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Message-ID: <20250728144340.711196-1-tmricht@linux.ibm.com>
Date: Mon, 28 Jul 2025 16:43:40 +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,
andriin@...com, irogers@...gle.com, iii@...ux.ibm.com,
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>, Jiri Olsa <jolsa@...hat.com>
Subject: [PATCH v2] perf/s390: Regression: Move uid filtering to BPF filters
V1 --> V2: Added Jiri Olsa's suggestion and introduced
member bpf_perf_event_opts::no_ioctl_enable.
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.
Make the change backward compatible and introduce a new structure
member bpf_perf_event_opts::no_ioctl_enable. It defaults to false and only
bpf_program__attach_perf_event() sets it to true. This way only
perf tool invocation do not enable the sampling event.
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>
Suggested-by: Jiri Olsa <jolsa@...hat.com>
---
tools/lib/bpf/libbpf.c | 19 +++++++++++++------
tools/lib/bpf/libbpf.h | 3 ++-
2 files changed, 15 insertions(+), 7 deletions(-)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index e067cb5776bd..8e0fb4391b54 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -10911,6 +10911,7 @@ struct bpf_link *bpf_program__attach_perf_event_opts(const struct bpf_program *p
struct bpf_link_perf *link;
int prog_fd, link_fd = -1, err;
bool force_ioctl_attach;
+ bool no_ioctl_enable;
if (!OPTS_VALID(opts, bpf_perf_event_opts))
return libbpf_err_ptr(-EINVAL);
@@ -10965,11 +10966,14 @@ 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;
+ no_ioctl_enable = OPTS_GET(opts, no_ioctl_enable, false);
+ if (!no_ioctl_enable) {
+ 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;
@@ -10982,7 +10986,10 @@ struct bpf_link *bpf_program__attach_perf_event_opts(const struct bpf_program *p
struct bpf_link *bpf_program__attach_perf_event(const struct bpf_program *prog, int pfd)
{
- return bpf_program__attach_perf_event_opts(prog, pfd, NULL);
+ DECLARE_LIBBPF_OPTS(bpf_perf_event_opts, pe_opts);
+
+ pe_opts.no_ioctl_enable = true;
+ return bpf_program__attach_perf_event_opts(prog, pfd, &pe_opts);
}
/*
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index d1cf813a057b..4be2b7664031 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -499,9 +499,10 @@ struct bpf_perf_event_opts {
__u64 bpf_cookie;
/* don't use BPF link when attach BPF program */
bool force_ioctl_attach;
+ bool no_ioctl_enable;
size_t :0;
};
-#define bpf_perf_event_opts__last_field force_ioctl_attach
+#define bpf_perf_event_opts__last_field no_ioctl_enable
LIBBPF_API struct bpf_link *
bpf_program__attach_perf_event(const struct bpf_program *prog, int pfd);
--
2.50.1
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