linux-kernel - Re: [PATCH v3 0/3] create sysfs representation of ACPI HMAT

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Date:   Wed, 20 Dec 2017 14:24:08 -0700
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org,
        "Anaczkowski, Lukasz" <lukasz.anaczkowski@...el.com>,
        "Box, David E" <david.e.box@...el.com>,
        "Kogut, Jaroslaw" <Jaroslaw.Kogut@...el.com>,
        "Koss, Marcin" <marcin.koss@...el.com>,
        "Koziej, Artur" <artur.koziej@...el.com>,
        "Lahtinen, Joonas" <joonas.lahtinen@...el.com>,
        "Moore, Robert" <robert.moore@...el.com>,
        "Nachimuthu, Murugasamy" <murugasamy.nachimuthu@...el.com>,
        "Odzioba, Lukasz" <lukasz.odzioba@...el.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "Schmauss, Erik" <erik.schmauss@...el.com>,
        "Verma, Vishal L" <vishal.l.verma@...el.com>,
        "Zheng, Lv" <lv.zheng@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Balbir Singh <bsingharora@...il.com>,
        Brice Goglin <brice.goglin@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jerome Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Len Brown <lenb@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>, devel@...ica.org,
        linux-acpi@...r.kernel.org, linux-mm@...ck.org,
        linux-nvdimm@...ts.01.org, linux-api@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
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:

/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?