timekeeping: Account for clocksource tick quantisation via NTP

cycle_interval is an integer number of counter cycles per NTP interval,
so the real time it represents differs from the nominal
NTP_INTERVAL_LENGTH by up to half a counter period. For coarse
clocksources this is significant: the 3.579545 MHz ACPI PM timer at
HZ=1000 rounds 3579.545 cycles up to 3580, making each tick 1.000127 ms
(+127 PPM).

Commit a386b5af8edd ("time: Compensate for rounding on odd-frequency
clocksources") introduced xtime_remainder to compensate for exactly
this, citing the same 127 PPM ACPI PM example. The compensation is
correct and necessary, but it was applied inside the timekeeping
accumulation in timekeeping.c: subtracted in the mult computation in
timekeeping_adjust() and folded into the ntp_error update in
logarithmic_accumulation(). That keeps the base rate correct and leaves
NTP its full symmetric +/-MAXFREQ range rather than +373/-627 PPM, but
the NTP code in ntp.c never sees it: tick_length is computed without the
correction, so ntp.c's notion of how long a tick is disagrees with the
rate timekeeping actually produces.

Make the offset an explicit part of the NTP tick_length instead. Add
ntp_data::cs_tick_adj, a fixed per-second addend that
ntp_update_frequency() includes alongside ntp_tick_adj and time_freq.
tk_setup_internals() computes it from the difference between the real
cycle_interval duration and the nominal interval, stores it in the
timekeeper, and hands it to NTP through a new argument to ntp_clear() --
which already recomputes the frequency and is invoked after every
clocksource (re)configuration. timekeeping_init() now uses TK_UPDATE_ALL
for this; clearing NTP there is otherwise redundant since ntp_init() has
just initialised it.

ntp.c now computes the true tick rate, giving a single source of truth.
Like ntp_tick_adj, cs_tick_adj stays internal to the kernel: userspace
still sees the nominal 1.000000 ms tick via adjtimex and is unaware of
the addends. timekeeping_adjust() and logarithmic_accumulation() use
ntp_tick / xtime_interval directly, and xtime_remainder is removed.

The base-rate arithmetic is unchanged: ntp_tick becomes
xtime_interval << ntp_error_shift, so the mult division yields the same
base mult and the ntp_error accumulation still nets to zero per tick.

Beyond the cleanup of treating all the tick_length contributions
(nominal interval, ntp_tick_adj, cs_tick_adj, time_freq) consistently
as addends in one place, it also prepares for feed-forward discipline:
a future timekeeping_set_reference() will set tick_length to track an
absolute external reference such as a vmclock, and that path needs
ntp.c to own a tick_length that already reflects the clocksource
quantisation, with no hidden correction applied elsewhere.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Assisted-by: Kiro:claude-opus-4.8
Acked-by: John Stultz <jstultz@google.com>
Link: https://patch.msgid.link/20260621220051.1030462-4-dwmw2@infradead.org

1 files changed, 5 insertions, 3 deletions

diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h
index 264db7c9c071..9c53f44537f0 100644
--- a/include/linux/timekeeper_internal.h
+++ b/include/linux/timekeeper_internal.h
@@ -84,8 +84,6 @@ struct tk_read_base {
  * @cycle_interval:		Number of clock cycles in one NTP interval
  * @xtime_interval:		Number of clock shifted nano seconds in one NTP
  *				interval.
- * @xtime_remainder:		Shifted nano seconds left over when rounding
- *				@cycle_interval
  * @raw_interval:		Shifted raw nano seconds accumulated per NTP interval.
  * @next_leap_ktime:		CLOCK_MONOTONIC time value of a pending leap-second
  * @ntp_tick:			The ntp_tick_length() value currently being
@@ -99,6 +97,10 @@ struct tk_read_base {
  * @ntp_error_shift:		Shift conversion between clock shifted nano seconds and
  *				ntp shifted nano seconds.
  * @ntp_err_mult:		Multiplication factor for scaled math conversion
+ * @cs_tick_adj:		Per-second adjustment handed to NTP via ntp_clear()
+ *				accounting for the difference between the nominal
+ *				NTP interval and the real time taken by the
+ *				clocksource's integer @cycle_interval (upscaled).
  * @skip_second_overflow:	Flag used to avoid updating NTP twice with same second
  * @tai_offset:			The current UTC to TAI offset in seconds
  *
@@ -178,7 +180,6 @@ struct timekeeper {
 
 	u64			cycle_interval;
 	u64			xtime_interval;
-	s64			xtime_remainder;
 	u64			raw_interval;
 
 	ktime_t			next_leap_ktime;
@@ -186,6 +187,7 @@ struct timekeeper {
 	s64			ntp_error;
 	u32			ntp_error_shift;
 	u32			ntp_err_mult;
+	s64			cs_tick_adj;
 	u32			skip_second_overflow;
 	s32			tai_offset;
 };