linux-kernel - perf probe issues

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Date:	Tue, 14 Jun 2016 14:05:04 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Masami Hiramatsu <mhiramat@...nel.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: perf probe issues

Hi Masami,

	While investigating a problem in the annotation code reported by
Linus, with "kaslr" on the kernel command line and this in .config:

[root@...et ~]# egrep 'CONFIG_(RANDOMIZE_BASE|DEBUG_INFO)' ~acme/git/build/v4.7.0-rc3/.config
CONFIG_RANDOMIZE_BASE=y
# CONFIG_DEBUG_INFO is not set
[root@...et ~]#

[root@...et ~]# grep -w vfs_write /proc/kallsyms 
ffffffffac2404c0 T vfs_write
[root@...et ~]# perf probe vfs_write
vfs_write is out of .text, skip it.
  Error: Failed to add events.
[root@...et ~]# perf probe -v vfs_write
probe-definition(0): vfs_write 
symbol:vfs_write file:(null) line:0 offset:0 return:0 lazy:(null)
0 arguments
Looking at the vmlinux_path (8 entries long)
Using /lib/modules/4.7.0-rc3/build/vmlinux for symbols
Could not open debuginfo. Try to use symbols.
vfs_write is out of .text, skip it.
  Error: Failed to add events. Reason: No such file or directory (Code: -2)
[root@...et ~]# 

Can you try to test with this config to investigate this problem?

We need to improve the error messages, for instance:

"Could not open debuginfo. Try to use symbols."

I think should be rewritten as: "Trying to use kallsyms", no? I.e. if it
doesn't manage to find a vmlinux file, it will try to use kallsyms as a
fallback, is that what is happening here?

- Arnaldo