Selective system profiling

[John Sigler <linux.kernel@free.fr>]

Hello everyone,

Here's my situation:

I'm pushing data in chunks of 1316 bytes to a PCI device at 38 Mbit/s.
In other words, I write 1316 bytes to the device every 277 microseconds.

I've noticed that the latency of this operation varies immensely. Most 
of the time it completes in 50-80 microseconds, but there are occasions 
when it takes several milliseconds (I've even logged 23 ms).

The pseudo-code looks like this:

  deadline = now;

  while ( 1 )
  {
    deadline += 277 µs
    sleep_until(deadline)
    t0 = now;
    write 1316 bytes to PCI device
    t1 = now;
    if (t1-t0 > 100 µs) scream & panic
  }

Relevant (?) information about the setup:

I'm running a PREEMPT_RT kernel with high-resolution timers.
(Specifically 2.6.20.7-rt8)
http://rt.wiki.kernel.org/index.php/Main_Page
HZ=100
The process is in SCHED_RR with priority 75.
The only "process" (?) with higher priority is posix_cpu_timer.
write() is implemented as an ioctl in the driver.
You might think that write() blocks when the buffers on the PCI board 
are full, but I am 99.9% sure that the buffers are never full.


Here's my question:

When my process comes back from a write operation, and I find that I 
have been blocked for more than X ms, can I call a function from a 
system profiler (oprofile perhaps?) to know exactly where in the kernel 
the CPU has been for the last X ms, and how much time has been spent in 
each function, either sleeping or doing something?

Regards.
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