Add start_new_session=True to mpstat Popen to fix perf hook teardown hang

Summary:
The perf hook's `after_job` iterates `AVAIL_MONITORS` in dict order — and `mpstat` is first. `Monitor.terminate()` (D107426991) now does `os.killpg(os.getpgid(self.proc.pid), signal.SIGINT)` to clean up the bash wrapper + perf-stat child cleanly. But when the Popen does NOT pass `start_new_session=True`, the child inherits the PARENT's process group (= benchpress's own group). The killpg then sends SIGINT to **benchpress itself**, raising KeyboardInterrupt inside `terminate()`, aborting cleanup before any subsequent monitor runs, and before `write_csv()` ever fires.

Symptom: Run A completes the benchmark (metrics.json + breakdown.csv + perf.data written) but the perf hook produces no CSVs (perf-stat.csv, amd-zen4-perf-collector.csv, etc.). benchpress eventually hits the outer `timeout 3600` and exits with `KeyboardInterrupt` traceback ending at `perf_monitors/__init__.py:115` (the `self.proc.wait(timeout=15)` line).

Other monitors that use `subprocess.Popen` (perfstat, topdown's main collectors at lines 187/277/346/505) already have `start_new_session=True` from D107426991. Thread-based monitors (cpufreq_*, memstat, netstat, power, vmstat) override `terminate()` themselves and bypass the killpg path. Only mpstat was unfixed.

Differential Revision: D107602859

Author: excelle08

benchpress/plugins/hooks/perf_monitors/mpstat.py

@@ -18,7 +18,17 @@ def __init__(self, interval, job_uuid):
 
     def run(self):
         args = ["mpstat", "-u", f"{self.interval}"]
-        self.proc = subprocess.Popen(args, stdout=subprocess.PIPE, encoding="utf-8")
+        self.proc = subprocess.Popen(
+            args,
+            stdout=subprocess.PIPE,
+            encoding="utf-8",
+            # CRITICAL: Run in its own process group so Monitor.terminate()
+            # can killpg() the bash wrapper + mpstat cleanly without
+            # signalling benchpress itself (which would raise
+            # KeyboardInterrupt and abort cleanup of all subsequent
+            # monitors).
+            start_new_session=True,
+        )
         super(MPStat, self).run()
 
     def process_output(self, line):