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perf/s390: Regression: Move uid filtering to BPF filters #9368

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Pull request for series with
subject: perf/s390: Regression: Move uid filtering to BPF filters
version: 2
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=986451

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Upstream branch: 5345e64
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=986451
version: 2

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Upstream branch: 5b4c54a
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=986451
version: 2

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Upstream branch: cd7c97f
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=986451
version: 2

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Upstream branch: e8d780d
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=986451
version: 2

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Upstream branch: 821c9e5
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=986451
version: 2

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Upstream branch: a6923c0
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=986451
version: 2

perf/s390: Regression: Move uid filtering to BPF filters
92e0fba 
V1 --> V2: Added Jiri Olsa's suggestion and introduced
           member bpf_perf_event_opts::no_ioctl_enable.

On linux-next
commit b4c658d ("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: 63f2f5e ("libbpf: add ability to attach/detach BPF program to perf event")
To: Andrii Nakryiko <[email protected]>
To: Ian Rogers <[email protected]>
To: Ilya Leoshkevich <[email protected]>
Signed-off-by: Thomas Richter <[email protected]>
Suggested-by: Jiri Olsa <[email protected]>
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At least one diff in series https://patchwork.kernel.org/project/netdevbpf/list/?series=986451 expired. Closing PR.

@kernel-patches-daemon-bpf kernel-patches-daemon-bpf bot deleted the series/986451=>bpf-next branch August 4, 2025 08:10
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