linux-kernel - Re: [RFC] perf probe: -x option position issue

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Message-ID: <20150330194849.GH32560@kernel.org>
Date:	Mon, 30 Mar 2015 16:48:49 -0300
From:	Arnaldo Carvalho de Melo <acme@...radead.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ingo Molnar <mingo@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	David Ahern <dsahern@...il.com>, linux-kernel@...r.kernel.org,
	Martin Cermak <mcermak@...hat.com>
Subject: Re: [RFC] perf probe: -x option position issue

Em Mon, Mar 30, 2015 at 07:46:55PM +0200, Jiri Olsa escreveu:
> hi,
> Martin found out following issue.. having following ex binary:
> 
> ---
> int main(void)
> {
>         return 0;
> }
> ---
> 
> following will create uprobe on main:
> 
>   [root@...l-per510-01 perf]# gcc -g -o ex ex.c
>   [root@...l-per510-01 perf]# ./perf probe -x ./ex -a main
>   Added new event:
>     probe_ex:main        (on main in /root/linux/tools/perf/ex)
> 
>   You can now use it in all perf tools, such as:
> 
>           perf record -e probe_ex:main -aR sleep 1
> 
>   [root@...l-per510-01 perf]# cat /sys/kernel/debug/tracing/uprobe_events 
>   p:probe_ex/main /root/linux/tools/perf/ex:0x00000000000004f6
> 
> 
> while following will create (?) kprobe with complain in dmesg:

Right, it looks like it will create the probe on the currently selected
DSO, which, if you have none, will be the kernel, thus the kprobe
(probe:main), while if you do a '-x ./ex -a main' you are selecting the
'ex' DSO and then asking for the probe to be added to a function named
'main', on that DSO, that 'perf probe' realizes is a userspace binary,
thus creates a uprobe: probe_%DSONAME:%FUNCTIONAME.

I wonder if I can do:

[root@...andy acme]# perf probe -a icmp_rcv -x ./ex -a main
Probe point 'icmp_rcv' not found.
  Error: Failed to add events.

No, I can't, I'd say we should support that, i.e. inserting multiple
probes per command line, for different DSOs, etc. I.e. the above would
be equivalent to these two calls:

[root@...andy acme]# perf probe -a icmp_rcv
Added new event:
  probe:icmp_rcv       (on icmp_rcv)

You can now use it in all perf tools, such as:

	perf record -e probe:icmp_rcv -aR sleep 1

[root@...andy acme]# perf probe -x ./ex -a main
Added new event:
  probe_ex:main        (on main in /home/acme/ex)

You can now use it in all perf tools, such as:

	perf record -e probe_ex:main -aR sleep 1

[root@...andy acme]#

But it isn't like that, so, yes, what you report is a bug, both for your
expectation (that I think is that it should put a uprobes with both your
examples) and for mine (that it would add the first to the kernel, and
the second to the selected DSO via -x).

- Arnaldo

>   [root@...l-per510-01 perf]# gcc -g -o ex ex.c
>   [root@...l-per510-01 perf]# ./perf probe -a main -x ./ex
>   Added new event:
>     probe:main           (on main in ex)
> 
>   You can now use it in all perf tools, such as:
> 
>           perf record -e probe:main -aR sleep 1
> 
>   [root@...l-per510-01 perf]# dmesg | tail -2
>   [16986.182159] Could not insert probe at ex:main+0: -2
>   [16986.187030] This probe might be able to register aftertarget module is loaded. Continue.
> 
> 
> that does not seem as an expected behaviour, or am I missing something?
> 
> thanks,
> jirka
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