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[37.188.255.121]) by smtp.gmail.com with ESMTPSA id q8sm81107wrc.8.2020.03.17.07.01.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 17 Mar 2020 07:01:24 -0700 (PDT) Date: Tue, 17 Mar 2020 15:01:22 +0100 From: Michal Hocko To: Vlastimil Babka Cc: Srikar Dronamraju , Sachin Sant , Linus Torvalds , LKML , linux-mm@kvack.org, Mel Gorman , "Kirill A. Shutemov" , Andrew Morton , linuxppc-dev@lists.ozlabs.org, Christopher Lameter , Joonsoo Kim , Kirill Tkhai Subject: Re: [PATCH 1/3] powerpc/numa: Set numa_node for all possible cpus Message-ID: <20200317140122.GQ26018@dhcp22.suse.cz> References: <20200311115735.GM23944@dhcp22.suse.cz> <20200312052707.GA3277@linux.vnet.ibm.com> <5e5c736a-a88c-7c76-fc3d-7bc765e8dcba@suse.cz> <20200312131438.GB3277@linux.vnet.ibm.com> <61437352-8b54-38fa-4471-044a65c9d05a@suse.cz> <20200312161310.GC3277@linux.vnet.ibm.com> <20200316090652.GC11482@dhcp22.suse.cz> <65b99db6-3bdf-6caa-74e5-6d6b681f16b5@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <65b99db6-3bdf-6caa-74e5-6d6b681f16b5@suse.cz> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 17-03-20 14:44:45, Vlastimil Babka wrote: > On 3/16/20 10:06 AM, Michal Hocko wrote: > > On Thu 12-03-20 17:41:58, Vlastimil Babka wrote: > > [...] > >> with nid present in: > >> N_POSSIBLE - pgdat might not exist, node_to_mem_node() must return some online > > > > I would rather have a dummy pgdat for those. Have a look at > > $ git grep "NODE_DATA.*->" | wc -l > > 63 > > > > Who knows how many else we have there. I haven't looked more closely. > > Besides that what is a real reason to not have pgdat ther and force all > > users of a $random node from those that the platform considers possible > > for special casing? Is that a memory overhead? Is that really a thing? > > I guess we can ignore memory overhead. I guess there only might be some concern > that for nodes that are initially offline, we will allocate the pgdat on a > different node, and after they are online, it will stay on a different node with > more access latency from local cpus. If we only allocate for online nodes, it > can always be local? But I guess it doesn't matter that much. This is not the case even now because of chicke&egg. You need a memory to allocate from and that memory has to be managed somewhere per node (pgdat). Keep in mind we do not have the bootmem allocator for the hotplug. Have a look at hotadd_new_pgdat and when it is called. There are some attempts to allocate memmap from the hotpluged memory but I am not sure we can do the whole thing without pgdat in place. If we can then can come up with some replace the pgdat magic. But still I am not even sure this is something we really have to optimize for. -- Michal Hocko SUSE Labs