On Mon, 2022-04-25 at 09:20 +0530, Aneesh Kumar K.V wrote: > "ying.huang@xxxxxxxxx" <ying.huang@xxxxxxxxx> writes: > > > 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. > > It is not just that the demotion can be disabled. We should be able to > use demotion on a system where we can find DRAM only NUMA nodes. That > cannot be achieved by /sys/kernel/mm/numa/demotion_enabled. It needs > something similar to to N_DEMOTION_TARGETS > Can you show NUMA information of your machines with DRAM-only nodes and PMEM nodes? We can try to find the proper demotion order for the system. If you can not show it, we can defer N_DEMOTION_TARGETS until the machine is available. > > 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 > > This is achieved by > > [PATCH v2 1/5] mm: demotion: Set demotion list differently > > > > > 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. > > > IMHO the above (2b and 3) can be done using per node demotion targets. Below is > what I think we could do with a single slow memory NUMA node 4. If we can use writable per-node demotion targets as ABI, then we don't need N_DEMOTION_TARGETS. > /sys/devices/system/node# cat node[0-4]/demotion_targets > 4 > 4 > 4 > 4 > > /sys/devices/system/node# echo 1 > node1/demotion_targets > bash: echo: write error: Invalid argument > /sys/devices/system/node# cat node[0-4]/demotion_targets > 4 > 4 > 4 > 4 > > /sys/devices/system/node# echo 0 > node1/demotion_targets > /sys/devices/system/node# cat node[0-4]/demotion_targets > 4 > 0 > 4 > 4 > > /sys/devices/system/node# echo 1 > node0/demotion_targets > bash: echo: write error: Invalid argument > /sys/devices/system/node# cat node[0-4]/demotion_targets > 4 > 0 > 4 > 4 > > Disable demotion for a specific node. > /sys/devices/system/node# echo > node1/demotion_targets > /sys/devices/system/node# cat node[0-4]/demotion_targets > 4 > > 4 > 4 > > Reset demotion to default > /sys/devices/system/node# echo -1 > node1/demotion_targets > /sys/devices/system/node# cat node[0-4]/demotion_targets > 4 > 4 > 4 > 4 > > When a specific device/NUMA node is used for demotion target via the user interface, it is taken > out of other NUMA node targets. IMHO, we should be careful about interaction between auto-generated and overridden demotion order. Best Regards, Huang, Ying > root@ubuntu-guest:/sys/devices/system/node# cat node[0-4]/demotion_targets > 4 > 4 > 4 > 4 > > /sys/devices/system/node# echo 4 > node1/demotion_targets > /sys/devices/system/node# cat node[0-4]/demotion_targets > > 4 > > > > If more than one node requies the same demotion target > /sys/devices/system/node# echo 4 > node0/demotion_targets > /sys/devices/system/node# cat node[0-4]/demotion_targets > 4 > 4 > > > > -aneesh
