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Date:   Wed, 20 Dec 2017 14:24:08 -0700
From:   Ross Zwisler <>
To:     Matthew Wilcox <>
Cc:     Dave Hansen <>,
        Ross Zwisler <>,
        Michal Hocko <>,,
        "Anaczkowski, Lukasz" <>,
        "Box, David E" <>,
        "Kogut, Jaroslaw" <>,
        "Koss, Marcin" <>,
        "Koziej, Artur" <>,
        "Lahtinen, Joonas" <>,
        "Moore, Robert" <>,
        "Nachimuthu, Murugasamy" <>,
        "Odzioba, Lukasz" <>,
        "Rafael J. Wysocki" <>,
        "Rafael J. Wysocki" <>,
        "Schmauss, Erik" <>,
        "Verma, Vishal L" <>,
        "Zheng, Lv" <>,
        Andrew Morton <>,
        Balbir Singh <>,
        Brice Goglin <>,
        Dan Williams <>,
        Jerome Glisse <>,
        John Hubbard <>,
        Len Brown <>,
        Tim Chen <>,,,,,,
Subject: Re: [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:


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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