Re: RFC: Memory Tiering Kernel Interfaces

[Aneesh Kumar K.V]

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