On Fri, Jul 25, 2025 at 11:34:05AM +0200, Thomas Richter wrote:
> On linux-next
> commit b4c658d4d63d61 ("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. This happens now after enabling the event:
>
> 2. The event's fd is mmap() 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() returns 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 call backs setup_aux() and
> free_aux() are defined. Without both call back 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 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: 63f2f5ee856ba ("libbpf: add ability to attach/detach BPF program to perf event")
> To: Andrii Nakryiko <andriin@xxxxxx>
> To: Ian Rogers <irogers@xxxxxxxxxx>
> To: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
> Signed-off-by: Thomas Richter <tmricht@xxxxxxxxxxxxx>
> Tested-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
> ---
> tools/lib/bpf/libbpf.c | 6 ------
> 1 file changed, 6 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 162ebd16a59f..5973412a1031 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -10960,12 +10960,6 @@ struct bpf_link *bpf_program__attach_perf_event_opts(const struct bpf_program *p
> }
> link->link.fd = pfd;
> }
> - if (ioctl(pfd, PERF_EVENT_IOC_ENABLE, 0) < 0) {
> - err = -errno;
> - pr_warn("prog '%s': failed to enable perf_event FD %d: %s\n",
> - prog->name, pfd, errstr(err));
> - goto err_out;
> - }
I think this might break existing users depending on this
could we instead add some 'enable' flag to bpf_perf_event_opts and perf
would use bpf_program__attach_perf_event_opts function instead?
jirka
