linux-kernel - Re: perf probe issues

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Date:	Wed, 15 Jun 2016 11:02:01 +0900
From:	Masami Hiramatsu <mhiramat@...nel.org>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: perf probe issues

On Tue, 14 Jun 2016 14:05:04 -0300
Arnaldo Carvalho de Melo <acme@...nel.org> wrote:

> 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?

Yes, I guess it missed to get start_text because of KASLR...

> 
> 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?

Let me check that the perf symbol/map object uses kallsyms for kernel.
Anyway, if the target is not the kernel, do we change the message
as "trying to use symbols"?


> 
> - Arnaldo


-- 
Masami Hiramatsu <mhiramat@...nel.org>