linux-kernel - Re: [PATCH 1/3] perf probe: Set retprobe flag when probe in address-based alternative mode

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Date:	Mon, 13 Apr 2015 11:39:03 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	He Kuang <hekuang@...wei.com>
Cc:	masami.hiramatsu.pt@...achi.com, a.p.zijlstra@...llo.nl,
	mingo@...hat.com, namhyung@...nel.org, wangnan0@...wei.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] perf probe: Set retprobe flag when probe in
 address-based alternative mode

Em Mon, Apr 13, 2015 at 07:41:28PM +0800, He Kuang escreveu:
> Perf probe misses to set retprobe flag back when falling back to
> address-based alternative mode.

And when is that happens, can you explain? Because I tried to follow your
instructions, but in my case it worked in the same way before and after your
patch.

Can you please provide more context for reviewing your patch?

Here are my results:

Before:

  [root@...andy ~]# perf probe -v --add='sys_write%return'
  probe-definition(0): sys_write%return
  symbol:sys_write file:(null) line:0 offset:0 return:1 lazy:(null)
  0 arguments
  Using /root/.debug/.build-id/dd/32e51921ede0fd46f034091b7f6a0f2e01ebda for symbols
  Could not open debuginfo. Try to use symbols.
  Opening /sys/kernel/debug/tracing/kprobe_events write=1
  Added new event:
  Writing event: r:probe/sys_write sys_write+0
      probe:sys_write      (on sys_write%return)

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

	  perf record -e probe:sys_write -aR sleep 1
  [root@...andy ~]# cat /sys/kernel/debug/tracing/kprobe_events
  r:probe/sys_write sys_write

Remove it:

  [root@...andy ~]# perf probe --del *:*
  Removed event: probe:sys_write

After:

  [root@...andy ~]# perf probe -v --add='sys_write%return'
  probe-definition(0): sys_write%return
  symbol:sys_write file:(null) line:0 offset:0 return:1 lazy:(null)
  0 arguments
  Using /root/.debug/.build-id/dd/32e51921ede0fd46f034091b7f6a0f2e01ebda for symbols
  Could not open debuginfo. Try to use symbols.
  Opening /sys/kernel/debug/tracing/kprobe_events write=1
  Added new event:
  Writing event: r:probe/sys_write sys_write+0
    probe:sys_write      (on sys_write%return)
  
  You can now use it in all perf tools, such as:

	  perf record -e probe:sys_write -aR sleep 1

  [root@...andy ~]# 
  [root@...andy ~]# cat /sys/kernel/debug/tracing/kprobe_events
  r:probe/sys_write sys_write

Humm, noticed one other problem, but not with your patch, about this message:

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

That is really not clear, specially that "try to use symbols" :-)

[root@...andy ~]# ls -la /root/.debug/.build-id/dd/32e51921ede0fd46f034091b7f6a0f2e01ebda
lrwxrwxrwx. 1 root root 86 Apr 10 18:02 /root/.debug/.build-id/dd/32e51921ede0fd46f034091b7f6a0f2e01ebda -> ../../home/acme/git/build/v4.0.0-rc6+/vmlinux/dd32e51921ede0fd46f034091b7f6a0f2e01ebda
[root@...andy ~]# ls -la /root/.debug/.build-id/dd/../../home/acme/git/build/v4.0.0-rc6+/vmlinux/dd32e51921ede0fd46f034091b7f6a0f2e01ebda 
-rwxr-xr-x. 1 root root 22698661 Apr 10 18:02 /root/.debug/.build-id/dd/../../home/acme/git/build/v4.0.0-rc6+/vmlinux/dd32e51921ede0fd46f034091b7f6a0f2e01ebda
[root@...andy ~]# ls -la /root/.debug/.build-id/dd/../../home/acme/git/build/v4.0.0-rc6+/vmlinux/dd32e51921ede0fd46f034091b7f6a0f2e01ebda

I.e. it managed to read the debuginfo, its just that it has no symbols in it :-)

Anyway, digression ended.

- Arnaldo
 
> Can be reproduced as following:
> 
>   $ perf probe -v -k vmlinux --add='sys_write%return'
>   ...
>   Added new event:
>   Writing event: p:probe/sys_write _stext+1584952
>     probe:sys_write      (on sys_write%return)
> 
>   $ cat /sys/kernel/debug/tracing/kprobe_events
>   p:probe/sys_write _stext+1584952
> 
> After this patch:
> 
>   $ perf probe -v -k vmlinux --add='sys_write%return'
>   Added new event:
>   Writing event: r:probe/sys_write SyS_write+0
>     probe:sys_write      (on sys_write%return)
> 
>   $ cat /sys/kernel/debug/tracing/kprobe_events
>   r:probe/sys_write SyS_write
> 
> Signed-off-by: He Kuang <hekuang@...wei.com>
> ---
>  tools/perf/util/probe-event.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
> index 30545ce..5483d98 100644
> --- a/tools/perf/util/probe-event.c
> +++ b/tools/perf/util/probe-event.c
> @@ -332,6 +332,7 @@ static int find_alternative_probe_point(struct debuginfo *dinfo,
>  	else {
>  		result->offset += pp->offset;
>  		result->line += pp->line;
> +		result->retprobe = pp->retprobe;
>  		ret = 0;
>  	}
>  
> -- 
> 2.3.3.220.g9ab698f
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