqcKZPm4KH+U7L6ndgiDJRPZeTq8hD47eBhm7g== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/28/26 00:06, Christoph Lameter (Ampere) wrote: > On Wed, 22 Jul 2026, Mark Rutland wrote: > >> I expect that should come with a reasonable benefit, but I don't have >> benchmark figures yet as I haven't finished converting the xchg and >> cmpxchg implementations. >> >>> It sounds like it just moved the cost from one place to the other >>> place and it also seems hacky TBH. > > > Yang Shi's patch has *no* critical section. There is no additional code > for the RMV instruction. The RMV instruction is executed on the correct > per cpu area. One of the reasons for the performance win is the > eliminattion of these critical sections. Your approach still has some form > of prologue and posthandling like the current preempt approach and > therefore will not be able to have the same performance gains. > > The code is more efficient, there is no restart necessary and the > technique is already widely used on x86 for a long time. > > Having the ability in general to map mmemory differently depending on the > cpu opens up a number of other optimization like > > 1. Per Node areas. Calculations of addresses for per node data and RMV > operations on per node data becomes as trivial as the per cpu data > handling. This will further reduce and eliminate critical sections > currently necessary to handle per node data modifications for NUMA > configurations which are increasingly becoming important for large core > configurations on ARM64. > > 2. Kernel text replication. Kernel text can be replicated per node or > per whatever memory is closer to the executing code. This avoids transfers > via the on chip memory busses and increases performance. We have seen > 20-50% on that one. > > 3. Readonly data replication. A similar approach is possible for kernel > read only data. > > 3. Read-mostly replication. This is a bit more complex and the writes > become more expensive since updates have to be made to all copies but > a read-mostly variable is rarely written and the replication will reduce > the latencies to reach these variables. > > > 4. Custom sets of cores that operate on shared data that is replicated > per node or some custom set of cores. This is for example > useful for network devices that have the ability to do I/O via a split > PCI bus or other construct where I/O lanes are duplicated to sets of > cores. > > > What we are proposing here is a basic new feature that simplifies code and > allows addititonal performance and functional features that are so far not > possible on ARM64. Most of the features you mentioned above are not supposed to be architecture-specific things. IIUC, Linus strongly objected per-cpu page tables in the past, which suggests to me that such an arm64-only thing is not the way to go? -- Cheers, David