On Wed, 2022-04-27 at 09:27 -0700, Wei Xu wrote: > On Wed, Apr 27, 2022 at 12:11 AM ying.huang@xxxxxxxxx > <ying.huang@xxxxxxxxx> wrote: > > > > On Mon, 2022-04-25 at 09:56 -0700, Wei Xu wrote: > > > On Sat, Apr 23, 2022 at 8:02 PM ying.huang@xxxxxxxxx > > > <ying.huang@xxxxxxxxx> wrote: > > > > > > > > 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. > > > > > > > > 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 > > > > > > > > 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. > > > > > > I think 2(a) should be the system-wide configuration and 2(b) can be > > > achieved with NUMA mempolicy (which needs to be added to demotion). > > > > Unfortunately, some NUMA mempolicy information isn't available at > > demotion time, for example, mempolicy enforced via set_mempolicy() is > > for thread. But I think that cpusets can work for demotion. > > > > > In general, we can view the demotion order in a way similar to > > > allocation fallback order (after all, if we don't demote or demotion > > > lags behind, the allocations will go to these demotion target nodes > > > according to the allocation fallback order anyway). If we initialize > > > the demotion order in that way (i.e. every node can demote to any node > > > in the next tier, and the priority of the target nodes is sorted for > > > each source node), we don't need per-node demotion order override from > > > the userspace. What we need is to specify what nodes should be in > > > each tier and support NUMA mempolicy in demotion. > > > > This sounds interesting. Tier sounds like a natural and general concept > > for these memory types. It's attracting to use it for user space > > interface too. For example, we may use that for mem_cgroup limits of a > > specific memory type (tier). > > > > And if we take a look at the N_DEMOTION_TARGETS again from the "tier" > > point of view. The nodes are divided to 2 classes via > > N_DEMOTION_TARGETS. > > > > - The nodes without N_DEMOTION_TARGETS are top tier (or tier 0). > > > > - The nodes with N_DEMOTION_TARGETS are non-top tier (or tier 1, 2, 3, > > ...) > > > > Yes, this is one of the main reasons why we (Google) want this interface. > > > So, another possibility is to fit N_DEMOTION_TARGETS and its overriding > > into "tier" concept too. !N_DEMOTION_TARGETS == TIER0. > > > > - All nodes start with TIER0 > > > > - TIER0 can be cleared for some nodes via e.g. kmem driver > > > > TIER0 node list can be read or overriden by the user space via the > > following interface, > > > > /sys/devices/system/node/tier0 > > > > In the future, if we want to customize more tiers, we can add tier1, > > tier2, tier3, ..... For now, we can add just tier0. That is, the > > interface is extensible in the future compared with > > .../node/demote_targets. > > > > This more explicit tier definition interface works, too. > In addition to make tiering definition explicit, more importantly, this makes it much easier to support more than 2 tiers. For example, for a system with HBM (High Bandwidth Memory), CPU+DRAM, DRAM only, and PMEM, that is, 3 tiers, we can put HBM in tier 0, CPU+DRAM and DRAM only in tier 1, and PMEM in tier 2, automatically, or via user space overridding. N_DEMOTION_TARGETS isn't natural to be extended to support this. Best Regards, Huang, Ying > > This isn't as flexible as the writable per-node demotion targets. But > > it may be enough for most requirements? > > I would think so. Besides, it doesn't really conflict with the > per-node demotion target interface if we really want to introduce the > latter. > > > Best Regards, > > Huang, Ying > > > > > Cross-socket demotion should not be too big a problem in practice > > > because we can optimize the code to do the demotion from the local CPU > > > node (i.e. local writes to the target node and remote read from the > > > source node). The bigger issue is cross-socket memory access onto the > > > demoted pages from the applications, which is why NUMA mempolicy is > > > important here. > > > > > > > 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. > > > > > > > > Best Regards, > > > > Huang, Ying > > > > > > > > > >
