[BUG] testing/sched/timerjitter: unexpected results under normal usage #3634

Description / Steps to reproduce the issue

Found while testing apps/testing/sched/timerjitter on QEMU RISC-V (rv-virt), NuttX-13.0.0-RC2. All three reproduce under normal, non-adversarial usage. A separate off-by-one in the sample loop was already
fixed in pull#3629

1. avg: nan with default args

  nsh> timerjitter
  timer jitter in 0 run:
  (latency/us) min: 4294967295, avg: nan, max 0

Running with no arguments at all completes 0 iterations and prints avg: nan. Should the default guarantee at least 1 iteration, or otherwise avoid printing nan?

2. Very large missed-frame counts, one run did not finish

  nsh> timerjitter 100 5
  time frame missed 1
  ...
  time frame missed 17363
  timer jitter in 5 run:
  (latency/us) min: 3129, avg: 347295, max 1269743

Another run with the same arguments printed missed-frame lines continuously and never completed; it had to be terminated externally:

  nsh> timerjitter 100 5
  time frame missed 1
  ...
  time frame missQEMU: Terminated

3. Negative jitter shown as a huge unsigned value

  nsh> timerjitter 10000 10
  time frame missed 1
  timer jitter in 10 run:
  (latency/us) min: 4294966445, avg: 1010, max 12854

4294966445 = 2^32 - 851. avg in the same run is a small, plausible number, while min is not — suggesting min/max end up holding a negative value that gets displayed as a large unsigned one.

On which OS does this issue occur?

[OS: Linux]

What is the version of your OS?

Ubuntu 20.04

NuttX Version

master

Issue Architecture

[Arch: all]

Issue Area

[Area: Testing]

Host information

Environment

• Board/config: boards/risc-v/qemu-rv/rv-virt/configs/knsh_romfs, running on QEMU
• CONFIG_USEC_PER_TICK changed from the board's default (1000) back to the Kconfig default 10000

Verification

• I have verified before submitting the report.