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SubjectRe: [PATCH v3 0/3] create sysfs representation of ACPI HMAT
On Wed, Dec 20, 2017 at 01:16:49PM -0800, Matthew Wilcox wrote:
> On Wed, Dec 20, 2017 at 12:22:21PM -0800, Dave Hansen wrote:
> > On 12/20/2017 10:19 AM, Matthew Wilcox wrote:
> > > I don't know what the right interface is, but my laptop has a set of
> > > /sys/devices/system/memory/memoryN/ directories. Perhaps this is the
> > > right place to expose write_bw (etc).
> >
> > Those directories are already too redundant and wasteful. I think we'd
> > really rather not add to them. In addition, it's technically possible
> > to have a memory section span NUMA nodes and have different performance
> > properties, which make it impossible to represent there.
> >
> > In any case, ACPI PXM's (Proximity Domains) are guaranteed to have
> > uniform performance properties in the HMAT, and we just so happen to
> > always create one NUMA node per PXM. So, NUMA nodes really are a good fit.
>
> I think you're missing my larger point which is that I don't think this
> should be exposed to userspace as an ACPI feature. Because if you do,
> then it'll also be exposed to userspace as an openfirmware feature.
> And sooner or later a devicetree feature. And then writing a portable
> program becomes an exercise in suffering.
>
> So, what's the right place in sysfs that isn't tied to ACPI? A new
> directory or set of directories under /sys/devices/system/memory/ ?

Oh, the current location isn't at all tied to acpi except that it happens to
be named 'hmat'. When it was all named 'hmem' it was just:

/sys/devices/system/hmem

Which has no ACPI-isms at all. I'm happy to move it under
/sys/devices/system/memory/hmat if that's helpful, but I think we still have
the issue that the data represented therein is still pulled right from the
HMAT, and I don't know how to abstract it into something more platform
agnostic until I know what data is provided by those other platforms.

For example, the HMAT provides latency information and bandwidth information
for both reads and writes. Will the devicetree/openfirmware/etc version have
this same info, or will it be just different enough that it won't translate
into whatever I choose to stick in sysfs?

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