feat: add GPU filter (--gpus) to nvlink_monitor

[staryxchen]

Summary

Third of the feature-enhancement batch. Adds -g, --gpus <id1,id2,...> to nvlink_monitor so the user can restrict monitoring to a subset of GPUs. On an 8-GPU system where a bandwidth test only touches 2 GPUs, this cuts irrelevant output and focuses on the links that matter.

Changes

monitor/arg_parser.{h,cpp}

• New std::vector<int> gpuFilter field on MonitorCliArgs (empty = all GPUs).
• Parses -g/--gpus as a comma-separated integer list, e.g. 0,1,3. Whitespace around tokens is tolerated. Negative ids, non-numeric tokens, missing value, and empty lists are all rejected with ok=false.

monitor/nvlink_monitor.{h,cpp}

• Constructor takes const std::vector<int>& gpuFilter (stored as std::set<int> for O(log n) lookup).
• New private helper isGpuSelected(id) — returns true if the filter is empty (all GPUs) or the id is in the set.
• getNvLinkData skips GPUs not selected by the filter, so CSV/JSON/text output and bandwidth calc only cover the selected GPUs. Snapshot indices stay aligned across samples (filter is stable for the monitor's lifetime), so calculateBandwidth's index-based matching is unaffected.
• Validation in the constructor after discovery: an out-of-range id throws std::runtime_error (surfaced as Error: GPU id N out of range (0-M) by main's catch) so a typo doesn't silently monitor nothing.
• A Monitoring K of N GPU(s) (filter applied) notice is printed to stderr when a filter is active.
• printHelp and main updated for the new flag.

test/test_arg_parser.cpp

+6 tests (72 total, was 66): single short (-g 3), multiple long (--gpus 0,1,3), whitespace tolerated (0, 1, 3), invalid token rejected, negative rejected, missing value rejected; Defaults asserts gpuFilter empty.

README.md

Documented -g, --gpus with the out-of-range-rejection note.

Verification

• make check-format — clean
• make test — 72/72 pass
• make monitor — builds with -Wall -Wextra, no warnings
• Smoke-tested on an 8x H20 system:
  • -g 0,1 -f csv → CSV output contains only GPU 0 and 1
  • -g 9 → errors with Error: GPU id 9 out of range (0-7)
  • stderr shows Monitoring 2 of 8 GPU(s) (filter applied)

Files changed

 README.md                  |  1 +
 monitor/arg_parser.cpp     | 38 ++++++++++++++++++++++++++++++++++++++
 monitor/arg_parser.h       |  3 +++
 monitor/nvlink_monitor.cpp | 34 +++++++++++++++++++++++++++++++---
 monitor/nvlink_monitor.h   | 16 ++++++++++++----
 test/test_arg_parser.cpp   | 39 +++++++++++++++++++++++++++++++++++++++
 6 files changed, 131 insertions(+), 3 deletions(-)