update_fast_timekeeper(9)

💖 update_fast_timekeeper(9) 💖

okay so bestie, this is the Kernel API Manual page for update_fast_timekeeper(9), and it's lowkey one of those tiny kernel helpers with main-character energy. It updates the fast and NMI safe monotonic timekeeper. no cap. 🎀

✨ NAME ✨

update_fast_timekeeper — update the fast and NMI safe monotonic timekeeper. That's the whole name-line vibe, and honestly it ate. 💅

🌸 SYNOPSIS 🌸

void update_fast_timekeeper (const struct tk_read_base *tkr , struct
tk_fast *tkf );

🎀 ARGUMENTS 🎀

• tkr — timekeeping readout base from which we take the update. It's giving "source info, but make it kernel." 🫶
• tkf — pointer to the NMI safe timekeeper. Just a pointer, bestie, but a very important one. ✨

🦋 DESCRIPTION 🦋

okay so the whole plot: we want to use this from any context, including NMI and tracing / instrumenting the timekeeping code itself. That's a big ask, and the function is literally built for it. 🔥

Employ the latch technique; see write_seqcount_latch. That's the iconic move. 🧚

So if a NMI hits the update of base[0] then it will use base[1] which is still consistent. That's the whole safety slay. 🦋

In the worst case this can result in a slightly wrong timestamp (a few nanoseconds). Yes, literally a few nanoseconds. See ktime_get_mono_fast_ns. It's a tiny caveat, not a whole crisis. 💖

💜 SEE ALSO 💜

Kernel file: ./kernel/time/timekeeping.c.

• struct tk_fast(9)
• tk_setup_internals(9)
• ktime_get_mono_fast_ns(9)
• ktime_get_raw_fast_ns(9)
• ktime_get_boot_fast_ns(9)
• ktime_get_tai_fast_ns(9)
• ktime_get_real_fast_ns(9)
• halt_fast_timekeeper(9)
• pvclock_gtod_register_notifier(9)
• pvclock_gtod_unregister_notifier(9)
• timekeeping_forward_now(9)
• ktime_get_real_ts64(9)
• ktime_mono_to_any(9)
• ktime_get_raw(9)
• ktime_get_ts64(9)
• ktime_get_seconds(9)
• ktime_get_real_seconds(9)
• __ktime_get_real_seconds(9)
• ktime_get_snapshot_id(9)
• adjust_historical_crosststamp(9)
• ktime_real_to_base_clock(9)
• get_device_system_crosststamp(9)
• timekeeping_clocksource_has_base(9)
• do_settimeofday64(9)
• __timekeeping_inject_offset(9)
• timekeeping_notify(9)
• ktime_get_raw_ts64(9)
• timekeeping_valid_for_hres(9)
• timekeeping_max_deferment(9)
• read_persistent_clock64(9)
• read_persistent_wall_and_boot_offset(9)
• __timekeeping_inject_sleeptime(9)
• timekeeping_inject_sleeptime64(9)
• timekeeping_resume(9)
• update_wall_time(9)
• getboottime64(9)
• ktime_get_coarse_real_ts64_mg(9)
• ktime_get_real_ts64_mg(9)
• ktime_get_update_offsets_now(9)
• random_get_entropy_fallback(9)
• do_adjtimex(9)
• hardpps(9)
• ktime_get_aux(9)
• ktime_get_aux_ts64(9)

mwah 💋 that's the page! This one is from source ./kernel/time, dated October 2026, and it's all about update_fast_timekeeper(9). go be fast, go be NMI safe, and remember: periodt. 🌈👑