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

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Date:   Mon, 16 May 2022 18:57:11 +0530
From:   Ravi Bangoria <ravi.bangoria@....com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     peterz@...radead.org, rrichter@....com, mingo@...hat.com,
        mark.rutland@....com, jolsa@...nel.org, namhyung@...nel.org,
        tglx@...utronix.de, bp@...en8.de, irogers@...gle.com,
        yao.jin@...ux.intel.com, 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, Ravi Bangoria <ravi.bangoria@....com>
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 <ravi.bangoria@....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.

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