From 5e2ac8e8571df54d0a9c9d08f287e006269a6674 Mon Sep 17 00:00:00 2001 From: Ilya Leoshkevich Date: Wed, 6 Aug 2025 18:22:42 +0200 Subject: [PATCH] perf bpf-filter: Enable events manually On s390, and, in general, on all platforms where the respective event supports auxiliary data gathering, the command: # ./perf record -u 0 -aB --synth=no -- ./perf test -w thloop [ perf record: Woken up 1 times to write data ] [ perf record: Captured and wrote 0.011 MB perf.data ] # ./perf report --stats | grep SAMPLE # does not generate samples in the perf.data file. On x86 the command: # sudo perf record -e intel_pt// -u 0 ls is broken too. Looking at the sequence of calls in 'perf record' reveals this behavior: 1. The event 'cycles' is created and enabled: record__open() +-> evlist__apply_filters() +-> perf_bpf_filter__prepare() +-> bpf_program.attach_perf_event() +-> bpf_program.attach_perf_event_opts() +-> __GI___ioctl(..., PERF_EVENT_IOC_ENABLE, ...) The event 'cycles' is enabled and active now. However the event's ring-buffer to store the samples generated by hardware is not allocated yet. 2. The event's fd is mmap()ed to create the ring buffer: record__open() +-> record__mmap() +-> record__mmap_evlist() +-> evlist__mmap_ex() +-> perf_evlist__mmap_ops() +-> mmap_per_cpu() +-> mmap_per_evsel() +-> mmap__mmap() +-> perf_mmap__mmap() +-> mmap() This allocates the ring buffer for the event 'cycles'. With mmap() the kernel creates the ring buffer: perf_mmap(): kernel function to create the event's ring | buffer to save the sampled data. | +-> ring_buffer_attach(): Allocates memory for ring buffer. | The PMU has auxiliary data setup function. The | has_aux(event) condition is true and the PMU's | stop() is called to stop sampling. It is not | restarted: | | if (has_aux(event)) | perf_event_stop(event, 0); | +-> cpumsf_pmu_stop(): Hardware sampling is stopped. No samples are generated and saved anymore. 3. After the event 'cycles' has been mapped, the event is enabled a second time in: __cmd_record() +-> evlist__enable() +-> __evlist__enable() +-> evsel__enable_cpu() +-> perf_evsel__enable_cpu() +-> perf_evsel__run_ioctl() +-> perf_evsel__ioctl() +-> __GI___ioctl(., PERF_EVENT_IOC_ENABLE, .) The second ioctl(fd, PERF_EVENT_IOC_ENABLE, 0); is just a NOP in this case. The first invocation in (1.) sets the event::state to PERF_EVENT_STATE_ACTIVE. The kernel functions perf_ioctl() +-> _perf_ioctl() +-> _perf_event_enable() +-> __perf_event_enable() return immediately because event::state is already set to PERF_EVENT_STATE_ACTIVE. This happens on s390, because the event 'cycles' offers the possibility to save auxilary data. The PMU callbacks setup_aux() and free_aux() are defined. Without both callback functions, cpumsf_pmu_stop() is not invoked and sampling continues. To remedy this, remove the first invocation of ioctl(..., PERF_EVENT_IOC_ENABLE, ...). in step (1.) Create the event in step (1.) and enable it in step (3.) after the ring buffer has been mapped. Output after: # ./perf record -aB --synth=no -u 0 -- ./perf test -w thloop 2 [ perf record: Woken up 3 times to write data ] [ perf record: Captured and wrote 0.876 MB perf.data ] # ./perf report --stats | grep SAMPLE SAMPLE events: 16200 (99.5%) SAMPLE events: 16200 # The software event succeeded both before and after the patch: # ./perf record -e cpu-clock -aB --synth=no -u 0 -- \ ./perf test -w thloop 2 [ perf record: Woken up 7 times to write data ] [ perf record: Captured and wrote 2.870 MB perf.data ] # ./perf report --stats | grep SAMPLE SAMPLE events: 53506 (99.8%) SAMPLE events: 53506 # Fixes: b4c658d4d63d61 ("perf target: Remove uid from target") Suggested-by: Jiri Olsa Tested-by: Thomas Richter Acked-by: Namhyung Kim Co-developed-by: Thomas Richter Signed-off-by: Thomas Richter Signed-off-by: Ilya Leoshkevich Link: https://lore.kernel.org/r/20250806162417.19666-3-iii@linux.ibm.com Signed-off-by: Alexei Starovoitov --- tools/perf/util/bpf-filter.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/tools/perf/util/bpf-filter.c b/tools/perf/util/bpf-filter.c index d0e013eeb0f7..a0b11f35395f 100644 --- a/tools/perf/util/bpf-filter.c +++ b/tools/perf/util/bpf-filter.c @@ -451,6 +451,8 @@ int perf_bpf_filter__prepare(struct evsel *evsel, struct target *target) struct bpf_link *link; struct perf_bpf_filter_entry *entry; bool needs_idx_hash = !target__has_cpu(target); + DECLARE_LIBBPF_OPTS(bpf_perf_event_opts, pe_opts, + .dont_enable = true); entry = calloc(MAX_FILTERS, sizeof(*entry)); if (entry == NULL) @@ -522,7 +524,8 @@ int perf_bpf_filter__prepare(struct evsel *evsel, struct target *target) prog = skel->progs.perf_sample_filter; for (x = 0; x < xyarray__max_x(evsel->core.fd); x++) { for (y = 0; y < xyarray__max_y(evsel->core.fd); y++) { - link = bpf_program__attach_perf_event(prog, FD(evsel, x, y)); + link = bpf_program__attach_perf_event_opts(prog, FD(evsel, x, y), + &pe_opts); if (IS_ERR(link)) { pr_err("Failed to attach perf sample-filter program\n"); ret = PTR_ERR(link); -- 2.47.2