2022-05-09 10:09:42

by Ravi Bangoria

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

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 <[email protected]>
---
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;
+
+ 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);
}

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

2022-05-16 17:42:34

by Arnaldo Carvalho de Melo

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

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 <[email protected]>
> ---
> 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

2022-05-17 02:28:16

by Ravi Bangoria

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

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 <[email protected]>
>> ---
>> 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