On Wed, 2022-04-27 at 08:27 +0530, Aneesh Kumar K V wrote: > On 4/27/22 6:59 AM, ying.huang@xxxxxxxxx wrote: > > On Mon, 2022-04-25 at 20:14 +0530, Aneesh Kumar K V 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. > > > > > > > > > Is there a driver for the device (memory from the remote machine)? If > > so, we can adjust demotion order for it in the driver. > > > > At this point, it is managed by hypervisor, is hotplugged into the the > LPAR with more additional properties specified via device tree. So there > is no inception specific device driver. Because there's information in device tree, I still think it's doable in the kernel. But it's up to you to choose the appropriate way. Best Regards, Huang, Ying > > In general, I think that we can adjust demotion order inside kernel from > > various information sources. In addition to ACPI SLIT, we also have > > HMAT, kmem driver, other drivers, etc. > > > > Managing inception memory will any way requires a userspace component to > track the owner machine for the remote memory. So we should be ok to > have userspace manage demotion order. > > -aneesh >
