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

Arnaldo Carvalho de Melo
2022-05-16T13:22:10Z

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.

> + 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?

> }
>
> int evsel__set_filter(struct evsel *evsel, const char *filter)
> diff --git a/tools/perf/util/evsel.h b/tools/perf/util/evsel.h
> index 041b42d33bf5..195ae30ec45b 100644
> --- a/tools/perf/util/evsel.h
> +++ b/tools/perf/util/evsel.h
> @@ -281,6 +281,7 @@ void evsel__set_sample_id(struct evsel *evsel, bool use_sample_identifier);
>
> void arch_evsel__set_sample_weight(struct evsel *evsel);
> void arch_evsel__fixup_new_cycles(struct perf_event_attr *attr);
> +void arch_evsel__warn_ambiguity(struct evsel *evsel, struct perf_event_attr *attr);
>
> int evsel__set_filter(struct evsel *evsel, const char *filter);
> int evsel__append_tp_filter(struct evsel *evsel, const char *filter);
> --
> 2.27.0

--

- Arnaldo