On Wed, Apr 20, 2022 at 11:24 PM ying.huang@xxxxxxxxx <ying.huang@xxxxxxxxx> wrote: > > On Wed, 2022-04-20 at 22:41 -0700, Wei Xu 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, > > > > I agree that a decent default policy is important. As that was enhanced > in [1/5] of this patchset. > > > 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. > > Do you mean the user space interface proposed by [3/5] of this patchset? Yes. > IMHO, if we want to add a user space interface, I think that it should > be powerful enough to address all existing issues and some potential > future issues, so that it can be stable. I don't think it's a good idea > to define a partial user space interface that works only for a specific > use case and cannot be extended for other use cases. I actually think that they can be viewed as two separate problems: one is to define which nodes can be used as demotion targets (this patch set), and the other is how to initialize the per-node demotion path (node_demotion[]). We don't have to solve both problems at the same time. If we decide to go with a per-node demotion path customization interface to indirectly set N_DEMOTION_TARGETS, I'd prefer that there is a single global control to turn off all demotion targets (for the machines that don't use memory-only nodes for demotion). > Best Regards, > Huang, Ying > > [snip] > > > > > >
