Messages in this thread | | | Date | Tue, 26 Apr 2022 14:32:39 +0530 | Subject | Re: [PATCH v2 0/5] mm: demotion: Introduce new node state N_DEMOTION_TARGETS | From | Aneesh Kumar K V <> |
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On 4/26/22 2:12 PM, ying.huang@intel.com wrote: > On Mon, 2022-04-25 at 13:39 +0530, Aneesh Kumar K V wrote: >> On 4/25/22 11:40 AM, ying.huang@intel.com wrote: >>> On Mon, 2022-04-25 at 09:20 +0530, Aneesh Kumar K.V wrote: >>>> "ying.huang@intel.com" <ying.huang@intel.com> 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@gmail.com/ >>>>> >>>>>> [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.
> 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?
> >>>> /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) - - -
-aneesh
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