Alistair Popple <apopple@xxxxxxxxxx> writes: > Wei Xu <weixugc@xxxxxxxxxx> writes: > >> On Thu, May 5, 2022 at 5:19 PM Alistair Popple <apopple@xxxxxxxxxx> wrote: >>> >>> Wei Xu <weixugc@xxxxxxxxxx> writes: >>> >>> [...] >>> >>> >> > >>> >> > >>> >> > Tiering Hierarchy Initialization >>> >> > `==============================' >>> >> > >>> >> > By default, all memory nodes are in the top tier (N_TOPTIER_MEMORY). >>> >> > >>> >> > A device driver can remove its memory nodes from the top tier, e.g. >>> >> > a dax driver can remove PMEM nodes from the top tier. >>> >> >>> >> With the topology built by firmware we should not need this. >>> >>> I agree that in an ideal world the hierarchy should be built by firmware based >>> on something like the HMAT. But I also think being able to override this will be >>> useful in getting there. Therefore a way of overriding the generated hierarchy >>> would be good, either via sysfs or kernel boot parameter if we don't want to >>> commit to a particular user interface now. >>> >>> However I'm less sure letting device-drivers override this is a good idea. How >>> for example would a GPU driver make sure it's node is in the top tier? By moving >>> every node that the driver does not know about out of N_TOPTIER_MEMORY? That >>> could get messy if say there were two drivers both of which wanted their node to >>> be in the top tier. >> >> The suggestion is to allow a device driver to opt out its memory >> devices from the top-tier, not the other way around. > > So how would demotion work in the case of accelerators then? In that > case we would want GPU memory to demote to DRAM, but that won't happen > if both DRAM and GPU memory are in N_TOPTIER_MEMORY and it seems the > only override available with this proposal would move GPU memory into a > lower tier, which is the opposite of what's needed there. How about we do 3 tiers now. dax kmem devices can be registered to tier 3. By default all numa nodes can be registered at tier 2 and HBM or GPU can be enabled to register at tier 1. ? -aneesh
