Cross-check /proc process-tree CPU sampling against cgroup v2 cpu.stat with systemd-run --user --scope

Goal: check that a /proc-based process-tree CPU sampler is accurate for a multi-process job (a coordinator plus worker processes that exit early).

Sampler: for the root and its live descendants, sum stat fields 14-17 (utime+stime+cutime+cstime) / CLK_TCK, polled every 100 ms. Parse fields after the last ')' so command names with spaces can't shift them. Fields 16-17 include reaped children, so workers that exited are still counted.

Reference: run the job in its own cgroup v2 leaf without root: systemd-run --user --scope --quiet --unit=cpu-$N bash -c 'job & echo $! > pidfile; wait; cat /sys/fs/cgroup$(cut -d: -f3 /proc/self/cgroup)/cpu.stat' Read cpu.stat from inside the scope, after wait; the scope unit disappears once its last process exits. Run the sampler outside the scope, reading the root pid from the pidfile.

Result (Linux, cgroup2fs, about 15 CPU-s over 5.6 s wall, n=3): the sampler read 0.981-0.982 of usage_usec every rep. The ~2% gap is the time after the last sample plus the wrapper shell inside the scope. ps -o times= is whole seconds only, so it is not usable for this.

Tip: report CPU-s together with cores busy (CPU-s / wall), so readers don't take it for wall time.