On Sun, Apr 24, 2022 at 11:02:47AM +0800, ying.huang@xxxxxxxxx wrote: > 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. /sys/kernel/mm/numa/demotion_target was introduced in v2, I was talking about removing it from next version of the series as the similar interface is available as a result of introducing N_DEMOTION_TARGETS at /sys/devices/system/node/demotion_targets, so instead of introducing duplicate interface to write N_DEMOTION_TARGETS, we can instead make /sys/devices/system/node/demotion_targets writable. > 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 > >
