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[2003:cb:c73d:2400:3be1:a856:724c:fd29]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-441b8a31695sm126153245e9.40.2025.05.05.01.12.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 05 May 2025 01:12:51 -0700 (PDT) Message-ID: <0e568e33-34fa-40f6-a20d-ebf653de123d@redhat.com> Date: Mon, 5 May 2025 10:12:48 +0200 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 v3 1/3] driver/base: Optimize memory block registration to reduce boot time To: Oscar Salvador Cc: Donet Tom , Mike Rapoport , Zi Yan , Greg Kroah-Hartman , Andrew Morton , rafael@kernel.org, Danilo Krummrich , Ritesh Harjani , Jonathan Cameron , Alison Schofield , Yury Norov , Dave Jiang , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <188fbfba-afb4-4db7-bbba-7689a96be931@redhat.com> <74c500dd-8d1c-4177-96c7-ddd51ca77306@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 05.05.25 09:57, Oscar Salvador wrote: > On Mon, May 05, 2025 at 09:38:43AM +0200, David Hildenbrand wrote: >> On 05.05.25 09:28, Oscar Salvador wrote: >>> On Mon, May 05, 2025 at 09:16:48AM +0200, David Hildenbrand wrote: >>>> memory hotplug code never calls register_one_node(), unless I am missing >>>> something. >>>> >>>> During add_memory_resource(), we call __try_online_node(nid, false), meaning >>>> we skip register_one_node(). >>>> >>>> The only caller of __try_online_node(nid, true) is try_online_node(), called >>>> from CPU hotplug code, and I *guess* that is not required. >>> >>> Well, I guess this is because we need to link the cpus to the node. >>> register_one_node() has two jobs: 1) register cpus belonging to the node >>> and 2) register memory-blocks belonging to the node (if any). >> >> Ah, via __register_one_node() ... >> >> I would assume that an offline node >> >> (1) has no memory >> (2) has no CPUs > > That is right. > >> When we *hotplug* either memory or CPUs, and we first online the node, there >> is nothing to register. Because if there would be something, the node would >> already be online. > > I think I do not understand this, but let us imagine the following > scenario: > > - You craft a VM with qemu that has a numa node which is memoryless and cpuless. > This node will be allocated in free_area_init()->alloc_offline_node_data() but > it will not be marked online because it does not have any resources. > > Then if you create a cpu device and hotplug in there, this will > trigger try_online_node() from the cpu callback and go all the way to > __register_one_node() to link the hotplugged cpu to the corresponding node. Assume you hotplug the second CPU. The node is already registered/online, so who does the register_cpu_under_node() call? It's register_cpu() I guess? But no idea in which order that is called with node onlining. The code has to be cleaned up such that onlining a node does not traverse any cpus / memory. Whoever adds a CPU / memory *after onlining the node* must register the device manually under the *now online* node. Maybe we have to adjust CPU hotplug code if the existing register_cpu_under_node() is misplaced. register_cpu_under_node() has this weird !node_online check in there ... ... it's a mess > > Now, I do not see an issue with that. > The only think that makes me go "meh", is that register_one_node() > calls register_memory_blocks_under_node() blindly when there might not > be any memory block to register (I am sure we bail out if we see that > somewhere, but still feels kinda of "wrong"? Yes -- Cheers, David / dhildenb