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Message-ID: <20250725093405.3629253-1-tmricht@linux.ibm.com>
Date: Fri, 25 Jul 2025 11:34:05 +0200
From: Thomas Richter <tmricht@...ux.ibm.com>
To: linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
linux-perf-users@...r.kernel.org, acme@...nel.org, namhyung@...nel.org,
bpf@...r.kernel.org
Cc: agordeev@...ux.ibm.com, gor@...ux.ibm.com, sumanthk@...ux.ibm.com,
hca@...ux.ibm.com, japo@...ux.ibm.com,
Thomas Richter <tmricht@...ux.ibm.com>,
Ilya Leoshkevich <iii@...ux.ibm.com>
Subject: [PATCH bpf] libbpf: eBPF fails on events with auxiliary data
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@...com>
To: Ian Rogers <irogers@...gle.com>
To: Ilya Leoshkevich <iii@...ux.ibm.com>
Signed-off-by: Thomas Richter <tmricht@...ux.ibm.com>
Tested-by: Ilya Leoshkevich <iii@...ux.ibm.com>
---
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;
- }
return &link->link;
err_out:
--
2.50.0
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