What prevents my RT thread from working?

Question

I have some data acquisition application running under Linux 2.6.37 on DM8148 with TI Linux. I have two threads:

• thread named IDE, scheduled as SCHED_RR, prio 114 (75), which collects data from HW FIFO arriving at 200KiB/s into 30MiB ring buffer each 2ms:

  while(1) {
    sleep(ms);
    while(DataInFIFO) {
        CollectToRingBuffer();
        SignalToWriter(); }
  }
  

• thread WriterIDE, scheduled as SCHED_RR, prio 113 (74), writing this ring buffer in to the USB disk-on-key.

  while(1) {
    WaitForSignal();
    writeToFileOnDOK();
  }
  

I know from measures of "write()" function that sometimes this USB writing may "hang" for some 1.5 and even 2 seconds, trying to write to the DOK. But I was sure that as I gave the collector task 30MiB, which is enough for 150s, everything will be OK.

No! It is not!
I put the time measuring code. And what I see is, that when writer hangs for a long time (f.e.1342ms), then entering the collector thread time also is very large (306ms). This causes HW FIFO overflow and data inconsistency.

I checked the spread of threads priority in the system (ps command) - nothing is real-time, except me. All system tasks are scheduled as OTHER (TS in ps output), even kernel USB threads. Only IRQ tasks are FF, but even them are of lower priority.

I don't know where to go from here...:-(
Please, help!