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[2003:d8:2f3e:5900:27aa:5f4a:b65c:3d3c]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-442cd3b7f18sm31317155e9.40.2025.05.08.02.18.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 08 May 2025 02:18:37 -0700 (PDT) Message-ID: <78bc6a1b-164e-4925-a624-a271a4499364@redhat.com> Date: Thu, 8 May 2025 11:18:35 +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: Mike Rapoport Cc: Oscar Salvador , Donet Tom , 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> <8180a50d-eebe-4f9b-9ce8-d886654a992d@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 15:24, Mike Rapoport wrote: > On Mon, May 05, 2025 at 10:18:43AM +0200, David Hildenbrand wrote: >> On 05.05.25 09:53, Mike Rapoport 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 >>>> >>>> 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. >>>> >>>> In particular, try_offline_node() will only offline a node if >>>> >>>> (A) No present pages: No pages are spanned anymore. This includes >>>> offline memory blocks. >>>> (B) No present CPUs. >>>> >>>> But maybe there is some case that I am missing ... >>> >>> I actually hoped you and Oscar know how that stuff works :) >> >> Well, I know how the memory side works, but the CPU side is giving me a hard >> time :) >> >>> >>> I tried to figure what is going on there and it all looks really convoluted. >> >> Jap ... >> >>> >>> So, on boot we have >>> cpu_up() -> >>> try_online_node() -> >>> bails out because all nodes are online (at least on >>> x86 AFAIU, see 1ca75fa7f19d ("arch/x86/mm/numa: Do >>> not initialize nodes twice")) >>> node_dev_init()i -> >>> register_one_node() -> >>> this one can use __register_one_node() and loop >>> over memblock regions. >>> >>> And for the hotplug/unplug path, it seems that >>> register_memory_blocks_under_node(MEMINIT_EARLY) is superfluous, because if >>> a node had memory it wouldn't get offlined, and if we are hotplugging an >>> node with memory and cpus, memory hotplug anyway calls >>> register_memory_blocks_under_node_hotplug(). >>> >>> So, IMHO, register_one_node() should not call >>> register_memory_blocks_under_node() at all, but again, I might have missed >>> something :) >> >> Hm, but someone has to create these links for the memory blocks. > > My understanding that the links for the memory blocks during hotplug are created in > > add_memory_resource() > register_memory_blocks_under_node() Yes, during hotplug it's exactly that, after registering the node + setting it online. > > So register_one_node() only calls register_memory_blocks_under_node() when > there are no actual memory resources under that node, isn't it? Except in early boot. That's why register_one_node() has: register_memory_blocks_under_node(nid, start_pfn, end_pfn, MEMINIT_EARLY); ^ early :) And that is triggered by node_dev_init()->register_one_node() > > Then we can drop the call to register_memory_blocks_under_node() from > register_one_node() and add creation of memory blocks to node_dev_init(), > i.e. > > node_dev_init() > for_each_node(nid) > __register_one_node(nid) > for_each_mem_region() > /* create memory block if node matches */ Yes exactly, that makes sense. -- Cheers, David / dhildenb