Hello, Michael. On Wed, Aug 23, 2017 at 09:00:39PM +1000, Michael Ellerman wrote: > > I don't think that's true. The CPU id used in kernel doesn't have to > > match the physical one and arch code should be able to pre-map CPU IDs > > to nodes and use the matching one when hotplugging CPUs. I'm not > > saying that's the best way to solve the problem tho. > > We already virtualise the CPU numbers, but not the node IDs. And it's > the node IDs that are really the problem. Yeah, it just needs to match up new cpus to the cpu ids assigned to the right node. > > It could be that the best way forward is making cpu <-> node mapping > > dynamic and properly synchronized. > > We don't need it to be dynamic (at least for this bug). The node mapping for that cpu id changes *dynamically* while the system is running and that can race with node-affinity sensitive operations such as memory allocations. > Laurent is booting Qemu with a fixed CPU <-> Node mapping, it's just > that because some CPUs aren't present at boot we don't know what the > node mapping is. (Correct me if I'm wrong Laurent). > > So all we need is: > - the workqueue code to cope with CPUs that are possible but not online > having NUMA_NO_NODE to begin with. > - a way to update the workqueue cpumask when the CPU comes online. > > Which seems reasonable to me? Please take a step back and think through the problem again. You can't bandaid it this way. Thanks. -- tejun
