On Mon, 25 Apr 2022 20:14:58 +0530 Aneesh Kumar K V <aneesh.kumar@xxxxxxxxxxxxx> wrote: > On 4/25/22 7:27 PM, Jonathan Cameron wrote: > > On Mon, 25 Apr 2022 16:45:38 +0530 > > Jagdish Gediya <jvgediya@xxxxxxxxxxxxx> wrote: > > > >> On Sun, Apr 24, 2022 at 11:19:53AM +0800, ying.huang@xxxxxxxxx wrote: > >>> On Sat, 2022-04-23 at 01:25 +0530, Jagdish Gediya wrote: > >>>> Some systems(e.g. PowerVM) can have both DRAM(fast memory) only > >>>> NUMA node which are N_MEMORY and slow memory(persistent memory) > >>>> only NUMA node which are also N_MEMORY. As the current demotion > >>>> target finding algorithm works based on N_MEMORY and best distance, > >>>> it will choose DRAM only NUMA node as demotion target instead of > >>>> persistent memory node on such systems. If DRAM only NUMA node is > >>>> filled with demoted pages then at some point new allocations can > >>>> start falling to persistent memory, so basically cold pages are in > >>>> fast memor (due to demotion) and new pages are in slow memory, this > >>>> is why persistent memory nodes should be utilized for demotion and > >>>> dram node should be avoided for demotion so that they can be used > >>>> for new allocations. > >>>> > >>>> Current implementation can work fine on the system where the memory > >>>> only numa nodes are possible only for persistent/slow memory but it > >>>> is not suitable for the like of systems mentioned above. > >>> > >>> Can you share the NUMA topology information of your machine? And the > >>> demotion order before and after your change? > >>> > >>> Whether it's good to use the PMEM nodes as the demotion targets of the > >>> DRAM-only node too? > >> > >> $ numactl -H > >> available: 2 nodes (0-1) > >> node 0 cpus: 0 1 2 3 4 5 6 7 > >> node 0 size: 14272 MB > >> node 0 free: 13392 MB > >> node 1 cpus: > >> node 1 size: 2028 MB > >> node 1 free: 1971 MB > >> node distances: > >> node 0 1 > >> 0: 10 40 > >> 1: 40 10 > >> > >> 1) without N_DEMOTION_TARGETS patch series, 1 is demotion target > >> for 0 even when 1 is DRAM node and there is no demotion targets for 1. > > > > I'm not convinced the distinction between DRAM and persistent memory is > > valid. There will definitely be systems with a large pool > > of remote DRAM (and potentially no NV memory) where the right choice > > is to demote to that DRAM pool. > > > > Basing the decision on whether the memory is from kmem or > > normal DRAM doesn't provide sufficient information to make the decision. > > > > Hence the suggestion for the ability to override this from userspace. > Now, for example, we could build a system with memory from the remote > machine (memory inception in case of power which will mostly be plugged > in as regular hotpluggable memory ) and a slow CXL memory or OpenCAPI > memory. > > In the former case, we won't consider that for demotion with this series > because that is not instantiated via dax kmem. So yes definitely we > would need the ability to override this from userspace so that we could > put these remote memory NUMA nodes as demotion targets if we want. Agreed. I would like to have a better 'guess' at the right default though if possible. With hindsight my instinct would have been to have a default of no demotion path at all and hence ensure distros will carry appropriate userspace setup scripts. Ah well, too late :) > > >> > >> $ cat /sys/bus/nd/devices/dax0.0/target_node > >> 2 > >> $ > >> # cd /sys/bus/dax/drivers/ > >> :/sys/bus/dax/drivers# ls > >> device_dax kmem > >> :/sys/bus/dax/drivers# cd device_dax/ > >> :/sys/bus/dax/drivers/device_dax# echo dax0.0 > unbind > >> :/sys/bus/dax/drivers/device_dax# echo dax0.0 > ../kmem/new_id > >> :/sys/bus/dax/drivers/device_dax# numactl -H > >> available: 3 nodes (0-2) > >> node 0 cpus: 0 1 2 3 4 5 6 7 > >> node 0 size: 14272 MB > >> node 0 free: 13380 MB > >> node 1 cpus: > >> node 1 size: 2028 MB > >> node 1 free: 1961 MB > >> node 2 cpus: > >> node 2 size: 0 MB > >> node 2 free: 0 MB > >> node distances: > >> node 0 1 2 > >> 0: 10 40 80 > >> 1: 40 10 80 > >> 2: 80 80 10 > >> > >> 2) Once this new node brought online, without N_DEMOTION_TARGETS > >> patch series, 1 is demotion target for 0 and 2 is demotion target > >> for 1. > >> > >> With this patch series applied, > >> 1) No demotion target for either 0 or 1 before dax device is online > > > > I'd argue that is wrong. At this state you have a tiered memory system > > be it one with just DRAM. Using it as such is correct behavior that > > we should not be preventing. Sure some usecases wouldn't want that > > arrangement but some do want it. > > > > For your case we could add a heuristic along the lines of the demotion > > target should be at least as big as the starting point but that would > > be a bit hacky. > > > > Hence the proposal to do a per node demotion target override with the > semantics that i explained here > > > https://lore.kernel.org/linux-mm/8735i1zurt.fsf@xxxxxxxxxxxxx/ > > Let me know if that interface would be good to handle all the possible > demotion target configs we would want to have. At first glance it looks good to me. Jonathan > > -aneesh
