[PATCHv1] timekeeping: Update multiplier when NTP frequency is set directly

[PATCHv1] timekeeping: Update multiplier when NTP frequency is set directly

[Miroslav Lichvar]

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

Re: [PATCHv1] timekeeping: Update multiplier when NTP frequency is set directly

[John Stultz]

On Tue, May 29, 2018 at 3:53 AM, Miroslav Lichvar <mlichvar@redhat.com> wrote:
> 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)

This is kind of a nit, but mind switching out bool for an enum?  Using
something like TK_ADV_NORMAL and TK_ADV_FORCE?

> +void update_wall_time(void)
> +{
> +       timekeeping_advance(false);
> +}

The enum makes usage like timekeeping_advance(false) a little less
opaque to the reader ("Wait, don't advance? Let me go look at the
function").

We got bitten by this earlier when we had the old
"timekeeping_update(tk, true, false, true)" usage.

thanks
-john

Re: [PATCHv1] timekeeping: Update multiplier when NTP frequency is set directly

[Miroslav Lichvar]

On Tue, May 29, 2018 at 10:42:05AM -0700, John Stultz wrote:
> On Tue, May 29, 2018 at 3:53 AM, Miroslav Lichvar <mlichvar@redhat.com> wrote:
> > -void update_wall_time(void)
> > +static void timekeeping_advance(bool force_update)
> 
> This is kind of a nit, but mind switching out bool for an enum?  Using
> something like TK_ADV_NORMAL and TK_ADV_FORCE?
> 
> > +void update_wall_time(void)
> > +{
> > +       timekeeping_advance(false);
> > +}
> 
> The enum makes usage like timekeeping_advance(false) a little less
> opaque to the reader ("Wait, don't advance? Let me go look at the
> function").
> 
> We got bitten by this earlier when we had the old
> "timekeeping_update(tk, true, false, true)" usage.

Ok. That make sense. I'll send a v2.

Thanks,

-- 
Miroslav Lichvar