Re: [Xenomai-help] [RT_PIPE] - User space read() not waking up

[Jan Kiszka <jan.kiszka@domain.hid>]

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Viktor Stark wrote:
> Hello,
> 
> I'm having a little issue with RT pipes and I was wondering if anyone could give me some pointers.
> 
> I wrote a RT kernel module for a DAQ card. The DAQ card gets interrupts every 200 us (5 kHz) and I've got a RT interrupt handler which wakes up a RT function (by sending an event to it). Inside that function I use rt_pripe_write to send some data (about 300 bytes) down to user space.
> 
> On the user space side, I've got a real small executable which basically opens the pipe via the Xenomai registry and then reads data from it in a tight loop. Inside that loop I read the TSC after each write and I compute a minimum / maximum for a 10 seconds cycle. The problem is that I get a maximum of about 395 us (which is 2 times 200 us approximately). On the kernel side I also do timings and I am 100% sure that my function using rt_pipe_write() is woken up every 200 us (+- 10 us jitter), that is no interrupts are missed.
> 
> After further reasearch I realised that I get a maximum value every 10000 cycles, that is every two seconds. I don't have anything in my system (to my knowledge) that has such a period so I'm really confused about what could be causing this.
> 
> I tried to have a look at pipe.c in src/nucleus and saw that the user space side is supposed to get woken up on each xnpipe_send() but I admit that I didn't quite catch all the details.
> 
> So basically, my question is:
> If I have rt_pipe_write() writing to a pipe from a kernel module with a 200 us period and I have a read() in a tight loop reading from that same pipe is user space, what could be the origin of the fact that sometimes my read() doesn't get woken up?
> 
> It's worth to mention too, that I'm not losing any data. That is, after the read which eats up 2 cycles, the next read finishes nearly instantaneously (within 5 us) as if only the second rt_pipe_write() woke up the user space read().
> Timing illustration (time between read()'s in us): 195 | 200 | 205 | 395 | 5 | 195 | 200 | 200
> 
> My setup:
> Xenomai 2.2-rc3
> Kernel 2.6.17 (with Xenomai compiled in - ie. not compiled as modules)
> P4 3GHz - 1Gb RAM
> No APM/ACPI/CpuFreq - SMI workaround enabled.
> Tried with 2.6.19 / Xenomai 2.3 - same results.
> 
> Thanks in advance for any pointers.

Have a look at the ipipe tracer:

http://www.xenomai.org/index.php/I-pipe:Tracer

Put a xntrace_user_freeze at the spot in your application where you
detect the misbehaviour and check the back-trace of the last few hundred
microseconds. If you don't understand what you see (the tracer's
information can be overwhelming...), feel free to post the trace.

> 
> Keep up the good work! Xenomai just *rocks*.

Nice to hear about yet another happy user. :)

Jan


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