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Message-ID: <aJPc2NvJqLOGaIKl@google.com>
Date: Wed, 6 Aug 2025 15:53:12 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Ilya Leoshkevich <iii@...ux.ibm.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Ian Rogers <irogers@...gle.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>, bpf@...r.kernel.org,
linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-s390@...r.kernel.org, Thomas Richter <tmricht@...ux.ibm.com>,
Jiri Olsa <jolsa@...nel.org>, Heiko Carstens <hca@...ux.ibm.com>,
Vasily Gorbik <gor@...ux.ibm.com>,
Alexander Gordeev <agordeev@...ux.ibm.com>
Subject: Re: [PATCH v4 2/2] perf bpf-filter: Enable events manually
Hello,
On Wed, Aug 06, 2025 at 01:40:35PM +0200, Ilya Leoshkevich wrote:
> 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 <jolsa@...nel.org>
> Tested-by: Thomas Richter <tmricht@...ux.ibm.com>
> Co-developed-by: Thomas Richter <tmricht@...ux.ibm.com>
> Signed-off-by: Thomas Richter <tmricht@...ux.ibm.com>
> Signed-off-by: Ilya Leoshkevich <iii@...ux.ibm.com>
Acked-by: Namhyung Kim <namhyung@...nel.org>
Thanks,
Namhyung
> ---
> 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);
> --
> 2.50.1
>
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