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

[Jerome Glisse <jglisse@redhat.com>]

On Tue, Dec 04, 2018 at 06:34:37PM -0800, Dan Williams wrote:
> On Tue, Dec 4, 2018 at 5:15 PM Logan Gunthorpe <logang@deltatee.com> wrote:
> >
> >
> >
> > 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?
> 
> This came up before for apis even better defined than HMS as well as
> more limited scope, i.e. experimental ABI availability only for -rc
> kernels. Linus said this:
> 
> "There are no loopholes. No "but it's been only one release". No, no,
> no. The whole point is that users are supposed to be able to *trust*
> the kernel. If we do something, we keep on doing it.
> 
> And if it makes it harder to add new user-visible interfaces, then
> that's a *good* thing." [1]
> 
> The takeaway being don't land work-in-progress ABIs in the kernel.
> Once an application depends on it, there are no more incompatible
> changes possible regardless of the warnings, experimental notices, or
> "staging" designation. DAX is experimental because there are cases
> where it currently does not work with respect to another kernel
> feature like xfs-reflink, RDMA. The plan is to fix those, not continue
> to hide behind an experimental designation, and fix them in a way that
> preserves the user visible behavior that has already been exposed,
> i.e. no regressions.
> 
> [1]: https://lists.linuxfoundation.org/pipermail/ksummit-discuss/2017-August/004742.html

So i guess i am heading down the vXX road ... such is my life :)

Cheers,
Jérôme