Hi Ilpo,
On 9/6/24 1:44 AM, Ilpo Järvinen wrote:
On Thu, 5 Sep 2024, Reinette Chatre wrote:
On 9/5/24 4:45 AM, Ilpo Järvinen wrote:
On Wed, 4 Sep 2024, Reinette Chatre wrote:
On 9/4/24 4:43 AM, Ilpo Järvinen wrote:
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@xxxxxxxxx>
---
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
Sure, will look into that.
because it does allocation -= ALLOCATION_STEP which could underflow if
the
defines are ever changed.
Is this not already covered in the following check:
if (allocation < ALLOCATION_MIN || allocation > ALLOCATION_MAX)
return END_OF_TESTS;
We are talking about hardcoded constants though.
Borderline yes. It is "covered" by the allocation > ALLOCATION_MAX but
it's also very non-intuitive to let the value underflow and then check for
that with the > operator.
My understanding is that this is the traditional way of checking overflow
(or more accurately wraparound). There are several examples of this pattern
in the kernel and it is also the pattern that I understand Linus referred
to as "traditional" in [1]. Even the compiler's intrinsic overflow checkers
do checking in this way (perform the subtraction and then check if it
overflowed) [2].
Fair enough. I've never come across that kind of claim before.
You're correct that they're constants so one would need to tweak the
source to end up into this condition in the first place.
Perhaps I'm being overly pendantic here but I in general don't like
leaving trappy and non-obvious logic like that lying around because one
day one of such will come back biting.
It is not clear to me what is "trappy" about this. The current checks will
catch the wraparound if somebody changes ALLOCATION_STEP in your scenario, no?
So, if a variable is unsigned and we ever do subtraction (or adding
negative numbers to it), I'd prefer additional check to prevent ever
underflowing it unexpectedly. Or leave them signed.
To support checking at the time of subtraction we either need to change the
flow of that function since it cannot exit with failure if that subtraction
fails because of overflow/wraparound, or we need to introduce more state that
will be an additional check that the existing
"if (allocation < ALLOCATION_MIN || allocation > ALLOCATION_MAX)"
would have caught anyway.
In either case, to do this checking at the time of subtraction the ideal way
would be to use check_sub_overflow() ... but it again does exactly what
you find to be non-intuitive since the implementation in
tools/include/linux/overflow.h uses the gcc intrinsics that does subtraction
first and then checks if overflow occurred.
It's trappy because by glance, that check looks unnecessary since
allocation starts from max and goes downwards. Also worth to note,
the check is not immediately after the decrement but done on the next
iteration.
Right. This is probably what causes most confusion.
Considering that, what do you think of below that avoids wraparound entirely:
---8<---
From: Reinette Chatre <reinette.chatre@xxxxxxxxx>
Subject: [PATCH] selftests/resctrl: Make wraparound handling obvious
Within mba_setup() the programmed bandwidth delay value starts
at the maximum (100, or rather ALLOCATION_MAX) and progresses
towards ALLOCATION_MIN by decrementing with ALLOCATION_STEP.
The programmed bandwidth delay should never be negative, so
representing it with an unsigned int is most appropriate. This
may introduce confusion because of the "allocation > ALLOCATION_MAX"
check used to check wraparound of the subtraction.
Modify the mba_setup() flow to start at the minimum, ALLOCATION_MIN,
and incrementally, with ALLOCATION_STEP steps, adjust the
bandwidth delay value. This avoids wraparound while making the purpose
of "allocation > ALLOCATION_MAX" clear and eliminates the
need for the "allocation < ALLOCATION_MIN" check.
Reported-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
---
Changes since V1:
- New patch
---
tools/testing/selftests/resctrl/mba_test.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)
diff --git a/tools/testing/selftests/resctrl/mba_test.c b/tools/testing/selftests/resctrl/mba_test.c
index ab8496a4925b..947d5699f0c8 100644
--- a/tools/testing/selftests/resctrl/mba_test.c
+++ b/tools/testing/selftests/resctrl/mba_test.c
@@ -39,7 +39,8 @@ static int mba_setup(const struct resctrl_test *test,
const struct user_params *uparams,
struct resctrl_val_param *p)
{
- static int runs_per_allocation, allocation = 100;
+ static unsigned int allocation = ALLOCATION_MIN;
+ static int runs_per_allocation = 0;
char allocation_str[64];
int ret;
@@ -50,7 +51,7 @@ static int mba_setup(const struct resctrl_test *test,
if (runs_per_allocation++ != 0)
return 0;
- if (allocation < ALLOCATION_MIN || allocation > ALLOCATION_MAX)
+ if (allocation > ALLOCATION_MAX)
return END_OF_TESTS;
sprintf(allocation_str, "%d", allocation);
@@ -59,7 +60,7 @@ static int mba_setup(const struct resctrl_test *test,
if (ret < 0)
return ret;
- allocation -= ALLOCATION_STEP;
+ allocation += ALLOCATION_STEP;
return 0;
}
@@ -72,8 +73,9 @@ static int mba_measure(const struct user_params *uparams,
static bool show_mba_info(unsigned long *bw_imc, unsigned long *bw_resc)
{
- int allocation, runs;
+ unsigned int allocation;
bool ret = false;
+ int runs;
ksft_print_msg("Results are displayed in (MB)\n");
/* Memory bandwidth from 100% down to 10% */
@@ -103,7 +105,7 @@ static bool show_mba_info(unsigned long *bw_imc, unsigned long *bw_resc)
avg_diff_per > MAX_DIFF_PERCENT ?
"Fail:" : "Pass:",
MAX_DIFF_PERCENT,
- ALLOCATION_MAX - ALLOCATION_STEP * allocation);
+ ALLOCATION_MIN + ALLOCATION_STEP * allocation);
ksft_print_msg("avg_diff_per: %d%%\n", avg_diff_per);
ksft_print_msg("avg_bw_imc: %lu\n", avg_bw_imc);
The risk here is that somebody removes allocation > ALLOCATION_MAX check.
Something called check_sub_overflow() is not subject to a similar risk
regardless of what operations occur inside it.
Reinette
