linux-kernel - Re: [PATCH v3 1/5] perf record ibs: Warn about sampling period skew

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Date:   Thu, 19 May 2022 15:14:32 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Ravi Bangoria <ravi.bangoria@....com>
Cc:     acme@...nel.org, peterz@...radead.org, rrichter@....com,
        mingo@...hat.com, mark.rutland@....com, jolsa@...nel.org,
        namhyung@...nel.org, tglx@...utronix.de, bp@...en8.de,
        james.clark@....com, leo.yan@...aro.org, kan.liang@...ux.intel.com,
        ak@...ux.intel.com, eranian@...gle.com, like.xu.linux@...il.com,
        x86@...nel.org, linux-perf-users@...r.kernel.org,
        linux-kernel@...r.kernel.org, sandipan.das@....com,
        ananth.narayan@....com, kim.phillips@....com,
        santosh.shukla@....com
Subject: Re: [PATCH v3 1/5] perf record ibs: Warn about sampling period skew

On Wed, May 18, 2022 at 10:44 PM Ravi Bangoria <ravi.bangoria@....com> 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@....com>

Acked-by: Ian Rogers <irogers@...gle.com>

Thanks,
Ian

> ---
>  tools/perf/arch/x86/util/evsel.c | 50 ++++++++++++++++++++++++++++++++
>  tools/perf/util/evsel.c          |  7 +++++
>  tools/perf/util/evsel.h          |  1 +
>  3 files changed, 58 insertions(+)
>
> diff --git a/tools/perf/arch/x86/util/evsel.c b/tools/perf/arch/x86/util/evsel.c
> index ac2899a25b7a..e8ff4ddb53c9 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 +31,51 @@ 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__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)) {
> +                       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..e9be9b94a062 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__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)
>  {
> @@ -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__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 041b42d33bf5..207de5082ee9 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__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);
> --
> 2.27.0
>