return probe not executed on SMP system

[Hien Nguyen]

Hi,
After looking deep into this issue I found out the problem is in the 
stpd. The sptd unloads the systemtap module a little too early before 
the return probes have a chance to fire their handlers.

If you have the systemtap src tree try this tempory_fix.patch. I will 
file a bugzilla tomorrow.

Hien.



Guang Lei Li wrote:

>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
>
>  
>

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