On Mon, 10 Aug 2009 16:07:27 +0200
Wolfgang Steinwender [off-list ref] wrote:

Carsten Emde wrote:

quoted

Does the attached patch help?

Sorry for the late reply. I now switched to linux-2.6.29.6-rt23
(which has the patch included) and verified that the problem is
solved. Reverting the patch gives the problem again.

Now the error messages have disappeared, but I really cannot
tell if the test is doing something at all.

Here's the output from running the python script from rt-tests-50:
$> hwlatdetect --debug
debugging prints turned on
looking for modules
module path: /lib/modules/2.6.29.6-rt23-pae-debug/kernel/drivers/misc
checking
/lib/modules/2.6.29.6-rt23-pae-debug/kernel/drivers/misc/hwlat_detector.ko
not mounting debugfs
test duration is 120s
hwlatdetect:  test duration 120 seconds
   parameters:
        Latency threshold: 10us
        Sample window:     1000000us
        Sample width:      500000us
     Non-sampling period:  500000us
        Output File:       None

Starting test
Starting hardware latency detection for 120 seconds
enabling detector module
first attempt at enable
detector module enabled
disabling detector module
first attempt at disable
detector module disabled
Hardware latency detection done (0 samples)
test finished
Max Latency: 0us
Samples recorded: 0
Samples exceeding threshold: 0
not umounting debugfs

The output from the hwlat_detector module is:
hwlat_detector: version 1.0.0

For me, the output "Samples recorded: 0" means that no samples have
been read at all. Or do I misinterpret the output?

Wolfgang,

The kernel module behavior changed on me. Originally the smi_detector.ko
module just streamed sample data out, most of it being samples of zero
(meaning no gaps in time seen). When Jon re-worked it to use the
ring-buffer structure and renamed it to hwlat_detector.ko, he only
provides sample data if it exceeds the specified threshold. 

So, long answer to a short question, yes you interpreted the output
correctly, there were no gaps in the TSC values read by the sampling
thread. 

It is also not possible for me to cat the sample entry
when the module is enabled: "strace cat sample"
just waits forever:
open("sample", O_RDONLY|O_LARGEFILE)    = 3
fstat64(3, {st_mode=S_IFREG|0444, st_size=0, ...}) = 0
read(3,

Is there anything else I can try?

Due to the change in behavior above, the hwlatdetect python script
now opens the "sample" entry with O_NDELAY and polls that descriptor. 

Clark