Re: [PATCH v2] perf stat: Introduce skippable evsels
From: Liang, Kan
Date: Fri Apr 14 2023 - 14:07:46 EST
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
> 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.