Re: [PATCH v5 1/8] perf record ibs: Warn about sampling period skew

[Namhyung Kim]

Hi Ravi,

On Tue, May 31, 2022 at 8:27 PM Ravi Bangoria <ravi.bangoria@xxxxxxx> wrote:
>
> 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@xxxxxxx>
> Acked-by: Ian Rogers <irogers@xxxxxxxxxx>
> ---
>  tools/perf/arch/x86/util/evsel.c | 49 ++++++++++++++++++++++++++++++++
>  tools/perf/util/evsel.c          |  7 +++++
>  tools/perf/util/evsel.h          |  1 +
>  3 files changed, 57 insertions(+)
>
> diff --git a/tools/perf/arch/x86/util/evsel.c b/tools/perf/arch/x86/util/evsel.c
> index 88306183d629..fceb904902ab 100644
> --- a/tools/perf/arch/x86/util/evsel.c
> +++ b/tools/perf/arch/x86/util/evsel.c
> @@ -5,6 +5,7 @@
>  #include "util/env.h"
>  #include "util/pmu.h"
>  #include "linux/string.h"
> +#include "util/debug.h"
>
>  void arch_evsel__set_sample_weight(struct evsel *evsel)
>  {
> @@ -60,3 +61,51 @@ bool arch_evsel__must_be_in_group(const struct evsel *evsel)
>                 (!strcasecmp(evsel->name, "slots") ||
>                  strcasestr(evsel->name, "topdown"));
>  }
> +
> +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__post_evsel_config(struct evsel *evsel, struct perf_event_attr *attr)
> +{
> +       struct perf_pmu *evsel_pmu, *ibs_fetch_pmu, *ibs_op_pmu;
> +       static int warned_once;
> +       /* 0: Uninitialized, 1: Yes, -1: No */
> +       static int is_amd;
> +
> +       if (warned_once || is_amd == -1)
> +               return;
> +
> +       if (!is_amd) {
> +               struct perf_env *env = evsel__env(evsel);
> +
> +               if (!perf_env__cpuid(env) || !env->cpuid ||
> +                   !strstarts(env->cpuid, "AuthenticAMD")) {
> +                       is_amd = -1;
> +                       return;
> +               }
> +               is_amd = 1;
> +       }
> +
> +       evsel_pmu = evsel__find_pmu(evsel);
> +       if (!evsel_pmu)
> +               return;
> +
> +       ibs_fetch_pmu = perf_pmu__find("ibs_fetch");
> +       ibs_op_pmu = perf_pmu__find("ibs_op");
> +
> +       if (ibs_fetch_pmu && ibs_fetch_pmu->type == evsel_pmu->type) {
> +               if (attr->config & (1ULL << 59)) {

It'd be nice if we used a macro or something instead of the
magic number.

> +                       ibs_l3miss_warn();
> +                       warned_once = 1;
> +               }
> +       } else if (ibs_op_pmu && ibs_op_pmu->type == evsel_pmu->type) {
> +               if (attr->config & (1ULL << 16)) {

Ditto.

Thanks,
Namhyung


> +                       ibs_l3miss_warn();
> +                       warned_once = 1;
> +               }
> +       }
> +}
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index ce499c5da8d7..8fea51a9cd90 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -1091,6 +1091,11 @@ void __weak arch_evsel__fixup_new_cycles(struct perf_event_attr *attr __maybe_un
>  {
>  }
>
> +void __weak arch__post_evsel_config(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)
>  {
> @@ -1366,6 +1371,8 @@ void evsel__config(struct evsel *evsel, struct record_opts *opts,
>          */
>         if (evsel__is_dummy_event(evsel))
>                 evsel__reset_sample_bit(evsel, BRANCH_STACK);
> +
> +       arch__post_evsel_config(evsel, attr);
>  }
>
>  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 73ea48e94079..92bed8e2f7d8 100644
> --- a/tools/perf/util/evsel.h
> +++ b/tools/perf/util/evsel.h
> @@ -297,6 +297,7 @@ 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__post_evsel_config(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);
> --
> 2.31.1
>