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Subject[PATCHv1] timekeeping: Update multiplier when NTP frequency is set directly
Date
When the NTP frequency is set directly from userspace using the
ADJ_FREQUENCY or ADJ_TICK timex mode, immediately update the
timekeeper's multiplier instead of waiting for the next tick.

This removes a hidden non-deterministic delay in setting of the
frequency and allows an extremely tight control of the system clock
with update rates close to or even exceeding the kernel HZ.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
---

Notes:
RFC->v1:
- added a new parameter to force the update of the timekeeper to the current
NTP tick length only from adjtimex()
- added timekeeping_advance() to keep the parameter local to timekeeping.c

kernel/time/timekeeping.c | 23 ++++++++++++++++++-----
1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 49cbceef5deb..5524c07d43e3 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -2021,11 +2021,11 @@ static u64 logarithmic_accumulation(struct timekeeper *tk, u64 offset,
return offset;
}

-/**
- * update_wall_time - Uses the current clocksource to increment the wall time
- *
+/*
+ * timekeeping_advance - Updates the timekeeper to the current time and
+ * current NTP tick length
*/
-void update_wall_time(void)
+static void timekeeping_advance(bool force_update)
{
struct timekeeper *real_tk = &tk_core.timekeeper;
struct timekeeper *tk = &shadow_timekeeper;
@@ -2048,7 +2048,7 @@ void update_wall_time(void)
#endif

/* Check if there's really nothing to do */
- if (offset < real_tk->cycle_interval)
+ if (offset < real_tk->cycle_interval && !force_update)
goto out;

/* Do some additional sanity checking */
@@ -2105,6 +2105,15 @@ void update_wall_time(void)
clock_was_set_delayed();
}

+/**
+ * update_wall_time - Uses the current clocksource to increment the wall time
+ *
+ */
+void update_wall_time(void)
+{
+ timekeeping_advance(false);
+}
+
/**
* getboottime64 - Return the real time of system boot.
* @ts: pointer to the timespec64 to be set
@@ -2332,6 +2341,10 @@ int do_adjtimex(struct timex *txc)
write_seqcount_end(&tk_core.seq);
raw_spin_unlock_irqrestore(&timekeeper_lock, flags);

+ /* Update the multiplier immediately if frequency was set directly */
+ if (txc->modes & (ADJ_FREQUENCY | ADJ_TICK))
+ timekeeping_advance(true);
+
if (tai != orig_tai)
clock_was_set();

--
2.14.3
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