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'perf trace -p <PID>' work on a syscall that is unaugmented, but doesn't
work on a syscall that's augmented (when it calls perf_event_output() in
BPF).
Let's take open() as an example. open() is augmented in perf trace.
Before:
$ perf trace -e open -p 3792392
? ( ): ... [continued]: open()) = -1 ENOENT (No such file or directory)
? ( ): ... [continued]: open()) = -1 ENOENT (No such file or directory)
We can see there's no output.
After:
$ perf trace -e open -p 3792392
0.000 ( 0.123 ms): a.out/3792392 open(filename: "DINGZHEN", flags: WRONLY) = -1 ENOENT (No such file or directory)
1000.398 ( 0.116 ms): a.out/3792392 open(filename: "DINGZHEN", flags: WRONLY) = -1 ENOENT (No such file or directory)
Reason:
bpf_perf_event_output() will fail when you specify a pid in 'perf trace' (EOPNOTSUPP).
When using 'perf trace -p 114', before perf_event_open(), we'll have PID
= 114, and CPU = -1.
This is bad for bpf-output event, because the ring buffer won't accept
output from BPF's perf_event_output(), making it fail. I'm still trying
to find out why.
If we open bpf-output for every cpu, instead of setting it to -1, like
this:
PID = <PID>, CPU = 0
PID = <PID>, CPU = 1
PID = <PID>, CPU = 2
PID = <PID>, CPU = 3
Everything works.
You can test it with this script (open.c):
#include <unistd.h>
#include <sys/syscall.h>
int main()
{
int i1 = 1, i2 = 2, i3 = 3, i4 = 4;
char s1[] = "DINGZHEN", s2[] = "XUEBAO";
while (1) {
syscall(SYS_open, s1, i1, i2);
sleep(1);
}
return 0;
}
save, compile:
make open
perf trace:
perf trace -e open <path-to-the-executable>
Signed-off-by: Howard Chu <howardchu95@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240815013626.935097-2-howardchu95@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.