Re: perf probe -L on ubuntu 16.04

[Masami Hiramatsu]

On Thu, 19 May 2016 09:06:42 -0300
Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> wrote:

> Em Thu, May 19, 2016 at 11:00:39AM +0900, Masami Hiramatsu escreveu:
> > On Mon, 16 May 2016 22:56:36 -0300
> > Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> wrote:
> > > 	Have you ever tried using 'perf probe' on ubuntu?
> 
> > > acme@ubuntu:~/git/linux$ sudo apt-cache search linux-image-$(uname -r)
> > > linux-image-4.4.0-21-generic - Linux kernel image for version 4.4.0 on
> > > 64 bit x86 SMP
> > > linux-image-4.4.0-21-generic-dbgsym - Linux kernel debug image for
> > > version 4.4.0 on 64 bit x86 SMP
> > > acme@ubuntu:~/git/linux$ 
> > > acme@ubuntu:~/git/linux$ 
> > > 
> > > So those are installed, but then I try:
> > > 
> > > root@ubuntu:~# perf probe -vvv -L getname_flags
> > > Looking at the vmlinux_path (8 entries long)
> > > Using /usr/lib/debug/boot/vmlinux-4.4.0-21-generic for symbols
> > > Open Debuginfo file: /usr/lib/debug/boot/vmlinux-4.4.0-21-generic
> > > fname: /build/linux-Ay7j_C/linux-4.4.0/fs/namei.c, lineno:125
> > > New line range: 125 to 2147483647
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> > > Get 3665 lines from this CU
> 
> Ah, why do we repeat the above line that many times? Are we somehow
> reading this compile unit that many times?!?

Yes, because the getname_flags is inlined by compiler, depends on the
parameter, some lines in the function are never compiled at some call
points. In this case, we have to check every call points so that all
the available lines are found.

BTW, "this CU" has no meaning, it should show the diename, or remove
the useless message.

> > > path: /build/linux-Ay7j_C/linux-4.4.0/fs/namei.c
> > > Failed to find source file path.
> > >   Error: Failed to show lines. Reason: No such file or directory (Code: -2)
> > > root@ubuntu:~# 
> 
> > > Have to dig deeper to see where the source code is, ideas?
>  
> > It shows the "path" is ;
> > > path: /build/linux-Ay7j_C/linux-4.4.0/fs/namei.c
>  
> > So, would you have the source code under this temporary directory?
> > (yeah, I guess No)
> 
> No, and I couldn't find a package that had it
> 
> > Anyway, ubuntu has "linux-source" package, or apt source linux-image-`uname -r`
> > will give you the source tree in current dir.
>  
> > After you get a proper kernel source code, you can use -s to show source code.
>  
> >     -s, --source <directory>
> >                           path to kernel source
>  
> > I ensured it is working correctly by below command.
> > $ perf probe -s ./linux-4.4.0 -L getname_flags
> 
> Ok, how about telling the user this package is needed, since we can
> figure out the distro and the package name?

Did we do that already?

> Couldn't it figure out where
> the source code is and also not require using '-s'?

No, actually, ubuntu/debian are not providing kernel source tree package
as redhat/fedora do. User can only download source tarball and extract
it under current working directory. There seems no standard directory
where the source code is extracted to. I think it's debian/ubuntu's
packaging policy.

Anyway, even though we can provide how-to guide for them via Documentation.
I think it's better way because we also have to add some entries to 
apt.conf for installing kernel debuginfo.

Thank you,

> 
> - Arnaldo


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>