Experiences Getting SystemTap work on OLPC Machine

[William Cohen]

William Cohen wrote:

<snip>

> Setup of SystemTap on OLPC Machine
> 
> To run systemtap kernel modules on olpc machine the hacked 
> systemtap-runtime
> module needs to be installed on the olpc machine:
> 
> rpm -Uvh systemtap-runtime-0.5.12-1olpc.i386.rpm
> 
> The instrumentation needs to be run as root with:
> 
> staprun idle1.ko
> 
> This particular script attempts to determine the amount of the time
> spent in the power saving halted mode. It will run until control-c is
> typed on the command line. When control-c is pressed, the script
> prints out information about how long it ran, the number of entries
> and exits from halt state, and some statistics about the time spent in
> halted state. Below is the output of idle1.stp running on a idle
> machine running the 314 build and the q2b76 rom:
> 
> Starting halt watch
> Ran for 60544934 us
> entered default_halt 9609
> exited default_halt 9609
> percent halted: 17
> count: 9609
> avg_time: 1081
> min_time: 3
> max_time: 3429
> usec                distribution
> value |-------------------------------------------------- count
>     0 |                                                      0
>     1 |                                                      0
>     2 |                                                     18
>     4 |@                                                   219
>     8 |@                                                   295
>    16 |                                                     56
>    32 |@                                                   279
>    64 |                                                    149
>   128 |                                                    147
>   256 |@                                                   275
>   512 |@@@                                                 542
>  1024 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@  7605
>  2048 |                                                     24
>  4096 |                                                      0
>  8192 |                                                      0
> 
> This data show that the script was run for about 1 minute
> (60,544,934us).  There were about 9600 halt entries and exits. The
> time actually spent halted is surprisingly small, 17% for the unloaded
> machine. The average time spent halted is slightly over
> 1millisecond. One can see the distribution on the histogram.

I installed olpc406_c11.zip on the machine and reran the experiment with a 
snapshot of systemtap. The processor is spending much more time halted. For an 
idle machine b2 machine in init 5 95% of the time is spent with the processor 
halted (vs 17% before). The average time in halt is much larger, 6216 micro 
seconds versus 1061 micro seconds. Below is the output from the script.

# staprun idle1.ko
Starting halt watch
Ran for 511858365 us
entered default_halt 78239
exited default_halt 78239
percent halted: 95
count: 78239
avg_time: 6216
min_time: 13
max_time: 8998
usec                distribution
value |-------------------------------------------------- count
     2 |                                                       0
     4 |                                                       0
     8 |                                                       7
    16 |                                                     109
    32 |                                                      84
    64 |                                                     196
   128 |                                                     464
   256 |@                                                   1823
   512 |@                                                   1165
  1024 |@                                                   2238
  2048 |@@@                                                 3930
  4096 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@  57120
  8192 |@@@@@@@@@                                          11103
16384 |                                                       0
32768 |                                                       0

-Will