On Wed, Apr 20, 2022 at 8:12 PM Yang Shi <shy828301@xxxxxxxxx> wrote: > > On Thu, Apr 14, 2022 at 12:00 AM ying.huang@xxxxxxxxx > <ying.huang@xxxxxxxxx> wrote: > > > > On Wed, 2022-04-13 at 14:52 +0530, Jagdish Gediya wrote: > > > Current implementation to find the demotion targets works > > > based on node state N_MEMORY, however some systems may have > > > dram only memory numa node which are N_MEMORY but not the > > > right choices as demotion targets. > > > > > > This patch series introduces the new node state > > > N_DEMOTION_TARGETS, which is used to distinguish the nodes which > > > can be used as demotion targets, node_states[N_DEMOTION_TARGETS] > > > is used to hold the list of nodes which can be used as demotion > > > targets, support is also added to set the demotion target > > > list from user space so that default behavior can be overridden. > > > > It appears that your proposed user space interface cannot solve all > > problems. For example, for system as follows, > > > > 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 > > > > Demotion order 1: > > > > node demotion_target > > 0 1 > > 1 X > > 2 X > > > > Demotion order 2: > > > > node demotion_target > > 0 1 > > 1 X > > 2 1 > > > > The demotion order 1 is preferred if we want to reduce cross-socket > > traffic. While the demotion order 2 is preferred if we want to take > > full advantage of the slow memory node. We can take any choice as > > automatic-generated order, while make the other choice possible via user > > space overridden. > > > > I don't know how to implement this via your proposed user space > > interface. How about the following user space interface? > > > > 1. Add a file "demotion_order_override" in > > /sys/devices/system/node/ > > > > 2. When read, "1" is output if the demotion order of the system has been > > overridden; "0" is output if not. > > > > 3. When write "1", the demotion order of the system will become the > > overridden mode. When write "0", the demotion order of the system will > > become the automatic mode and the demotion order will be re-generated. > > > > 4. Add a file "demotion_targets" for each node in > > /sys/devices/system/node/nodeX/ > > > > 5. When read, the demotion targets of nodeX will be output. > > > > 6. When write a node list to the file, the demotion targets of nodeX > > will be set to the written nodes. And the demotion order of the system > > will become the overridden mode. > > TBH I don't think having override demotion targets in userspace is > quite useful in real life for now (it might become useful in the > future, I can't tell). Imagine you manage hundred thousands of > machines, which may come from different vendors, have different > generations of hardware, have different versions of firmware, it would > be a nightmare for the users to configure the demotion targets > properly. So it would be great to have the kernel properly configure > it *without* intervening from the users. > > So we should pick up a proper default policy and stick with that > policy unless it doesn't work well for the most workloads. I do > understand it is hard to make everyone happy. My proposal is having > every node in the fast tier has a demotion target (at least one) if > the slow tier exists sounds like a reasonable default policy. I think > this is also the current implementation. > This is reasonable. I agree that with a decent default policy, the overriding of per-node demotion targets can be deferred. The most important problem here is that we should allow the configurations where memory-only nodes are not used as demotion targets, which this patch set has already addressed. > > > > To reduce the complexity, the demotion order of the system is either in > > overridden mode or automatic mode. When converting from the automatic > > mode to the overridden mode, the existing demotion targets of all nodes > > will be retained before being changed. When converting from overridden > > mode to automatic mode, the demotion order of the system will be re- > > generated automatically. > > > > In overridden mode, the demotion targets of the hot-added and hot- > > removed node will be set to empty. And the hot-removed node will be > > removed from the demotion targets of any node. > > > > This is an extention of the interface used in the following patch, > > > > https://lore.kernel.org/lkml/20191016221149.74AE222C@xxxxxxxxxxxxxxxxxx/ > > > > What do you think about this? > > > > > node state N_DEMOTION_TARGETS is also set from the dax kmem > > > driver, certain type of memory which registers through dax kmem > > > (e.g. HBM) may not be the right choices for demotion so in future > > > they should be distinguished based on certain attributes and dax > > > kmem driver should avoid setting them as N_DEMOTION_TARGETS, > > > however current implementation also doesn't distinguish any > > > such memory and it considers all N_MEMORY as demotion targets > > > so this patch series doesn't modify the current behavior. > > > > > > > Best Regards, > > Huang, Ying > > > > [snip] > >
