Re: [RFC PATCH 02/14] mm/hms: heterogenenous memory system (HMS) documentation

[Logan Gunthorpe]

On 2018-12-04 4:56 p.m., Jerome Glisse wrote:
> One example i have is 4 nodes (CPU socket) each nodes with 8 GPUs and
> two 8 GPUs node connected through each other with fast mesh (ie each
> GPU can peer to peer to each other at the same bandwidth). Then this
> 2 blocks are connected to the other block through a share link.
> 
> So it looks like:
>     SOCKET0----SOCKET1-----SOCKET2----SOCKET3
>     |          |           |          |
>     S0-GPU0====S1-GPU0     S2-GPU0====S1-GPU0
>     ||     \\//            ||     \\//
>     ||     //\\            ||     //\\
>     ...    ====...    -----...    ====...
>     ||     \\//            ||     \\//
>     ||     //\\            ||     //\\
>     S0-GPU7====S1-GPU7     S2-GPU7====S3-GPU7

Well the existing NUMA node stuff tells userspace which GPU belongs to
which socket (every device in sysfs already has a numa_node attribute).
And if that's not good enough we should work to improve how that works
for all devices. This problem isn't specific to GPUS or devices with
memory and seems rather orthogonal to an API to bind to device memory.

> How the above example would looks like ? I fail to see how to do it
> inside current sysfs. Maybe by creating multiple virtual device for
> each of the inter-connect ? So something like
> 
> link0 -> device:00 which itself has S0-GPU0 ... S0-GPU7 has child
> link1 -> device:01 which itself has S1-GPU0 ... S1-GPU7 has child
> link2 -> device:02 which itself has S2-GPU0 ... S2-GPU7 has child
> link3 -> device:03 which itself has S3-GPU0 ... S3-GPU7 has child

I think the "links" between GPUs themselves would be a bus. In the same
way a NUMA node is a bus. Each device in sysfs would then need a
directory or something to describe what "link bus(es)" they are a part
of. Though there are other ways to do this: a GPU driver could simply
create symlinks to other GPUs inside a "neighbours" directory under the
device path or something like that.

The point is that this seems like it is specific to GPUs and could
easily be solved in the GPU community without any new universal concepts
or big APIs.

And for applications that need topology information, a lot of it is
already there, we just need to fill in the gaps with small changes that
would be much less controversial. Then if you want to create a libhms
(or whatever) to help applications parse this information out of
existing sysfs that would make sense.

> My proposal is to do HMS behind staging for a while and also avoid
> any disruption to existing code path. See with people living on the
> bleeding edge if they get interested in that informations. If not then
> i can strip down my thing to the bare minimum which is about device
> memory.

This isn't my area or decision to make, but it seemed to me like this is
not what staging is for. Staging is for introducing *drivers* that
aren't up to the Kernel's quality level and they all reside under the
drivers/staging path. It's not meant to introduce experimental APIs
around the kernel that might be revoked at anytime.

DAX introduced itself by marking the config option as EXPERIMENTAL and
printing warnings to dmesg when someone tries to use it. But, to my
knowledge, DAX also wasn't creating APIs with the intention of changing
or revoking them -- it was introducing features using largely existing
APIs that had many broken corner cases.

Do you know of any precedents where big APIs were introduced and then
later revoked or radically changed like you are proposing to do?

Logan