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

Le 20/12/2017 à 23:41, Ross Zwisler a écrit :

On Wed, Dec 20, 2017 at 02:29:56PM -0800, Dan Williams wrote:

quoted

On Wed, Dec 20, 2017 at 1:24 PM, Ross Zwisler
[off-list ref] wrote:

quoted

On Wed, Dec 20, 2017 at 01:16:49PM -0800, Matthew Wilcox wrote:

quoted

On Wed, Dec 20, 2017 at 12:22:21PM -0800, Dave Hansen wrote:

quoted

On 12/20/2017 10:19 AM, Matthew Wilcox wrote:

quoted

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.

We don't represent all the performance data, we only represent the data for
local initiator/target pairs.  I do think that this is useful to have in sysfs
because it provides a way to easily answer the most commonly asked questions
(or at least what I'm guessing will be the most commmonly asked queststions),
i.e. "given a CPU, what are the speeds of the various types of memory attached
to it", and "given a chunk of memory, how fast is it and to which CPU is it
local"?  By providing this base level of information I'm hoping to prevent
most applications from having to parse the HMAT directly.

The question of whether or not to include this local performance information
was one of the main questions of the initial RFC patch series, and I did get
feedback (albiet off-list) that the local performance information was
valuable to at least some users.  I did intentionally structure my (now very
short) set so that the performance information was added as a separate patch,
so we can get to the place you're talking about where we only provide firmware
id <=> proximity domain mappings by just leaving off the last patch in the
series.

Hello

I can confirm that HPC runtimes are going to use these patches (at least
all runtimes that use hwloc for topology discovery, but that's the vast
majority of HPC anyway).

We really didn't like KNL exposing a hacky SLIT table [1]. We had to
explicitly detect that specific crazy table to find out which NUMA nodes
were local to which cores, and to find out which NUMA nodes were
HBM/MCDRAM or DDR. And then we had to hide the SLIT values to the
application because the reported latencies didn't match reality. Quite
annoying.

With Ross' patches, we can easily get what we need:
* which NUMA nodes are local to which CPUs? /sys/devices/system/node/
can only report a single local node per CPU (doesn't work for KNL and
upcoming architectures with HBM+DDR+...)
* which NUMA nodes are slow/fast (for both bandwidth and latency)
And we can still look at SLIT under /sys/devices/system/node if really
needed.

And of course having this in sysfs is much better than parsing ACPI
tables that are only accessible to root :)

Regards
Brice

[1] local DDR = 10, remote DDR = 20, local HBM = 31, remote HBM = 41