M7 Phase 3: Per-process /proc Enrichment

Phase 3 adds CPU time accounting, process start time, and thread counting to the Process struct, then wires real values into /proc/[pid]/stat and /proc/[pid]/status.

The problem

The existing /proc/[pid]/stat emitted 52 fields but almost all were hardcoded zeros. The state was always 'S', utime/stime were always 0, num_threads was always 1, and vsize/rss were always 0. Similarly, /proc/[pid]/status hardcoded State: S (sleeping), Uid: 0 0 0 0, and Threads: 1 regardless of the actual process. Tools like ps, top, and htop rely on these fields being accurate.

CPU time accounting

Three new fields on the Process struct:

#![allow(unused)]
fn main() {
start_ticks: u64,       // monotonic ticks at creation
utime: AtomicU64,       // user-mode ticks
stime: AtomicU64,       // kernel-mode ticks
}

User time is approximated by incrementing utime in the timer IRQ handler for whichever non-idle process was running when the tick fired. Kernel time is approximated by incrementing stime once per syscall entry. Neither is high-precision, but both are the standard approach for tick-based kernels and match what Linux does with its statistical sampling.

The fields are initialized in all four process creation paths: new_idle_thread, new_init_process, fork, and new_thread. Each captures monotonic_ticks() as the start time.

Thread counting and VM size

Two new methods on Process:

count_threads() — locks PROCESSES and counts entries sharing the same TGID. This replaces the hardcoded 1 in /proc/[pid]/status and the zero in /proc/[pid]/stat field 20.
vm_size_bytes() — sums VMA lengths from the process's Vm. This was previously computed inline in the status file handler; extracting it to Process lets both stat and status share the same logic.

/proc/[pid]/stat fields

The stat file now reports real values for:

Field	Name	Source
3	state	ProcessState -> R/S/T/Z
14	utime	process.utime() atomic
15	stime	process.stime() atomic
20	num_threads	count_threads()
22	starttime	process.start_ticks()
23	vsize	vm_size_bytes()
24	rss	vsize / PAGE_SIZE (approx)

/proc/[pid]/status fields

The status file now reports:

State — mapped from ProcessState (R (running), S (sleeping), T (stopped), Z (zombie))
Uid/Gid — read from the process's uid/euid/gid/egid atomics instead of hardcoded zeros
VmSize/VmRSS — from vm_size_bytes() (shared implementation)
Threads — from count_threads() instead of hardcoded 1

Contract test

The new proc_pid.c test verifies:

/proc/self/stat field 1 (pid) matches getpid()
/proc/self/stat field 3 (state) is 'R' while actively running
/proc/self/stat field 20 (num_threads) is >= 1
/proc/self/status contains Name: and Pid: matching getpid()

Results

22/22 contract tests pass (5/5 subsystem tests including the new proc_pid).

What's next

Phase 4 adds /proc/[pid]/mountinfo and /proc/[pid]/cgroup — two files that glibc and systemd read during early init to discover the mount namespace and cgroup membership.

Kevlar Documentation