linux-kernel - Re: [PATCH v2 1/4] perf probe: Generate event name with line number

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Date:   Mon, 11 Nov 2019 11:04:50 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
        Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH v2 1/4] perf probe: Generate event name with line number

Em Fri, Nov 08, 2019 at 01:27:58AM +0900, Masami Hiramatsu escreveu:
> Generate event name from function name with line number
> as <function>_L<line_number>. Note that this is only for
> the new event which is defined by the line number of
> function (except for line 0).
> 
> If there is another event on same line, you have to use
> "-f" option. In that case, the new event has "_1" suffix.
> 
>  e.g.
>   # perf probe -a kernel_read:1
>   Added new events:
>     probe:kernel_read_L1 (on kernel_read:1)

While testing this, using the same function (kernel_read), I found it
confusing that it is possible to insert probes in lines seemingly with
no code, for instance:

[root@...co ~]# perf probe -a kernel_read:1
Added new event:
  probe:kernel_read_L1 (on kernel_read:1)

You can now use it in all perf tools, such as:

	perf record -e probe:kernel_read_L1 -aR sleep 1

[root@...co ~]# perf probe -a kernel_read:2
Added new event:
  probe:kernel_read_L2 (on kernel_read:2)

You can now use it in all perf tools, such as:

	perf record -e probe:kernel_read_L2 -aR sleep 1

[root@...co ~]#
[root@...co ~]# perf probe --list
  probe:kernel_read_L1 (on kernel_read@...read_write.c)
  probe:kernel_read_L2 (on kernel_read:1@...read_write.c)
[root@...co ~]# perf probe -L kernel_read
<kernel_read@...r/src/debug/kernel-5.3.fc30/linux-5.3.8-200.fc30.x86_64/fs/read_write.c:0>
      0  ssize_t kernel_read(struct file *file, void *buf, size_t count, loff_t *pos)
      1  {
      2         mm_segment_t old_fs;
      3         ssize_t result;

      5         old_fs = get_fs();
      6         set_fs(KERNEL_DS);
                /* The cast to a user pointer is valid due to the set_fs() */
      8         result = vfs_read(file, (void __user *)buf, count, pos);
      9         set_fs(old_fs);
     10         return result;
         }
         EXPORT_SYMBOL(kernel_read);


[root@...co ~]#


What is the point of putting a probe on line 2? I is not initializing,
etc, 1 as well, notthing there and we already have 0 (or not specifying
a line number) to put a probe at the start of a function, can you
clarify?

I'll apply this patch, the problem above isn't strictly related to it.

- Arnaldo