perf/s390: Regression: Move uid filtering to BPF filters #5684
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Automatically enabling a perf event after attaching a BPF prog to it is not always desirable. Add a new no_ioctl_enable field to struct bpf_perf_event_opts. While introducing ioctl_enable instead would be nicer in that it would avoid a double negation in the implementation, it would make DECLARE_LIBBPF_OPTS() less efficient. Suggested-by: Jiri Olsa <[email protected]> Co-developed-by: Thomas Richter <[email protected]> Signed-off-by: Thomas Richter <[email protected]> Signed-off-by: Ilya Leoshkevich <[email protected]> Acked-by: Eduard Zingerman <[email protected]>
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On linux-next commit b4c658d ("perf target: Remove uid from target") introduces a regression on s390. In fact the regression exists on all platforms when the event supports auxiliary data gathering. 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 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: 63f2f5e ("libbpf: add ability to attach/detach BPF program to perf event") Suggested-by: Jiri Olsa <[email protected]> Co-developed-by: Thomas Richter <[email protected]> Signed-off-by: Thomas Richter <[email protected]> Signed-off-by: Ilya Leoshkevich <[email protected]>
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Pull request for series with
subject: perf/s390: Regression: Move uid filtering to BPF filters
version: 3
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=988450