Hi, All, On Fri, 2022-04-22 at 16:30 +0530, Jagdish Gediya wrote: [snip] > I think it is necessary to either have per node demotion targets > configuration or the user space interface supported by this patch > series. As we don't have clear consensus on how the user interface > should look like, we can defer the per node demotion target set > interface to future until the real need arises. > > Current patch series sets N_DEMOTION_TARGET from dax device kmem > driver, it may be possible that some memory node desired as demotion > target is not detected in the system from dax-device kmem probe path. > > It is also possible that some of the dax-devices are not preferred as > demotion target e.g. HBM, for such devices, node shouldn't be set to > N_DEMOTION_TARGETS. In future, Support should be added to distinguish > such dax-devices and not mark them as N_DEMOTION_TARGETS from the > kernel, but for now this user space interface will be useful to avoid > such devices as demotion targets. > > We can add read only interface to view per node demotion targets > from /sys/devices/system/node/nodeX/demotion_targets, remove > duplicated /sys/kernel/mm/numa/demotion_target interface and instead > make /sys/devices/system/node/demotion_targets writable. > > Huang, Wei, Yang, > What do you suggest? We cannot remove a kernel ABI in practice. So we need to make it right at the first time. Let's try to collect some information for the kernel ABI definitation. The below is just a starting point, please add your requirements. 1. Jagdish has some machines with DRAM only NUMA nodes, but they don't want to use that as the demotion targets. But I don't think this is a issue in practice for now, because demote-in-reclaim is disabled by default. 2. For machines with PMEM installed in only 1 of 2 sockets, for example, Node 0 & 2 are cpu + dram nodes and node 1 are slow memory node near node 0, available: 3 nodes (0-2) node 0 cpus: 0 1 node 0 size: n MB node 0 free: n MB node 1 cpus: node 1 size: n MB node 1 free: n MB node 2 cpus: 2 3 node 2 size: n MB node 2 free: n MB node distances: node 0 1 2 0: 10 40 20 1: 40 10 80 2: 20 80 10 We have 2 choices, a) node demotion targets 0 1 2 1 b) node demotion targets 0 1 2 X a) is good to take advantage of PMEM. b) is good to reduce cross-socket traffic. Both are OK as defualt configuration. But some users may prefer the other one. So we need a user space ABI to override the default configuration. 3. For machines with HBM (High Bandwidth Memory), as in https://lore.kernel.org/lkml/39cbe02a-d309-443d-54c9-678a0799342d@xxxxxxxxx/ > [1] local DDR = 10, remote DDR = 20, local HBM = 31, remote HBM = 41 Although HBM has better performance than DDR, in ACPI SLIT, their distance to CPU is longer. We need to provide a way to fix this. The user space ABI is one way. The desired result will be to use local DDR as demotion targets of local HBM. Best Regards, Huang, Ying
