Re: [PATCH v2 4/4] kernel/time/clockevents: compensate for monotonic clock's dynamic frequency

[Nicolai Stange <nicstange@gmail.com>]

Nicolai Stange <nicstange@gmail.com> writes:

> With NOHZ_FULL and one single well-isolated, CPU consumptive task, one
> would expect approximately one clockevent interrupt per second. However, on
> my Intel Haswell where the monotonic clock is the TSC monotonic clock and
> the clockevent device is the TSC deadline device, it turns out that every
> second, there are two such interrupts: the first one arrives always
> approximately ~50us before the scheduled deadline as programmed by
> tick_nohz_stop_sched_tick() through the hrtimer API. The
> __hrtimer_run_queues() called in this interrupt detects that the queued
> tick_sched_timer hasn't expired yet and simply does nothing except
> reprogramming the clock event device to fire shortly after again.
>
> These too early programmed deadlines are explained as follows:
> clockevents_program_event() programs the clockevent device to fire
> after
>   f_event * delta_t_progr
> clockevent device cycles where f_event is the clockevent device's hardware
> frequency and delta_t_progr is the requested time interval. After that many
> clockevent device cycles have elapsed, the device underlying the monotonic
> clock, that is the monotonic raw clock has seen f_raw / f_event as many
> cycles.
> The ktime_get() called from __hrtimer_run_queues() interprets those
> cycles to run at the frequency of the monotonic clock. Summarizing:
>   delta_t_perc = 1/f_mono * f_raw/f_event * f_event * delta_t_progr
>                = f_raw / f_mono * delta_t_progr
> with f_mono being the monotonic clock's frequency and delta_t_perc being
> the elapsed time interval as perceived by __hrtimer_run_queues().
>
> Now, f_mono is not a constant, but is dynamically adjusted in
> timekeeping_adjust() in order to compensate for the NTP error. With the
> large values of delta_t_progr of 10^9ns with NOHZ_FULL, the error made
> becomes significant and results in the double timer interrupts described
> above.
>
> Compensate for this error by multiplying delta_t_progr with f_mono / f_raw
> in clockevents_program_event() before actually programming the clockevent
> device.
>
> Namely, introduce a helper, timekeeping_mono_interval_to_raw(), which
> converts a given time interval from the monotonic clock's perception to
> that of the raw monotonic clock by multiplying the value by f_mono / f_raw.
> Call that helper from clockevents_program_event() in order to obtain a
> suitable time interval to program the clockevent device with.
>
> Signed-off-by: Nicolai Stange <nicstange@gmail.com>
> ---
>  kernel/time/clockevents.c |  1 +
>  kernel/time/timekeeping.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++
>  kernel/time/timekeeping.h |  1 +
>  3 files changed, 61 insertions(+)
>
> diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
> index a9b76a4..4bccf04 100644
> --- a/kernel/time/clockevents.c
> +++ b/kernel/time/clockevents.c
> @@ -329,6 +329,7 @@ int clockevents_program_event(struct clock_event_device *dev, ktime_t expires,
>  		return dev->set_next_ktime(expires, dev);
>  
>  	delta = ktime_to_ns(ktime_sub(expires, ktime_get()));
> +	delta = timekeeping_mono_interval_to_raw(delta);
>  	if (delta <= 0)
>  		return force ? clockevents_program_min_delta(dev) : -ETIME;
>  
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index dcd5ce6..51dfbbb 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -23,6 +23,7 @@
>  #include <linux/stop_machine.h>
>  #include <linux/pvclock_gtod.h>
>  #include <linux/compiler.h>
> +#include <asm/div64.h>
>  
>  #include "tick-internal.h"
>  #include "ntp_internal.h"
> @@ -2133,6 +2134,64 @@ out:
>  }
>  
>  /**
> + * timekeeper_mono_interval_to_raw - Convert mono interval to raw's perception.
> + * @interval: Time interval as measured by the mono clock.
> + *
> + * Converts the given time interval as measured by the monotonic clock to
> + * what would have been measured by the raw monotonic clock in the meanwhile.
> + * The monotonic clock's frequency gets dynamically adjusted every now and then
> + * in order to compensate for the differences to NTP. OTOH, the clockevents
> + * devices are not affected by this adjustment, i.e. they keep ticking at some
> + * fixed hardware frequency which may be assumed to have a constant ratio to
> + * the fixed raw monotonic clock's frequency. This function provides a means
> + * to convert time intervals from the dynamic frequency monotonic clock to
> + * the fixed frequency hardware world.
> + *
> + * If interval < 0, zero is returned. If an overflow happens during the
> + * calculation, KTIME_MAX is returned.
> + */
> +s64 timekeeping_mono_interval_to_raw(s64 interval)
> +{
> +	struct timekeeper *tk = &tk_core.timekeeper;
> +	u32 raw_mult = tk->tkr_raw.mult, mono_mult = tk->tkr_mono.mult;
> +	u64 raw, tmp;
> +
> +	/* The overflow checks below can't deal with negative intervals. */
> +	if (interval <= 0)
> +		return 0;
> +
> +	/*
> +	 * Calculate
> +	 *   raw = f_mono / f_raw * interval
> +	 *       = (raw_mult / 2^raw_shift) / (mono_mult / 2^mono_shift)
> +	 *            * interval
> +	 * where f_mono and f_raw denote the frequencies of the monotonic
> +	 * and raw clock respectively.
> +	 *
> +	 * Note that the monotonic and raw clocks' shifts are equal and fixed,
> +	 * that is they cancel.
> +	 */
> +
> +	/* First, calculate interval * raw_mult while checking for overflow. */

After thinking further about this, I had to recognize that
  (raw_mult - mono_mult) * interval
is *much* less likely to overflow.

So, I'll send a v3 doing

  raw = interval + (((raw_mult - mono_mult) * interval) / mono_mult)

during the course of the day.



> +	raw = ((u64)interval >> 32) * raw_mult; /* Upper half of interval */
> +	if (raw >> 32)
> +		return KTIME_MAX;
> +	raw <<= 32;
> +	tmp = ((u64)interval & U32_MAX) * raw_mult; /* Lower half of interval */
> +	if (U64_MAX - raw < tmp)
> +		return KTIME_MAX;
> +	raw += tmp;
> +
> +	/* Finally, do raw /= mono_mult with proper rounding. */
> +	if (U64_MAX - raw < mono_mult / 2)
> +		return KTIME_MAX;
> +	raw += mono_mult / 2;
> +	do_div(raw, mono_mult);
> +
> +	return (s64)raw;
> +}
> +
> +/**
>   * getboottime64 - Return the real time of system boot.
>   * @ts:		pointer to the timespec64 to be set
>   *
> diff --git a/kernel/time/timekeeping.h b/kernel/time/timekeeping.h
> index 704f595..40a0fa9 100644
> --- a/kernel/time/timekeeping.h
> +++ b/kernel/time/timekeeping.h
> @@ -18,6 +18,7 @@ extern void timekeeping_resume(void);
>  
>  extern void do_timer(unsigned long ticks);
>  extern void update_wall_time(void);
> +extern s64 timekeeping_mono_interval_to_raw(s64 interval);
>  
>  extern seqlock_t jiffies_lock;