Re: [PATCH 16/24] selftests/resctrl: Rewrite Cache Allocation Technology (CAT) test

[Ilpo Järvinen]

On Thu, 2 Nov 2023, Reinette Chatre wrote:
> On 10/24/2023 2:26 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 allocated 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.
> 
> Thank you very much for doing this.
> 
> > 
> > Suggested-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> > Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> > ---
> >  tools/testing/selftests/resctrl/cat_test.c  | 286 +++++++++-----------
> >  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, 137 insertions(+), 204 deletions(-)
> > 
> > diff --git a/tools/testing/selftests/resctrl/cat_test.c b/tools/testing/selftests/resctrl/cat_test.c
> > index e71690a9bbb3..7518c520c5cc 100644
> > --- a/tools/testing/selftests/resctrl/cat_test.c
> > +++ b/tools/testing/selftests/resctrl/cat_test.c
> > @@ -11,65 +11,68 @@
> >  #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 mask to
> > + * the test with n bits. 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.
> 
> This formula is not clear to me. In the code the formula is always:
> MIN_DIFF_PERCENT_PER_BIT * (bits - 1) ... is the "-1" because it always
> decrements the number of bits tested by one?

No, -1 is not related to decrementing bits by one, but setting a boundary 
which workds for 1 bit masks. In general,  the smaller the number of bits 
in the allocation mask is, less change there will be between n+1 -> n bits
results. When n is 1, the result with some platforms is close to zero so I 
just had to make the min diff to allow it. Thus, n-1 to set the failure 
threshold at 0%.

> So, for example, if testing
> 5 then 3 bits it would be  MIN_DIFF_PERCENT_PER_BIT * (3 - 2)?
> Would above example thus be:
> MIN_DIFF_PERCENT_PER_BIT * (4 - (5 - 4)) = 3 ?

I suspect you're overthinking it here. The CAT selftest currently doesn't 
jump from 5 to 3 bits so I don't know what you're trying to calculate 
here.

> > - * Return:	0 on success. non-zero on failure.
> > + * Return:		0 on success. non-zero on failure.
> 
> Is non-zero specific enough? Does that mean that <0 and >0 are failure?

I suspect it is, after all the cleanups and fixes that have been done.
The wording is from the original though.

> >   */
> > -static int cat_test(struct resctrl_val_param *param, size_t span)
> > +static int cat_test(struct resctrl_val_param *param, size_t span, unsigned long current_mask)
> >  {
> > -	int memflush = 1, operation = 0, ret = 0;
> >  	char *resctrl_val = param->resctrl_val;
> >  	static struct perf_event_read pe_read;
> >  	struct perf_event_attr pea;
> > +	unsigned char *buf;
> > +	char schemata[64];
> > +	int ret, i, pe_fd;
> >  	pid_t bm_pid;
> > -	int pe_fd;
> >  
> >  	if (strcmp(param->filename, "") == 0)
> >  		sprintf(param->filename, "stdio");
> > @@ -143,54 +168,64 @@ static int cat_test(struct resctrl_val_param *param, size_t span)
> >  	if (ret)
> >  		return ret;
> >  
> > +	buf = alloc_buffer(span, 1);
> > +	if (buf == NULL)
> > +		return -1;
> > +
> >  	perf_event_attr_initialize(&pea, PERF_COUNT_HW_CACHE_MISSES);
> >  	perf_event_initialize_read_format(&pe_read);
> >  
> > -	/* Test runs until the callback setup() tells the test to stop. */
> > -	while (1) {
> > -		ret = param->setup(param);
> > -		if (ret == END_OF_TESTS) {
> > -			ret = 0;
> > -			break;
> > -		}
> > -		if (ret < 0)
> > -			break;
> > -		pe_fd = perf_event_reset_enable(&pea, bm_pid, param->cpu_no);
> > -		if (pe_fd < 0) {
> > -			ret = -1;
> > -			break;
> > -		}
> > +	while (current_mask) {
> > +		snprintf(schemata, sizeof(schemata), "%lx", param->mask & ~current_mask);
> > +		ret = write_schemata("", schemata, param->cpu_no, param->resctrl_val);
> > +		if (ret)
> > +			goto free_buf;
> > +		snprintf(schemata, sizeof(schemata), "%lx", current_mask);
> > +		ret = write_schemata(param->ctrlgrp, schemata, param->cpu_no, param->resctrl_val);
> > +		if (ret)
> > +			goto free_buf;
> > +
> > +		for (i = 0; i < NUM_OF_RUNS; i++) {
> > +			mem_flush(buf, span);
> > +			ret = fill_cache_read(buf, span, true);
> > +			if (ret)
> > +				goto free_buf;
> > +
> > +			pe_fd = perf_event_reset_enable(&pea, bm_pid, param->cpu_no);
> > +			if (pe_fd < 0) {
> > +				ret = -1;
> > +				goto free_buf;
> > +			}
> 
> It seems to me that the perf counters are reconfigured at every iteration.
> Can it not just be configured once and then the counters just reset and
> enabled at each iteration? I'd expect this additional work to impact
> values measured.

So you suggest I undo one of the changes made in 10/24 and just call the 
function which does only the ioctl() calls? I don't know why it has been 
done the way it has been, I can try to change it and see what happens.


-- 
 i.