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Date: Tue, 24 Feb 2015 15:49:47 -0300
From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jiri Olsa <jolsa@redhat.com>, David Ahern <dsahern@gmail.com>
Subject: Trying to use 'perf probe' to debug perf itself
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Hi Masami,

	I'm adding thread refcounting in perf, and in this process I am
investigating a problem, so thought about using perf itself to match
thread__get with thread__put:

[root@ssdandy ~]# perf probe -x ~/bin/perf -V thread__put 
Available variables at thread__put
        @<thread__put+0>
                struct thread*  thread
[root@ssdandy ~]# perf probe -x ~/bin/perf -V thread__get
Available variables at thread__get
        @<thread__get+0>
                struct thread*  thread
[root@ssdandy ~]#

cool, so I thought it would be just a matter of asking to put the probes
and get the value of 'thread', then match things, but:


[root@ssdandy ~]# perf probe -v ~/bin/perf thread__put thread
probe-definition(0): thread__put thread 
symbol:thread__put file:(null) line:0 offset:0 return:0 lazy:(null)
parsing arg: thread into thread
1 arguments
Open Debuginfo file: /root/bin/perf
Try to find probe point from debuginfo.
Probe point found: thread__put+0
Searching 'thread' variable in context.
Converting variable thread into trace event.
thread type is (null).
Found 1 probe_trace_events.
Opening /sys/kernel/debug/tracing/uprobe_events write=1
Added new event:
Writing event: p:probe_perf/thread__put /root/bin/perf:0xd03d2 thread=-32(%sp):u64
Failed to write event: Invalid argument
  Error: Failed to add events. Reason: Invalid argument (Code: -22)
[root@ssdandy ~]#

Not possible :-\ 

please let me know if you need some file, here is the readelf -wi for
those two routines:

# readelf -wi ~/bin/perf
<SNIP>
 <1><e25f1>: Abbrev Number: 35 (DW_TAG_subprogram)
    <e25f2>   DW_AT_external    : 1     
    <e25f2>   DW_AT_name        : (indirect string, offset: 0x4c5c3): thread__get       
    <e25f6>   DW_AT_decl_file   : 6     
    <e25f7>   DW_AT_decl_line   : 84    
    <e25f8>   DW_AT_prototyped  : 1     
    <e25f8>   DW_AT_type        : <0xe15cc>     
    <e25fc>   DW_AT_low_pc      : 0x4d038c      
    <e2604>   DW_AT_high_pc     : 0x46  
    <e260c>   DW_AT_frame_base  : 1 byte block: 9c      (DW_OP_call_frame_cfa)
    <e260e>   DW_AT_GNU_all_tail_call_sites: 1  
    <e260e>   DW_AT_sibling     : <0xe2621>     
 <2><e2612>: Abbrev Number: 32 (DW_TAG_formal_parameter)
    <e2613>   DW_AT_name        : (indirect string, offset: 0x43dc7): thread    
    <e2617>   DW_AT_decl_file   : 6     
    <e2618>   DW_AT_decl_line   : 84    
    <e2619>   DW_AT_type        : <0xe15cc>     
    <e261d>   DW_AT_location    : 2 byte block: 91 58   (DW_OP_fbreg: -40)
 <2><e2620>: Abbrev Number: 0
 <1><e2621>: Abbrev Number: 39 (DW_TAG_subprogram)
    <e2622>   DW_AT_external    : 1     
    <e2622>   DW_AT_name        : (indirect string, offset: 0x4c699): thread__put       
    <e2626>   DW_AT_decl_file   : 6     
    <e2627>   DW_AT_decl_line   : 90    
    <e2628>   DW_AT_prototyped  : 1     
    <e2628>   DW_AT_low_pc      : 0x4d03d2      
    <e2630>   DW_AT_high_pc     : 0x6c  
    <e2638>   DW_AT_frame_base  : 1 byte block: 9c      (DW_OP_call_frame_cfa)
    <e263a>   DW_AT_GNU_all_tail_call_sites: 1  
    <e263a>   DW_AT_sibling     : <0xe264d>     
 <2><e263e>: Abbrev Number: 32 (DW_TAG_formal_parameter)
    <e263f>   DW_AT_name        : (indirect string, offset: 0x43dc7): thread    
    <e2643>   DW_AT_decl_file   : 6     
    <e2644>   DW_AT_decl_line   : 90    
    <e2645>   DW_AT_type        : <0xe15cc>     
<SNIP>

Will see what I can do with just:

[root@ssdandy ~]# perf probe ~/bin/perf thread__get
Added new event:
  probe_perf:thread__get (on thread__get in /root/bin/perf)

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

	perf record -e probe_perf:thread__get -aR sleep 1

[root@ssdandy ~]# perf probe ~/bin/perf thread__put
Added new event:
  probe_perf:thread__put (on thread__put in /root/bin/perf)

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

	perf record -e probe_perf:thread__put -aR sleep 1

[root@ssdandy ~]# perf list probe*:*

List of pre-defined events (to be used in -e):

  probe_perf:thread__put                             [Tracepoint event]
  probe_perf:thread__get                             [Tracepoint event]
[root@ssdandy ~]# perf probe -l
  probe_perf:thread__get (on thread__get@util/thread.c in /root/bin/perf)
  probe_perf:thread__put (on thread__put@util/thread.c in /root/bin/perf)
[root@ssdandy ~]# 

- Arnaldo
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