Re: [PATCH 5/6] selftests/resctrl: Do not compare performance counters and resctrl at low bandwidth

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

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On Fri, 30 Aug 2024, Reinette Chatre wrote:
> On 8/30/24 4:42 AM, Ilpo Järvinen wrote:
> > On Thu, 29 Aug 2024, Reinette Chatre wrote:
> > 
> > > The MBA test incrementally throttles memory bandwidth, each time
> > > followed by a comparison between the memory bandwidth observed
> > > by the performance counters and resctrl respectively.
> > > 
> > > While a comparison between performance counters and resctrl is
> > > generally appropriate, they do not have an identical view of
> > > memory bandwidth. 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. As a ratio, this different view of memory
> > > bandwidth becomes more apparent at low memory bandwidths.
> > 
> > Interesting.
> > 
> > I did some time back prototype with a change to MBM test such that instead
> > of using once=false I changed fill_buf to be able to run N passes through
> > the buffer which allowed me to know how many reads were performed by the
> > benchmark. This yielded numerical difference between all those 3 values
> > (# of reads, MBM, perf) which also varied from arch to another so it
> > didn't end up making an usable test.
> > 
> > I guess I now have an explanation for at least a part of the differences.
> > 
> > > It is not practical to enable/disable the various features that
> > > may generate memory bandwidth to give performance counters and
> > > resctrl an identical view. Instead, do not compare performance
> > > counters and resctrl view of memory bandwidth when the memory
> > > bandwidth is low.
> > > 
> > > Bandwidth throttling behaves differently across platforms
> > > so it is not appropriate to drop measurement data simply based
> > > on the throttling level. Instead, use a threshold of 750MiB
> > > that has been observed to support adequate comparison between
> > > performance counters and resctrl.
> > > 
> > > Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
> > > ---
> > >   tools/testing/selftests/resctrl/mba_test.c | 7 +++++++
> > >   tools/testing/selftests/resctrl/resctrl.h  | 6 ++++++
> > >   2 files changed, 13 insertions(+)
> > > 
> > > diff --git a/tools/testing/selftests/resctrl/mba_test.c
> > > b/tools/testing/selftests/resctrl/mba_test.c
> > > index cad473b81a64..204b9ac4b108 100644
> > > --- a/tools/testing/selftests/resctrl/mba_test.c
> > > +++ b/tools/testing/selftests/resctrl/mba_test.c
> > > @@ -96,6 +96,13 @@ static bool show_mba_info(unsigned long *bw_imc,
> > > unsigned long *bw_resc)
> > >     		avg_bw_imc = sum_bw_imc / (NUM_OF_RUNS - 1);
> > >   		avg_bw_resc = sum_bw_resc / (NUM_OF_RUNS - 1);
> > > +		if (avg_bw_imc < THROTTLE_THRESHOLD || avg_bw_resc <
> > > THROTTLE_THRESHOLD) {
> > > +			ksft_print_msg("Bandwidth below threshold (%d MiB).
> > > Dropping results from MBA schemata %u.\n",
> > > +					THROTTLE_THRESHOLD,
> > > +					ALLOCATION_MAX - ALLOCATION_STEP *
> > > allocation);
> > 
> > The second one too should be %d.
> > 
> 
> hmmm ... I intended to have it be consistent with the ksft_print_msg() that
> follows. Perhaps allocation can be made unsigned instead?

If you go that way, then it would be good to make the related defines and 
allocation in mba_setup() unsigned too, although the latter is a bit scary 
because it does allocation -= ALLOCATION_STEP which could underflow if the 
defines are ever changed.

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 i.