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

Arnaldo Carvalho de Melo
2019-11-11T14:06:31Z

Em Mon, Nov 11, 2019 at 11:04:50AM -0300, Arnaldo Carvalho de Melo escreveu:
> 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
>
> #
> # 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)

Also look above at the listing, I would expect this instead:

# perf probe --list
probe:kernel_read_l1 (on kernel_read:1@fs/read_write.c)
probe:kernel_read_l2 (on kernel_read:2@fs/read_write.c)

Right?

- Arnaldo

> [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

--

- Arnaldo