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Subject: return probe not executed on SMP system
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Message-ID: <OF7105678A.DFBAFE34-ON482570AE.004CDE37-482570AE.004D0753@cn.ibm.com>
From: Guang Lei Li <liguangl@cn.ibm.com>
Date: Thu, 03 Nov 2005 14:01:00 -0000
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Hi,

  I met some difficulties when dealing with the return probe on a 
multi-processor system(Power5 System, 4 CPU).

  This is the stap script I used:

global counter

function info()
%{
  struct task_struct *cur = current;
  _stp_printf("\n|%ld|%ld|%ld|%u|", cur->pid, cur->tgid, 
cur->thread_info->cpu);
%}

probe kernel.function("sys_read")
{
  if(pid() == target())
  {
    counter--
    info()
    log("pid:".string(pid())." target:".string(target())."entry")
  }
}

probe kernel.function("sys_read").return
{
  if(pid() == target())
  {
    counter++
    info()
    log("pid:".string(pid())." target:".string(target())."return")
  }
}

probe begin
{
  counter=100
}

probe end
{
  log("counter: ".string(counter))
}

then I run:
  stap -g a.stp -c "ls > a"

The output:

root:/root/temp>stap -g b.stp -c "ls > a"

|3713|3713|3|0|pid:3713 target:3713entry

|3713|3713|3|0|pid:3713 target:3713entry

|3713|3713|3|0|pid:3713 target:3713entry

|3713|3713|3|0|pid:3713 target:3713entry

|3713|3713|3|0|pid:3713 target:3713entry
counter: 95

 It seemed that the return probe didn't work for me.
 I tried the same script on a uni-processor x86 system, it worked fine.

 And I also tried to write a simple c application which will open a file, 
and read some data from this file. I run it:
   stap -g b.stp -c "./a.out"
 It gave the output like:

...
|3881|3881|0|0|pid:3881 target:3881entry

|3881|3881|0|0|pid:3881 target:3881entry

|3881|3881|0|0|pid:3881 target:3881entry

|3881|3881|0|0|pid:3881 target:3881return

|3881|3881|0|0|pid:3881 target:3881entry

|3881|3881|0|0|pid:3881 target:3881return

|3881|3881|0|0|pid:3881 target:3881entry
....

|3881|3881|3|0|pid:3881 target:3881entry

|3881|3881|3|0|pid:3881 target:3881return
counter: 33

 You can see that there are still some return probes not be executed at 
all(if all are executed, the counter should be 100).

 Could anybody give me a hint about this problem?

Best Regards,

Li Guanglei