Re: [PATCH v2] perf stat: Introduce skippable evsels

[Liang, Kan]

On 2023-04-14 7:03 p.m., Ian Rogers wrote:
> On Fri, Apr 14, 2023 at 11:07 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>>
>>
>>
>> On 2023-04-14 1:19 a.m., Ian Rogers wrote:
>>> Perf stat with no arguments will use default events and metrics. These
>>> events may fail to open even with kernel and hypervisor disabled. When
>>> these fail then the permissions error appears even though they were
>>> implicitly selected. This is particularly a problem with the automatic
>>> selection of the TopdownL1 metric group on certain architectures like
>>> Skylake:
>>>
>>> ```
>>> $ perf stat true
>>> Error:
>>> Access to performance monitoring and observability operations is limited.
>>> Consider adjusting /proc/sys/kernel/perf_event_paranoid setting to open
>>> access to performance monitoring and observability operations for processes
>>> without CAP_PERFMON, CAP_SYS_PTRACE or CAP_SYS_ADMIN Linux capability.
>>> More information can be found at 'Perf events and tool security' document:
>>> https://www.kernel.org/doc/html/latest/admin-guide/perf-security.html
>>> perf_event_paranoid setting is 2:
>>>   -1: Allow use of (almost) all events by all users
>>>       Ignore mlock limit after perf_event_mlock_kb without CAP_IPC_LOCK
>>>> = 0: Disallow raw and ftrace function tracepoint access
>>>> = 1: Disallow CPU event access
>>>> = 2: Disallow kernel profiling
>>> To make the adjusted perf_event_paranoid setting permanent preserve it
>>> in /etc/sysctl.conf (e.g. kernel.perf_event_paranoid = <setting>)
>>> ```
>>>
>>> This patch adds skippable evsels that when they fail to open won't
>>> fail and won't appear in output. The TopdownL1 events, from the metric
>>> group, are marked as skippable. This turns the failure above to:
>>>
>>> ```
>>> $ perf stat true
>>>
>>>  Performance counter stats for 'true':
>>>
>>>               1.26 msec task-clock:u                     #    0.328 CPUs utilized
>>>                  0      context-switches:u               #    0.000 /sec
>>>                  0      cpu-migrations:u                 #    0.000 /sec
>>>                 49      page-faults:u                    #   38.930 K/sec
>>>            176,449      cycles:u                         #    0.140 GHz                         (48.99%)
>>
>> Multiplexing?
>>
>> Thanks,
>> Kan
> 
> I may have been running a test in the background otherwise I can't
> explain it. Repeating the test yields no multiplexing:


The above multiplexing should be on a Skylake (since there is no
topdownL1 printed), but the test which you repeat seems on a Tigerlake
(has topdownL1). Could you please double check on a Skylake?


Thanks,
Kan
> 
> ```
> $ perf stat true
> 
> Performance counter stats for 'true':
> 
>              0.78 msec task-clock:u                     #    0.383
> CPUs utilized
>                 0      context-switches:u               #    0.000
> /sec
>                 0      cpu-migrations:u                 #    0.000
> /sec
>                47      page-faults:u                    #   60.174
> K/sec
>           233,420      cycles:u                         #    0.299 GHz
>           133,318      instructions:u                   #    0.57
> insn per cycle
>            31,396      branches:u                       #   40.196
> M/sec
>             2,334      branch-misses:u                  #    7.43% of
> all branches
>         1,167,100      TOPDOWN.SLOTS:u                  #     12.2 %
> tma_retiring
>                                                  #     28.9 %
> tma_backend_bound
>                                                  #     41.0 %
> tma_frontend_bound
>                                                  #     18.0 %
> tma_bad_speculation
>           141,882      topdown-retiring:u
>           480,570      topdown-fe-bound:u
>           320,380      topdown-be-bound:u
>           224,266      topdown-bad-spec:u
