On 01.07.20 13:01, Srikar Dronamraju wrote: > * David Hildenbrand <david@xxxxxxxxxx> [2020-07-01 12:15:54]: > >> On 01.07.20 12:04, Srikar Dronamraju wrote: >>> * Michal Hocko <mhocko@xxxxxxxxxx> [2020-07-01 10:42:00]: >>> >>>> >>>>> >>>>> 2. Also existence of dummy node also leads to inconsistent information. The >>>>> number of online nodes is inconsistent with the information in the >>>>> device-tree and resource-dump >>>>> >>>>> 3. When the dummy node is present, single node non-Numa systems end up showing >>>>> up as NUMA systems and numa_balancing gets enabled. This will mean we take >>>>> the hit from the unnecessary numa hinting faults. >>>> >>>> I have to say that I dislike the node online/offline state and directly >>>> exporting that to the userspace. Users should only care whether the node >>>> has memory/cpus. Numa nodes can be online without any memory. Just >>>> offline all the present memory blocks but do not physically hot remove >>>> them and you are in the same situation. If users are confused by an >>>> output of tools like numactl -H then those could be updated and hide >>>> nodes without any memory&cpus. >>>> >>>> The autonuma problem sounds interesting but again this patch doesn't >>>> really solve the underlying problem because I strongly suspect that the >>>> problem is still there when a numa node gets all its memory offline as >>>> mentioned above. >>>> >>>> While I completely agree that making node 0 special is wrong, I have >>>> still hard time to review this very simply looking patch because all the >>>> numa initialization is so spread around that this might just blow up >>>> at unexpected places. IIRC we have discussed testing in the previous >>>> version and David has provided a way to emulate these configurations >>>> on x86. Did you manage to use those instruction for additional testing >>>> on other than ppc architectures? >>>> >>> >>> I have tried all the steps that David mentioned and reported back at >>> https://lore.kernel.org/lkml/20200511174731.GD1961@xxxxxxxxxxxxxxxxxx/t/#u >>> >>> As a summary, David's steps are still not creating a memoryless/cpuless on >>> x86 VM. >> >> Now, that is wrong. You get a memoryless/cpuless node, which is *not >> online*. Once you hotplug some memory, it will switch online. Once you >> remove memory, it will switch back offline. >> > > Let me clarify, we are looking for a node 0 which is cpuless/memoryless at > boot. The code in question tries to handle a cpuless/memoryless node 0 at > boot. I was just correcting your statement, because it was wrong. Could be that x86 code maps PXM 1 to node 0 because PXM 1 does neither have CPUs nor memory. That would imply that we can, in fact, never have node 0 offline during boot. -- Thanks, David / dhildenb
