Internals: the kernel log (klog)

kernel/lib/lib_klog.{c,h} - 29 declarations; the numbered 6:

klog_write(level, fmt, ...)      /* the entry point */
klog_set_level(lvl)
klog_set_level_enabled(lvl, on)
klog_flush()
klog_rotate()
klog_filter_set(min_level)

Behaviour (E2E-verified, not aspirational)

• Levels DEBUG/INFO/WARN/ERROR/FATAL; per-level enable + a global minimum filter.
• The ring buffer mirrors to serial; the persistent file (kernel.log on /etc) flushes on demand.
• Rotation at the 4096-byte threshold: kernel.log becomes kernel.log.1 (a real generational pair - S5 asserts both sizes).
• Filter proof: level >= 3 admits ERR/FATAL only while dbg/inf/wrn counts go to zero (S6's exact assertion).

Commands

dmesg (ring buffer), logcat (live follow), logctl (levels/filters), crashlog (crash records).

Who writes

Every subsystem's error paths + the health alarms + the watchdog policy kills - the log IS the system narrative; logctl is how you turn the noise down without losing the errors.