>             2,173      INT_MISC.UOP_DROPPING:u          #    2.782
> M/sec
>             3,323      cpu/INT_MISC.RECOVERY_CYCLES,cmask=1,edge/u #
>  4.254 M/sec
> 
> 
>       0.002036744 seconds time elapsed
> 
>       0.002252000 seconds user
>       0.000000000 seconds sys
> ```
> 
> Thanks,
> Ian
> 
>>>            122,905      instructions:u                   #    0.70  insn per cycle
>>>             28,264      branches:u                       #   22.456 M/sec
>>>              2,405      branch-misses:u                  #    8.51% of all branches
>>>
>>>        0.003834565 seconds time elapsed
>>>
>>>        0.000000000 seconds user
>>>        0.004130000 seconds sys
>>> ```
>>>
>>> When the events can have kernel/hypervisor disabled, like on
>>> Tigerlake, then it continues to succeed as:
>>>
>>> ```
>>> $ perf stat true
>>>
>>>  Performance counter stats for 'true':
>>>
>>>               0.57 msec task-clock:u                     #    0.385 CPUs utilized
>>>                  0      context-switches:u               #    0.000 /sec
>>>                  0      cpu-migrations:u                 #    0.000 /sec
>>>                 47      page-faults:u                    #   82.329 K/sec
>>>            287,017      cycles:u                         #    0.503 GHz
>>>            133,318      instructions:u                   #    0.46  insn per cycle
>>>             31,396      branches:u                       #   54.996 M/sec
>>>              2,442      branch-misses:u                  #    7.78% of all branches
>>>            998,790      TOPDOWN.SLOTS:u                  #     14.5 %  tma_retiring
>>>                                                   #     27.6 %  tma_backend_bound
>>>                                                   #     40.9 %  tma_frontend_bound
>>>                                                   #     17.0 %  tma_bad_speculation
>>>            144,922      topdown-retiring:u
>>>            411,266      topdown-fe-bound:u
>>>            258,510      topdown-be-bound:u
>>>            184,090      topdown-bad-spec:u
>>>              2,585      INT_MISC.UOP_DROPPING:u          #    4.528 M/sec
>>>              3,434      cpu/INT_MISC.RECOVERY_CYCLES,cmask=1,edge/u #    6.015 M/sec
>>>
>>>        0.001480954 seconds time elapsed
>>>
>>>        0.000000000 seconds user
>>>        0.001686000 seconds sys
>>> ```
>>>
>>> And this likewise works if paranoia allows or running as root.
>>>
>>> v2. Don't display the skipped events as <not counted> or <not supported>.
>>>
>>> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
>>> ---
>>>  tools/perf/builtin-stat.c      | 39 ++++++++++++++++++++++++++--------
>>>  tools/perf/util/evsel.c        | 15 +++++++++++--
>>>  tools/perf/util/evsel.h        |  1 +
>>>  tools/perf/util/stat-display.c |  4 ++++
>>>  4 files changed, 48 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
>>> index d3cbee7460fc..7a641a67486d 100644
>>> --- a/tools/perf/builtin-stat.c
>>> +++ b/tools/perf/builtin-stat.c
>>> @@ -667,6 +667,13 @@ static enum counter_recovery stat_handle_error(struct evsel *counter)
>>>                       evsel_list->core.threads->err_thread = -1;
>>>                       return COUNTER_RETRY;
>>>               }
>>> +     } else if (counter->skippable) {
>>> +             if (verbose > 0)
>>> +                     ui__warning("skipping event %s that kernel failed to open .\n",
>>> +                                 evsel__name(counter));
>>> +             counter->supported = false;
>>> +             counter->errored = true;
>>> +             return COUNTER_SKIP;
>>>       }
>>>
>>>       evsel__open_strerror(counter, &target, errno, msg, sizeof(msg));
>>> @@ -1885,15 +1892,29 @@ static int add_default_attributes(void)
>>>                * Add TopdownL1 metrics if they exist. To minimize
>>>                * multiplexing, don't request threshold computation.
