Re: [PATCH v2 19/26] selftests/resctrl: Rewrite Cache Allocation Technology (CAT) test

[Ilpo Järvinen]

On Tue, 28 Nov 2023, Reinette Chatre wrote:
> On 11/20/2023 3:13 AM, Ilpo Järvinen wrote:
> > CAT test spawns two processes into two different control groups with
> > exclusive schemata. Both the processes alloc a buffer from memory
> > matching their allocated LLC block size and flush the entire buffer out
> > of caches. Since the processes are reading through the buffer only once
> > during the measurement and initially all the buffer was flushed, the
> > test isn't testing CAT.
> > 
> > Rewrite the CAT test to allocate a buffer sized to half of LLC. Then
> > perform a sequence of tests with different LLC alloc sizes starting
> > from half of the CBM bits down to 1-bit CBM. Flush the buffer before
> > each test and read the buffer twice. Observe the LLC misses on the
> > second read through the buffer. As the allocated LLC block gets smaller
> > and smaller, the LLC misses will become larger and larger giving a
> > strong signal on CAT working properly.
> > 
> > The new CAT test is using only a single process because it relies on
> > measured effect against another run of itself rather than another
> > process adding noise. The rest of the system is set to use the CBM bits
> > not used by the CAT test to keep the test isolated.
> > 
> > Replace count_bits() with count_contiguous_bits() to get the first bit
> > position in order to be able to calculate masks based on it.
> > 
> > This change has been tested with a number of systems from different
> > generations.
> > 
> > Suggested-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> > Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> > ---
> >  tools/testing/selftests/resctrl/cat_test.c  | 282 +++++++++-----------
> >  tools/testing/selftests/resctrl/fill_buf.c  |   6 +-
> >  tools/testing/selftests/resctrl/resctrl.h   |   5 +-
> >  tools/testing/selftests/resctrl/resctrlfs.c |  44 +--
> >  4 files changed, 138 insertions(+), 199 deletions(-)
> > 
> > diff --git a/tools/testing/selftests/resctrl/cat_test.c b/tools/testing/selftests/resctrl/cat_test.c
> > index cfda87667b46..4169b17b8f91 100644
> > --- a/tools/testing/selftests/resctrl/cat_test.c
> > +++ b/tools/testing/selftests/resctrl/cat_test.c
> > @@ -11,65 +11,69 @@
> >  #include "resctrl.h"
> >  #include <unistd.h>
> >  
> > -#define RESULT_FILE_NAME1	"result_cat1"
> > -#define RESULT_FILE_NAME2	"result_cat2"
> > +#define RESULT_FILE_NAME	"result_cat"
> >  #define NUM_OF_RUNS		5
> > -#define MAX_DIFF_PERCENT	4
> > -#define MAX_DIFF		1000000
> >  
> >  /*
> > - * Change schemata. Write schemata to specified
> > - * con_mon grp, mon_grp in resctrl FS.
> > - * Run 5 times in order to get average values.
> > + * Minimum difference in LLC misses between a test with n+1 bits CBM to the
> > + * test with n bits is MIN_DIFF_PERCENT_PER_BIT * (n - 1). With e.g. 5 vs 4
> > + * bits in the CBM mask, the minimum difference must be at least
> > + * MIN_DIFF_PERCENT_PER_BIT * (4 - 1) = 3 percent.
> > + *
> > + * The relationship between number of used CBM bits and difference in LLC
> > + * misses is not expected to be linear. With a small number of bits, the
> > + * margin is smaller than with larger number of bits. For selftest purposes,
> > + * however, linear approach is enough because ultimately only pass/fail
> > + * decision has to be made and distinction between strong and stronger
> > + * signal is irrelevant.
> >   */
> > -static int cat_setup(struct resctrl_val_param *p)
> > -{
> > -	char schemata[64];
> > -	int ret = 0;
> > -
> > -	/* Run NUM_OF_RUNS times */
> > -	if (p->num_of_runs >= NUM_OF_RUNS)
> > -		return END_OF_TESTS;
> > -
> > -	if (p->num_of_runs == 0) {
> > -		sprintf(schemata, "%lx", p->mask);
> > -		ret = write_schemata(p->ctrlgrp, schemata, p->cpu_no,
> > -				     p->resctrl_val);
> > -	}
> > -	p->num_of_runs++;
> > -
> > -	return ret;
> > -}
> > +#define MIN_DIFF_PERCENT_PER_BIT	1
> >  
> >  static int show_results_info(__u64 sum_llc_val, int no_of_bits,
> > -			     unsigned long cache_span, unsigned long max_diff,
> > -			     unsigned long max_diff_percent, unsigned long num_of_runs,
> > -			     bool platform)
> > +			     unsigned long cache_span, long min_diff_percent,
> 
> With all care taken in unsigned use I wonder why min_diff_percent is 
> just long? 

This was a leftover from the time when I still wasn't using floats so the
compare typing was easier. But's it's long gone now so I'll make that 
unsigned long.

> It looks to me as though this test impacts the affinity of main program
> since it is only one process, changes its affinity, but never change it back.

Ah, right.

It looks pre-existing problem though as despite more than one process in 
the old CAT test, it altered affinity of both of them. I'll need to look 
into fixing this.

-- 
 i.