[Bug bpf/25218] New: stapbpf losing perf_events

["sapatel at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=25218

            Bug ID: 25218
           Summary: stapbpf losing perf_events
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: bpf
          Assignee: systemtap at sourceware dot org
          Reporter: sapatel at redhat dot com
  Target Milestone: ---

When there is frequent output from a script, it appears that stapbpf loses some
perf_events which leads to errors. Consider the script below.

global count

probe kernel.function("vfs_read") {

    count++

    printf("Beginning: %d\n", count)
    printf("Middle: %d\n", count)
    printf("End: %d\n", count)

}

Output:

...
...
Beginning: 13700
Middle: 13700
End: 13700
Beginning: 13701
Middle: 13701
End: 13701
WARNING: lost 359 perf_events on cpu 2
bpfinterp.cxx:421: printf already started
WARNING: /home/sapatel/stap_head/install/bin/stapbpf exited with signal: 6
(Aborted)

However, if the output is redirected to a file instead, the perf_events are not
lost, and the script proceeds as expected.

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