Re: return probe not executed on SMP system

[Hien Nguyen <hien at us dot ibm dot com>]

You probably run out out kretprobe instances. When the return probe runs 
out kretprobe instances (maxactive), it just skips and increase an 
internal counter "nmissed".

Systemtap does not have the interface to change kretprobe->maxactive (in 
this case increase the maxactive would help) or examine kretprobe->nmissed.

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
>
>