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

[Dan Williams]

On Wed, Dec 20, 2017 at 1:24 PM, Ross Zwisler
<ross.zwisler@xxxxxxxxxxxxxxx> wrote:
> 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?

For the initial implementation do we need to have a representation of
all the performance data? Given that
/sys/devices/system/node/nodeX/distance is the only generic
performance attribute published by the kernel today it is already the
case that applications that need to target specific memories need to
go parse information that is not provided by the kernel by default.
The question is can those specialized applications stay special and go
parse the platform specific data sources, like raw HMAT, directly, or
do we expect general purpose applications to make use of this data? I
think a firmware-id to numa-node translation facility
(/sys/devices/system/node/nodeX/fwid) is a simple start that we can
build on with more information as specific use cases arise.