>>>                */
>>> -             if (metricgroup__has_metric("TopdownL1") &&
>>> -                 metricgroup__parse_groups(evsel_list, "TopdownL1",
>>> -                                         /*metric_no_group=*/false,
>>> -                                         /*metric_no_merge=*/false,
>>> -                                         /*metric_no_threshold=*/true,
>>> -                                         stat_config.user_requested_cpu_list,
>>> -                                         stat_config.system_wide,
>>> -                                         &stat_config.metric_events) < 0)
>>> -                     return -1;
>>> +             if (metricgroup__has_metric("TopdownL1")) {
>>> +                     struct evlist *metric_evlist = evlist__new();
>>> +                     struct evsel *metric_evsel;
>>> +
>>> +                     if (!metric_evlist)
>>> +                             return -1;
>>> +
>>> +                     if (metricgroup__parse_groups(metric_evlist, "TopdownL1",
>>> +                                                     /*metric_no_group=*/false,
>>> +                                                     /*metric_no_merge=*/false,
>>> +                                                     /*metric_no_threshold=*/true,
>>> +                                                     stat_config.user_requested_cpu_list,
>>> +                                                     stat_config.system_wide,
>>> +                                                     &stat_config.metric_events) < 0)
>>> +                             return -1;
>>> +
>>> +                     evlist__for_each_entry(metric_evlist, metric_evsel) {
>>> +                             metric_evsel->skippable = true;
>>> +                     }
>>> +                     evlist__splice_list_tail(evsel_list, &metric_evlist->core.entries);
>>> +                     evlist__delete(metric_evlist);
>>> +             }
>>> +
>>>               /* Platform specific attrs */
>>>               if (evlist__add_default_attrs(evsel_list, default_null_attrs) < 0)
>>>                       return -1;
>>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
>>> index a85a987128aa..83a65f771666 100644
>>> --- a/tools/perf/util/evsel.c
>>> +++ b/tools/perf/util/evsel.c
>>> @@ -290,6 +290,7 @@ void evsel__init(struct evsel *evsel,
>>>       evsel->per_pkg_mask  = NULL;
>>>       evsel->collect_stat  = false;
>>>       evsel->pmu_name      = NULL;
>>> +     evsel->skippable     = false;
>>>  }
>>>
>>>  struct evsel *evsel__new_idx(struct perf_event_attr *attr, int idx)
>>> @@ -1720,9 +1721,13 @@ static int get_group_fd(struct evsel *evsel, int cpu_map_idx, int thread)
>>>               return -1;
>>>
>>>       fd = FD(leader, cpu_map_idx, thread);
>>> -     BUG_ON(fd == -1);
>>> +     BUG_ON(fd == -1 && !leader->skippable);
>>>
>>> -     return fd;
>>> +     /*
>>> +      * When the leader has been skipped, return -2 to distinguish from no
>>> +      * group leader case.
>>> +      */
>>> +     return fd == -1 ? -2 : fd;
>>>  }
>>>
>>>  static void evsel__remove_fd(struct evsel *pos, int nr_cpus, int nr_threads, int thread_idx)
>>> @@ -2104,6 +2109,12 @@ static int evsel__open_cpu(struct evsel *evsel, struct perf_cpu_map *cpus,
>>>
>>>                       group_fd = get_group_fd(evsel, idx, thread);
>>>
>>> +                     if (group_fd == -2) {
>>> +                             pr_debug("broken group leader for %s\n", evsel->name);
>>> +                             err = -EINVAL;
>>> +                             goto out_close;
>>> +                     }
>>> +
>>>                       test_attr__ready();
>>>
>>>                       /* Debug message used by test scripts */
>>> diff --git a/tools/perf/util/evsel.h b/tools/perf/util/evsel.h
>>> index 68072ec655ce..98afe3351176 100644
>>> --- a/tools/perf/util/evsel.h
>>> +++ b/tools/perf/util/evsel.h
>>> @@ -95,6 +95,7 @@ struct evsel {
>>>               bool                    weak_group;
>>>               bool                    bpf_counter;
>>>               bool                    use_config_name;
>>> +             bool                    skippable;
>>>               int                     bpf_fd;
>>>               struct bpf_object       *bpf_obj;
>>>               struct list_head        config_terms;
>>> diff --git a/tools/perf/util/stat-display.c b/tools/perf/util/stat-display.c
>>> index e6035ecbeee8..6b46bbb3d322 100644
>>> --- a/tools/perf/util/stat-display.c
>>> +++ b/tools/perf/util/stat-display.c
>>> @@ -810,6 +810,10 @@ static bool should_skip_zero_counter(struct perf_stat_config *config,
>>>       struct perf_cpu cpu;
>>>       int idx;
>>>
>>> +     /* Skip counters that were speculatively/default enabled rather than requested. */
>>> +     if (counter->skippable)
>>> +             return true;
>>> +
>>>       /*
>>>        * Skip value 0 when enabling --per-thread globally,
>>>        * otherwise it will have too many 0 output.