On Mon, 4 Jun 2018, Miroslav Lichvar 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.
>
> The update is limited to archs using modern timekeeping
> (!ARCH_USES_GETTIMEOFFSET).
John ?????
>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: John Stultz <john.stu...@linaro.org>
> Cc: Richard Cochran <richardcoch...@gmail.com>
> Cc: Prarit Bhargava <pra...@redhat.com>
> Signed-off-by: Miroslav Lichvar <mlich...@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
> v1->v2:
> - replaced bool parameter with enum
> - changed timekeeping_advance() to not make any updates for TK_ADV_FREQ
> with
> !ARCH_USES_GETTIMEOFFSET (an alternative might be to update with zero
> offset)
>
> kernel/time/timekeeping.c | 36 ++++++++++++++++++++++++++++++------
> 1 file changed, 30 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 49cbceef5deb..a08ef3771ab5 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -34,6 +34,14 @@
> #define TK_MIRROR (1 << 1)
> #define TK_CLOCK_WAS_SET (1 << 2)
>
> +enum timekeeping_adv_mode {
> + /* Update timekeeper when a tick has passed */
> + TK_ADV_TICK,
> +
> + /* Update timekeeper on a direct frequency change */
> + TK_ADV_FREQ
> +};
> +
> /*
> * The most important data for readout fits into a single 64 byte
> * cache line.
> @@ -2021,11 +2029,11 @@ 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(enum timekeeping_adv_mode mode)
> {
> struct timekeeper *real_tk = &tk_core.timekeeper;
> struct timekeeper *tk = &shadow_timekeeper;
> @@ -2042,14 +2050,17 @@ void update_wall_time(void)
>
> #ifdef CONFIG_ARCH_USES_GETTIMEOFFSET
> offset = real_tk->cycle_interval;
> +
> + if (mode != TK_ADV_TICK)
> + goto out;
> #else
> offset = clocksource_delta(tk_clock_read(&tk->tkr_mono),
> tk->tkr_mono.cycle_last, tk->tkr_mono.mask);
> -#endif
>
> /* Check if there's really nothing to do */
> - if (offset < real_tk->cycle_interval)
> + if (offset < real_tk->cycle_interval && mode == TK_ADV_TICK)
> goto out;
> +#endif
>
> /* Do some additional sanity checking */
> timekeeping_check_update(tk, offset);
> @@ -2105,6 +2116,15 @@ 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(TK_ADV_TICK);
> +}
> +
> /**
> * getboottime64 - Return the real time of system boot.
> * @ts: pointer to the timespec64 to be set
> @@ -2332,6 +2352,10 @@ 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(TK_ADV_FREQ);
> +
> if (tai != orig_tai)
> clock_was_set();
>
> --
> 2.14.3
>
>
