[PATCH V2 00/13] selftests/resctrl: Support diverse platforms with MBM and MBA tests

Reinette Chatre
2024-09-12T18:04:52Z

Changes since V1:
- V1: https://lore.kernel.org/cover.1724970211.git.reinette.chatre@intel.com/
- V2 contains the same general solutions to stated problem as V1 but these
are now preceded by more fixes (patches 1 to 5) and improved robustness
(patches 6 to 9) to existing tests before the series gets back
to solving the original problem with more confidence in patches 10 to 13.
- The posibility of making "memflush = false" for CMT test was discussed
during V1. Modifying this setting does not have a significant impact on the
observed results that are already well within acceptable range and this
version thus keeps original default. If performance was a goal it may
be possible to do further experimentation where "memflush = false" could
eliminate the need for the sleep(1) within the test wrapper, but
improving the performance is not a goal of this work.
- (New) Support what seems to be unintended ability for user space to provide
parameters to "fill_buf" by making the parsing robust and only support
changing parameters that are supported to be changed. Drop support for
"write" operation since it has never been measured.
- (New) Improve wraparound handling. (Ilpo)
- (New) A couple of new fixes addressing issues discovered during development.
- (Change from V1) To support fill_buf parameters provided by user space as
well as test specific fill_buf parameters struct fill_buf_param is no longer
just a member of struct resctrl_val_param, instead there could be at most
two instances of struct fill_buf_param, the immutable parameters provided
by user space and the parameters used by individual tests. (Ilpo)
- Please see individual patches for detailed changes.

V1 cover:

The resctrl selftests for Memory Bandwidth Allocation (MBA) and Memory
Bandwidth Monitoring (MBM) are failing on some (for example [1]) Emerald
Rapids systems. The test failures result from the following two
properties of these systems:
1) Emerald Rapids systems can have up to 320MB L3 cache. The resctrl
MBA and MBM selftests measure memory traffic for which a hardcoded
250MB buffer has been sufficient so far. On platforms with L3 cache
larger than the buffer, the buffer fits in the L3 cache and thus
no/very little memory traffic is generated during the "memory
bandwidth" tests.
2) Some platform features, for example RAS features or memory
performance features that generate memory traffic may drive accesses
that are counted differently by performance counters and MBM
respectively, for instance generating "overhead" traffic which is not
counted against any specific RMID. Until now these counting
differences have always been "in the noise". On Emerald Rapids
systems the maximum MBA throttling (10% memory bandwidth)
throttles memory bandwidth to where memory accesses by these other
platform features push the memory bandwidth difference between
memory controller performance counters and resctrl (MBM) beyond the
tests' hardcoded tolerance.

Make the tests more robust against platform variations:
1) Let the buffer used by memory bandwidth tests be guided by the size
of the L3 cache.
2) Larger buffers require longer initialization time before the buffer can
be used to measurement. Rework the tests to ensure that buffer
initialization is complete before measurements start.
3) Do not compare performance counters and MBM measurements at low
bandwidth. The value of "low" is hardcoded to 750MiB based on
measurements on Emerald Rapids, Sapphire Rapids, and Ice Lake
systems. This limit is not applicable to AMD systems since it
only applies to the MBA and MBM tests that are isolated to Intel.

[1]
https://ark.intel.com/content/www/us/en/ark/products/237261/intel-xeon-platinum-8592-processor-320m-cache-1-9-ghz.html

Reinette Chatre (13):
selftests/resctrl: Make functions only used in same file static
selftests/resctrl: Print accurate buffer size as part of MBM results
selftests/resctrl: Fix memory overflow due to unhandled wraparound
selftests/resctrl: Protect against array overrun during iMC config
parsing
selftests/resctrl: Make wraparound handling obvious
selftests/resctrl: Remove "once" parameter required to be false
selftests/resctrl: Only support measured read operation
selftests/resctrl: Remove unused measurement code
selftests/resctrl: Make benchmark parameter passing robust
selftests/resctrl: Ensure measurements skip initialization of default
benchmark
selftests/resctrl: Use cache size to determine "fill_buf" buffer size
selftests/resctrl: Do not compare performance counters and resctrl at
low bandwidth
selftests/resctrl: Keep results from first test run

tools/testing/selftests/resctrl/cmt_test.c | 37 +-
tools/testing/selftests/resctrl/fill_buf.c | 40 +-
tools/testing/selftests/resctrl/mba_test.c | 52 +-
tools/testing/selftests/resctrl/mbm_test.c | 38 +-
tools/testing/selftests/resctrl/resctrl.h | 73 ++-
.../testing/selftests/resctrl/resctrl_tests.c | 95 +++-
tools/testing/selftests/resctrl/resctrl_val.c | 445 +++++-------------
tools/testing/selftests/resctrl/resctrlfs.c | 17 -
8 files changed, 339 insertions(+), 458 deletions(-)

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2.46.0