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Message-ID: <20250806162417.19666-3-iii@linux.ibm.com>
Date: Wed, 6 Aug 2025 18:22:42 +0200
From: Ilya Leoshkevich <iii@...ux.ibm.com>
To: 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>
Cc: 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>,
Ilya Leoshkevich <iii@...ux.ibm.com>
Subject: [PATCH v5 2/2] 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 <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>
---
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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