Re: 3.12.6-rt9 on AM335x (armv7)

[Tim Sander <tim@krieglstein.org>]

Hi
> after some chatting with Nicholas and being sure that I didn't do anything
> totally stupid I would like to provide my current kernel config and a trace
> generated by "cyclictest ... -b500".
> 
> kernel config -> https://www.dropbox.com/s/zsud0b77xoosjm1/config
> trace ->
> https://www.dropbox.com/s/5uf58oqo0qwcn4x/trace-cyclictest-2508us.txt
> 
> I still get max latencies of about 4000us on this TI AM335x system running
> 3.12.6-rt9 with timers-do-not-raise-softirq-unconditionally.patch reverse
> applied.
> 
> sched_rt_runtime_us and sched_rt_period_us are identical. The system is idle
> during the test. Rootfilesystem comes via NFS.
> 
> root@generic-armv7a:~# cat /proc/sys/kernel/sched_rt_runtime_us
> 1000000
> root@generic-armv7a:~# cat /proc/sys/kernel/sched_rt_period_us
> 1000000
> 
> root@generic-armv7a:~# uname -a
> Linux generic-armv7a 3.12.6-rt9-dirty #3 SMP PREEMPT RT Sat Jan 4 23:01:19
> CET 2014 armv7l GNU/Linux
> 
> root@generic-armv7a:~# ~/cyclictest -p99 -m -n -i200 -h400 -q -l100000 -b
> 500 # /dev/cpu_dma_latency set to 0us
> INFO: debugfs mountpoint: /sys/kernel/debug/tracing/
> could not set ftrace_enabled to 0
> could not set tracing_max_latency to 0
> # Histogram
> ...
> # Total: 000018604
> # Min Latencies: 00024
> # Avg Latencies: 00158
> # Max Latencies: 02508
Wohaa, thats bad for a latency patched kernel. Have you disabled 
powermanagement? I have an older rt patched kernel and it was not too good.
If i remember correctly it was between 100-200µs latency. But thats
far better than this. You could also load the system with hackbench, if its 
the idle pm than latency would get better.

Best regards
Tim

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