Re: [PATCH v2 5/8] perf record ibs: Warn about sampling period skew

Ravi Bangoria
2022-05-16T13:27:41Z

On 16-May-22 6:52 PM, Arnaldo Carvalho de Melo wrote:
> Em Mon, May 09, 2022 at 10:19:11AM +0530, Ravi Bangoria escreveu:
>> Samples without an L3 miss are discarded and counter is reset with
>> random value (between 1-15 for fetch pmu and 1-127 for op pmu) when
>> IBS L3 miss filtering is enabled. This causes a sampling period skew
>> but there is no way to reconstruct aggregated sampling period. So
>> print a warning at perf record if user sets l3missonly=1.
>>
>> Ex:
>> # perf record -c 10000 -C 0 -e ibs_op/l3missonly=1/
>> WARNING: Hw internally resets sampling period when L3 Miss Filtering is enabled
>> and tagged operation does not cause L3 Miss. This causes sampling period skew.
>>
>> Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
>> ---
>> tools/perf/arch/x86/util/evsel.c | 34 ++++++++++++++++++++++++++++++++
>> tools/perf/util/evsel.c | 7 +++++++
>> tools/perf/util/evsel.h | 1 +
>> 3 files changed, 42 insertions(+)
>>
>> diff --git a/tools/perf/arch/x86/util/evsel.c b/tools/perf/arch/x86/util/evsel.c
>> index ac2899a25b7a..6399faa70a88 100644
>> --- a/tools/perf/arch/x86/util/evsel.c
>> +++ b/tools/perf/arch/x86/util/evsel.c
>> @@ -4,6 +4,8 @@
>> #include "util/evsel.h"
>> #include "util/env.h"
>> #include "linux/string.h"
>> +#include "util/pmu.h"
>> +#include "util/debug.h"
>>
>> void arch_evsel__set_sample_weight(struct evsel *evsel)
>> {
>> @@ -29,3 +31,35 @@ void arch_evsel__fixup_new_cycles(struct perf_event_attr *attr)
>>
>> free(env.cpuid);
>> }
>> +
>> +static void ibs_l3miss_warn(void)
>> +{
>> + pr_warning(
>> +"WARNING: Hw internally resets sampling period when L3 Miss Filtering is enabled\n"
>> +"and tagged operation does not cause L3 Miss. This causes sampling period skew.\n");
>> +}
>> +
>> +void arch_evsel__warn_ambiguity(struct evsel *evsel, struct perf_event_attr *attr)
>> +{
>> + struct perf_env *env = evsel__env(evsel);
>> + struct perf_pmu *evsel_pmu = evsel__find_pmu(evsel);
>> + struct perf_pmu *ibs_fetch_pmu = perf_pmu__find("ibs_fetch");
>> + struct perf_pmu *ibs_op_pmu = perf_pmu__find("ibs_op");
>> + static int warned_once;
>
> Please check first if the warning was emitted (warned_once is true)
> before calling all the find routines above.

Sure.

>
>> + if (warned_once || !perf_env__cpuid(env) || !env->cpuid ||
>> + !strstarts(env->cpuid, "AuthenticAMD") || !evsel_pmu)
>> + return;
>> +
>> + if (ibs_fetch_pmu && ibs_fetch_pmu->type == evsel_pmu->type) {
>> + if (attr->config & (1ULL << 59)) {
>> + ibs_l3miss_warn();
>> + warned_once = 1;
>> + }
>> + } else if (ibs_op_pmu && ibs_op_pmu->type == evsel_pmu->type) {
>> + if (attr->config & (1ULL << 16)) {
>> + ibs_l3miss_warn();
>> + warned_once = 1;
>> + }
>> + }
>> +}
>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
>> index 2a1729e7aee4..4f8b72d4a521 100644
>> --- a/tools/perf/util/evsel.c
>> +++ b/tools/perf/util/evsel.c
>> @@ -1064,6 +1064,11 @@ void __weak arch_evsel__fixup_new_cycles(struct perf_event_attr *attr __maybe_un
>> {
>> }
>>
>> +void __weak arch_evsel__warn_ambiguity(struct evsel *evsel __maybe_unused,
>> + struct perf_event_attr *attr __maybe_unused)
>> +{
>> +}
>> +
>> static void evsel__set_default_freq_period(struct record_opts *opts,
>> struct perf_event_attr *attr)
>> {
>> @@ -1339,6 +1344,8 @@ void evsel__config(struct evsel *evsel, struct record_opts *opts,
>> */
>> if (evsel__is_dummy_event(evsel))
>> evsel__reset_sample_bit(evsel, BRANCH_STACK);
>> +
>> + arch_evsel__warn_ambiguity(evsel, attr);
>
> Wouldn't this be better as a single arch__post_evsel_config() function that
> could do arch specific fixups or emit such warnings _after_ (thus the
> "post") the common code evsel__config() does its thing?

Will rename accordingly.

Thanks,
Ravi