Feedback requested: Exposing NVDIMM performance statistics in a generic way

[Vaibhav Jain <vaibhav@linux.ibm.com>]

Hello,

I am looking for some community feedback on these two Problem-statements:

1.How to expose NVDIMM performance statistics in an arch or nvdimm vendor
agnostic manner ?

2. Is there a common set of performance statistics for NVDIMMs that all
vendors should provide ?

Problem context
===============
While working on bring up of PAPR SCM based NVDIMMs[1] for arch/powerpc
we want to expose certain dimm performance statistics like "Media
Read/Write Counts", "Power-on Seconds" etc to user-space [2]. These
performance statistics are similar to what ipmctl[3] reports for Intel®
Optane™ persistent memory via the '-show performance' command line
arg. However the reported set of performance stats doesn't cover the
entirety of all performance stats supported by PAPR SCM based NVDimms.

For example here is a subset of performance stats which are specific to
PAPR SCM NVDimms and that not reported by ipmctl:

* Controller Reset Count
* Controller Reset Elapsed Time
* Power-on Seconds
* Cache Read Hit Count
* Cache Write Hit Count

Possibility of updating ipmctl to add support for these performance
statistics is greatly hampered by no support for ACPI on Powerpc
arch. Secondly vendors who dont support ACPI/NFIT command set
similar to Intel® Optane™ (Example MSFT) are also left out in
lurch. Problem-statement#1 points to this specific problem.

Additionally in absence of any pre-agreed set of performance statistics
which all vendors should support, adding support for such a
functionality in ipmctl may not bode well of other nvdimm vendors. For
example if support for reporting "Controller Reset Count" is added to
ipmctl then it may not be applicable to other vendors such as Intel®
Optane™. This issue is what Problem-statement#2 refers to.

Possible Solution for Problem#1
===============================

One possible solution to Problem#1 can to add support for reporting
NVDIMM performance statistics in 'ndtcl'. 'libndctl' already has a layer
that abstracts underlying NVDIMM vendors (via struct ndctl_dimm_ops),
making supporting different NVDIMM vendors fairly easy. Also ndctl is
more widely used compared to 'ipmctl', hence adding such a functionality
to ndctl would make it more widely used.

Above solution was implemented as RFC patch-set[2] that exposes these
performance statistics through a generic abstraction in libndctl and
added a presentation layer for this data in ndctl[4]. It added a new
command line flags '--stat' to ndctl to report *all* nvdimm vendor
reported performance stats. The output is similar to one below:

# ndctl list -D --stats
[
  {
    "dev":"nmem0",
    "stats":{
      "Power-on Seconds":603931,
      "Media Read Count":0,
      "Media Write Count":6313,
    }
  }
]

This was done by adding two new dimm-ops callbacks that were
implemented by the papr_scm implementation within libndctl. These
callbacks are invoked by newly introduce code in 'util/json-smart.c'
that format the returned stats from these new dimm-ops and transform
them into a json-object to later presentation. I would request you to
look at RFC patch-set[2] to understand the implementation details.

Possibled Solution for Problem#2
================================

Solution to Problem-statement#2 is what eludes me though. If there is a
minimal set of performance stats (similar to what ndctl enforces for
health-stats) then implementation of such a functionality in
ndctl/ipmctl would be easy to implement. But is it really possible to
have such a common set of performance stats that NVDIMM vendors can
expose.

Patch-set[2] though tries to bypass this problem by letting the vendor
descide which performance stats to expose. This opens up a possibility
of this functionality to abused by dimm vendors to reports arbirary data
through this flag that may not be performance-stats.

Summing-up
==========

In light of above, requesting your feedback as to how
problem-statements#{1, 2} can be addressed within ndctl subsystem. Also
are these problems even worth solving.

References
==========
[1] https://github.com/torvalds/linux/blob/master/Documentation/powerpc/papr_hcalls.rst

[2] "[ndctl RFC-PATCH 0/4] Add support for reporting PAPR NVDIMM
Statistics"
https://lore.kernel.org/linux-nvdimm/20200518110814.145644-1-vaibhav@linux.ibm.com/

[3] https://docs.pmem.io/ipmctl-user-guide/instrumentation/show-device-performance

[4] "[RFC-PATCH 1/4] ndctl,libndctl: Implement new dimm-ops 'new_stats'
and 'get_stat'"
https://lore.kernel.org/linux-nvdimm/20200514225258.508463-2-vaibhav@linux.ibm.com

Thanks,
~ Vaibhav
_______________________________________________
Linux-nvdimm mailing list -- linux-nvdimm@lists.01.org
To unsubscribe send an email to linux-nvdimm-leave@lists.01.org