Re: Problems with FTrace

[gowrishankar <gowrishankar.m@linux.vnet.ibm.com>]

Hi Paolo,

Paolo wrote:
> I need to know if a task is periodic or not and other informations (as
> the duration). I used the sched_switch function of ftrace to trace
> this behaviour.
> In order to have a proof of the tracing I used the cyclictest program.
> 
> The first time I tested the tracing goddness with a single cpu and the
> second time with all cpu online (2).
> In the first call the times was correct, in the second not: the
> periodicity of the cyclictask doesn't seem correct.
> 
> The periodicity is the interval between two wakes up of the same task
> and has to be  more or less constant, right?
> The difference between two wakes up in my test (with 2 cpu) is not
> constant, and the misured time is very different from the time set.
> 

I suspect if sirq-timer is taking up the chance to slightly disturb the
periodicity. To confirm more, you can try with sched_switch visualizer

http://www.osadl.org/Visualize-the-temporal-relationship-of-L.taks-visualizer.0.html

Thanks,
Gowri

> The test was:
> 
> mount -t debugfs nodev /tmp
> 
> echo nop > /tmp/tracing/current_tracer
> echo 1 > /tmp/tracing/tracing_enabled
> echo 0 > /tmp/tracing/tracing_enabled
> 
> echo sched_switch > /tmp/tracing/current_tracer
> echo 1 > /tmp/tracing/tracing_enabled
> 
> ./cyclictest -l 1000 -i 500 -n -t1
> 
> echo 0 > /tmp/tracing/tracing_enabled
> 
> The trace file shows (cat trace | grep + | grep 5036, where 5036 is
> the pid of the cyclictest thread):
> ...
>           <idle>-0     [001]   174.705470:      0:140:R   + [001]  5036:120:S
>           <idle>-0     [001]   174.705559:      0:140:R   + [001]  5036:120:S
>           <idle>-0     [001]   174.705648:      0:140:R   + [001]  5036:120:S
>           <idle>-0     [001]   174.705738:      0:140:R   + [001]  5036:120:S
>           <idle>-0     [001]   174.705966:      0:140:R   + [001]  5036:120:S
>           <idle>-0     [001]   174.706056:      0:140:R   + [001]  5036:120:S
>           <idle>-0     [001]   174.706500:      0:140:R   + [001]  5036:120:S
>           <idle>-0     [001]   174.706999:      0:140:R   + [001]  5036:120:S
>           <idle>-0     [001]   174.707505:      0:140:R   + [001]  5036:120:S
>           <idle>-0     [001]   174.707602:      0:140:R   + [001]  5036:120:S
>           <idle>-0     [001]   174.707691:      0:140:R   + [001]  5036:120:S
>           <idle>-0     [001]   174.707781:      0:140:R   + [001]  5036:120:S
>           <idle>-0     [001]   174.707868:      0:140:R   + [001]  5036:120:S
>           <idle>-0     [001]   174.707957:      0:140:R   + [001]  5036:120:S
> ...
> 
> Maybe I didn't set correctly the parameter of cyclictest and/or sched_switch.
> Is it possible a bug? Notice I use the 2.6.29.rc6 kernel.
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