From 0f72027bb9fb77a2b2ea7d73e6d58bb9c0efb1e8 Mon Sep 17 00:00:00 2001 From: Howard Chu Date: Wed, 30 Apr 2025 19:28:00 -0700 Subject: [PATCH] perf record --off-cpu: Parse off-cpu event Parse the off-cpu event using parse_event(), as bpf-output. Call evlist__enable_evsel() on off-cpu event. This fixes the inability to collect direct off-cpu samples on a workload, as reported by Arnaldo Carvalho de Melo . The reason being, workload sets enable_on_exec instead of calling evlist__enable(), but off-cpu event does not attach to an executable and execve won't be called, so the fds from perf_event_open() are not enabled. no-inherit should be set to 1, here's the reason: We update the BPF perf_event map for direct off-cpu sample dumping (in following patches), it executes as follows: bpf_map_update_value() bpf_fd_array_map_update_elem() perf_event_fd_array_get_ptr() perf_event_read_local() In perf_event_read_local(), there is: int perf_event_read_local(struct perf_event *event, u64 *value, u64 *enabled, u64 *running) { ... /* * It must not be an event with inherit set, we cannot read * all child counters from atomic context. */ if (event->attr.inherit) { ret = -EOPNOTSUPP; goto out; } Which means no-inherit has to be true for updating the BPF perf_event map. Moreover, for bpf-output events, we primarily want a system-wide event instead of a per-task event. The reason is that in BPF's bpf_perf_event_output(), BPF uses the CPU index to retrieve the perf_event file descriptor it outputs to. Making a bpf-output event system-wide naturally satisfies this requirement by mapping CPU appropriately. Suggested-by: Namhyung Kim Reviewed-by: Ian Rogers Signed-off-by: Howard Chu Tested-by: Arnaldo Carvalho de Melo Tested-by: Gautam Menghani Tested-by: Ian Rogers Acked-by: Namhyung Kim Cc: Adrian Hunter Cc: Alexander Shishkin Cc: Ingo Molnar Cc: James Clark Cc: Jiri Olsa Cc: Kan Liang Cc: Mark Rutland Cc: Peter Zijlstra Link: https://lore.kernel.org/r/20241108204137.2444151-4-howardchu95@gmail.com Link: https://lore.kernel.org/r/20250501022809.449767-3-howardchu95@gmail.com Signed-off-by: Arnaldo Carvalho de Melo --- tools/perf/builtin-record.c | 7 +++++++ tools/perf/util/bpf_off_cpu.c | 33 +++++++++++---------------------- tools/perf/util/evsel.c | 4 +++- 3 files changed, 21 insertions(+), 23 deletions(-) diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c index 6637a3acb1f1..4194ea5ac729 100644 --- a/tools/perf/builtin-record.c +++ b/tools/perf/builtin-record.c @@ -2568,6 +2568,13 @@ static int __cmd_record(struct record *rec, int argc, const char **argv) if (!target__none(&opts->target) && !opts->target.initial_delay) evlist__enable(rec->evlist); + /* + * offcpu-time does not call execve, so enable_on_exe wouldn't work + * when recording a workload, do it manually + */ + if (rec->off_cpu) + evlist__enable_evsel(rec->evlist, (char *)OFFCPU_EVENT); + /* * Let the child rip */ diff --git a/tools/perf/util/bpf_off_cpu.c b/tools/perf/util/bpf_off_cpu.c index 4269b41d1771..2101aa2b7c42 100644 --- a/tools/perf/util/bpf_off_cpu.c +++ b/tools/perf/util/bpf_off_cpu.c @@ -38,32 +38,21 @@ union off_cpu_data { static int off_cpu_config(struct evlist *evlist) { + char off_cpu_event[64]; struct evsel *evsel; - struct perf_event_attr attr = { - .type = PERF_TYPE_SOFTWARE, - .config = PERF_COUNT_SW_BPF_OUTPUT, - .size = sizeof(attr), /* to capture ABI version */ - }; - char *evname = strdup(OFFCPU_EVENT); - - if (evname == NULL) - return -ENOMEM; - evsel = evsel__new(&attr); - if (!evsel) { - free(evname); - return -ENOMEM; + scnprintf(off_cpu_event, sizeof(off_cpu_event), "bpf-output/name=%s/", OFFCPU_EVENT); + if (parse_event(evlist, off_cpu_event)) { + pr_err("Failed to open off-cpu event\n"); + return -1; } - evsel->core.attr.freq = 1; - evsel->core.attr.sample_period = 1; - /* off-cpu analysis depends on stack trace */ - evsel->core.attr.sample_type = PERF_SAMPLE_CALLCHAIN; - - evlist__add(evlist, evsel); - - free(evsel->name); - evsel->name = evname; + evlist__for_each_entry(evlist, evsel) { + if (evsel__is_offcpu_event(evsel)) { + evsel->core.system_wide = true; + break; + } + } return 0; } diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c index 99eee3a2c762..2415e067b892 100644 --- a/tools/perf/util/evsel.c +++ b/tools/perf/util/evsel.c @@ -1555,8 +1555,10 @@ void evsel__config(struct evsel *evsel, struct record_opts *opts, if (evsel__is_dummy_event(evsel)) evsel__reset_sample_bit(evsel, BRANCH_STACK); - if (evsel__is_offcpu_event(evsel)) + if (evsel__is_offcpu_event(evsel)) { evsel->core.attr.sample_type &= OFFCPU_SAMPLE_TYPES; + attr->inherit = 0; + } arch__post_evsel_config(evsel, attr); } -- 2.25.1