[LITMUS^RT] Missing st_trace records

[Mikyung Kang]

Hello all,

I'm trying to get whole tracing information of RT task sets using LITMUS-RT
Version 2014.1.

* System has 8 Cores, no hyper-threading, 16G memory
* Tested both Bare-metal case and Virtualization case (Xen): similar result
* Ubuntu 12.04 (Linux 3.10.5)
* Generated 10 tasks w/ Utilization=[1.0, 8.0] using rtspin
* Run 10 seconds using GSN-EDF scheduler

When I spawned only 1 task (Period=100ms, WCET=10ms) during 10 seconds, all
records are being saved into .bin file correctly w/o missing records.
But, more than 1 task, always records are being missed a lot.

To avoid record-loss, I tried the following options based on the thread:
https://lists.litmus-rt.org/pipermail/litmus-dev/2013/000480.html.

* Kernel config: CONFIG_SCHED_TASK_TRACE_SHIFT=13 (up to 8K events)
* Used /dev/shm/* instead of disk for the binary record file
* Removed unnecessary events for the calculation of deadline miss ratio
(switch_to/from, block, resume, action, np_enter/exit)
* Current KERNEL_IMAGE_SIZE 512*1024*1024

Then, around 4K events are being saved into one task-assigned core
(st-*.bin).
When I got the information through st_job_stats, I could see that the
number of recorded events per task is very different even though tasks have
the same period.
Moreover, usually 5~20% records are being missed for each task set, even
though utilization is very low. Sometimes, more than that.

Is this expected record-loss ratio using st_trace tool?
What should I check more? Is there any other way to reduce/remove
record-loss?

Thanks,
Mikyung
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