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

Arnaldo Carvalho de Melo
2019-11-11T14:04:55Z

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@quaco ~]# 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@quaco ~]# 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@quaco ~]#
[root@quaco ~]# perf probe --list
probe:kernel_read_L1 (on kernel_read@fs/read_write.c)
probe:kernel_read_L2 (on kernel_read:1@fs/read_write.c)
[root@quaco ~]# perf probe -L kernel_read
<kernel_read@/usr/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@quaco ~]#

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