* Peter Zijlstra <pet...@infradead.org> [2021-05-20 20:56:31]: > On Thu, May 20, 2021 at 09:14:25PM +0530, Srikar Dronamraju wrote: > > Currently scheduler populates the distance map by looking at distance > > of each node from all other nodes. This should work for most > > architectures and platforms. > > > > However there are some architectures like POWER that may not expose > > the distance of nodes that are not yet onlined because those resources > > are not yet allocated to the OS instance. Such architectures have > > other means to provide valid distance data for the current platform. > > > > For example distance info from numactl from a fully populated 8 node > > system at boot may look like this. > > > > node distances: > > node 0 1 2 3 4 5 6 7 > > 0: 10 20 40 40 40 40 40 40 > > 1: 20 10 40 40 40 40 40 40 > > 2: 40 40 10 20 40 40 40 40 > > 3: 40 40 20 10 40 40 40 40 > > 4: 40 40 40 40 10 20 40 40 > > 5: 40 40 40 40 20 10 40 40 > > 6: 40 40 40 40 40 40 10 20 > > 7: 40 40 40 40 40 40 20 10 > > > > However the same system when only two nodes are online at boot, then the > > numa topology will look like > > node distances: > > node 0 1 > > 0: 10 20 > > 1: 20 10 > > > > It may be implementation dependent on what node_distance(0,3) where > > node 0 is online and node 3 is offline. In POWER case, it returns > > LOCAL_DISTANCE(10). Here at boot the scheduler would assume that the max > > distance between nodes is 20. However that would not be true. > > > > When Nodes are onlined and CPUs from those nodes are hotplugged, > > the max node distance would be 40. > > > > To handle such scenarios, let scheduler allow architectures to populate > > the distance map. Architectures that like to populate the distance map > > can overload arch_populate_distance_map(). > > Why? Why can't your node_distance() DTRT? The arch interface is > nr_node_ids and node_distance(), I don't see why we need something new > and then replace one special use of it. > > By virtue of you being able to actually implement this new hook, you > supposedly can actually do node_distance() right too.
Since for an offline node, arch interface code doesn't have the info. As far as I know/understand, in POWER, unless there is an active memory or CPU that's getting onlined, arch can't fetch the correct node distance. Taking the above example: node 3 is offline, then node_distance of (3,X) where X is anything other than 3, is not reliable. The moment node 3 is onlined, the node distance is reliable. This problem will not happen even on POWER if all the nodes have either memory or CPUs active at the time of boot. -- Thanks and Regards Srikar Dronamraju
