On Thu, 2022-04-21 at 10:56 -0700, Yang Shi wrote: > On Wed, Apr 20, 2022 at 10:41 PM Wei Xu <weixugc@xxxxxxxxxx> wrote: > > > > 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. > > Yes, I agree. Fixing the bug and allowing override by userspace are > totally two separate things. > Yes. I agree with the separating thing, although [1/5] doesn't fix the bug, but improve the automatic order generation method. So I think it's better to separate this patchset into 2 patchsets. [1/5] is for improving the automatic order generation. The [2-5/5] is for user space overriding. Best Regards, Huang, Ying > > > > > > > > > > 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] > > > >
