On Tue, 2022-04-26 at 14:32 +0530, Aneesh Kumar K V wrote: > On 4/26/22 2:12 PM, ying.huang@xxxxxxxxx wrote: > > On Mon, 2022-04-25 at 13:39 +0530, Aneesh Kumar K V wrote: > > > On 4/25/22 11:40 AM, ying.huang@xxxxxxxxx wrote: > > > > 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. > > > > > > > > > Sure will find one such config. As you might have noticed this is very > > > easy to have in a virtualization setup because the hypervisor can assign > > > memory to a guest VM from a numa node that doesn't have CPU assigned to > > > the same guest. This depends on the other guest VM instance config > > > running on the system. So on any virtualization config that has got > > > persistent memory attached, this can become an easy config to end up with. > > > > > > > Why they want to do that? I am looking forward to a real issue, not > > theoritical possibility. > > > > > Can you elaborate this more? That is a real config. > > > > > > > > > > > 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. > > > > > > > > > Not sure I understand that. Yes, once you have a writeable per node > > > demotion target it is easy to build any demotion order. > > > > Yes. > > > > > But that doesn't > > > mean we should not improve the default unless you have reason to say > > > that using N_DEMOTTION_TARGETS breaks any existing config. > > > > > > > Becuase N_DEMOTTION_TARGETS is a new kernel ABI to override the default, > > not the default itself. [1/5] of this patchset improve the default > > behavior itself, and I think that's good. > > > > we are improving the default by using N_DEMOTION_TARGETS because the > current default breaks configs which can get you memory only NUMA nodes. > I would not consider it an override. > OK. I guess that there is some misunderstanding here. I thought that you refer to N_DEMOTION_TARGETS overriden via make the following file writable, /sys/devices/system/node/demotion_targets Now, I think you are referring to setting N_DEMOTION_TARGETS in kmem driver by default. Sorry if I misunderstood you. So, to be clear. I am OK to restrict default demotion targets via kmem driver (we can improve this in the future with more source). But I don't think it's good to make /sys/devices/system/node/demotion_targets writable. Instead, I think it's better to make /sys/devices/system/node/nodeX/demotion_targets writable. > > Because we must maintain the kernel ABI almost for ever, we need to be > > careful about adding new ABI and add less if possible. If writable per- > > node demotion targets can address your issue. Then it's unnecessary to > > add another redundant kernel ABI for that. > > This means on platform like powerpc, we would always need to have a > userspace managed demotion because we can end up with memory only numa > nodes for them. Why force that? Please take a look at the above. > > > > > > > /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. > > > > > > > > > > yes, we should avoid loop between that. > > > > In addition to that, we need to get same result after hot-remove then > > hot-add the same node. That is, the result should be stable after NOOP. > > I guess we can just always, > > > > - Generate the default demotion order automatically without any > > overriding. > > > > - Apply the overriding, after removing the invalid targets, etc. > > > > > But if you agree for the above > > > ABI we could go ahead and share the implementation code. > > > > I think we need to add a way to distinguish auto-generated and overriden > > demotion targets in the output of nodeX/demotion_targets. Otherwise it > > looks good to me. > > > > > something like: > > /sys/devices/system/node# echo 4 > node1/demotion_targets > /sys/devices/system/node# cat node[0-4]/demotion_targets > - > 4 (userspace override) > - > - > - > Or /sys/devices/system/node# echo 4 > node1/demotion_targets /sys/devices/system/node# cat node[0-4]/demotion_targets - *4 - - - Best Regards, Huang, Ying
