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Message-ID: <CAP-5=fU_=v8CXNg5ovK2XcFv4e4KSXNLaP-7k9y1yqQ_NhzgSg@mail.gmail.com>
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
>
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