svn commit: r352059 - head/sys/kern

[Conrad Meyer]

What’s the motivation for using more timehands?

On Mon, Sep 9, 2019 at 04:30 Konstantin Belousov <kib at freebsd.org> wrote:

> Author: kib
> Date: Mon Sep  9 11:29:58 2019
> New Revision: 352059
> URL: https://svnweb.freebsd.org/changeset/base/352059
>
> Log:
>   Make timehands count selectable at boottime.
>
>   Tested by:    O'Connor, Daniel <darius at dons.net.au>
>   Sponsored by: The FreeBSD Foundation
>   MFC after:    1 week
>   Differential revision:        https://reviews.freebsd.org/D21563
>
> Modified:
>   head/sys/kern/kern_tc.c
>
> Modified: head/sys/kern/kern_tc.c
>
> ==============================================================================
> --- head/sys/kern/kern_tc.c     Mon Sep  9 11:22:38 2019        (r352058)
> +++ head/sys/kern/kern_tc.c     Mon Sep  9 11:29:58 2019        (r352059)
> @@ -83,19 +83,16 @@ struct timehands {
>         struct timehands        *th_next;
>  };
>
> -static struct timehands th0;
> -static struct timehands th1 = {
> -       .th_next = &th0
> -};
> -static struct timehands th0 = {
> +static struct timehands ths[16] = {
> +    [0] =  {
>         .th_counter = &dummy_timecounter,
>         .th_scale = (uint64_t)-1 / 1000000,
>         .th_offset = { .sec = 1 },
>         .th_generation = 1,
> -       .th_next = &th1
> +    },
>  };
>
> -static struct timehands *volatile timehands = &th0;
> +static struct timehands *volatile timehands = &ths[0];
>  struct timecounter *timecounter = &dummy_timecounter;
>  static struct timecounter *timecounters = &dummy_timecounter;
>
> @@ -115,6 +112,10 @@ static int timestepwarnings;
>  SYSCTL_INT(_kern_timecounter, OID_AUTO, stepwarnings, CTLFLAG_RW,
>      &timestepwarnings, 0, "Log time steps");
>
> +static int timehands_count = 2;
> +SYSCTL_INT(_kern_timecounter, OID_AUTO, timehands_count, CTLFLAG_RDTUN,
> +    &timehands_count, 0, "Count of timehands in rotation");
> +
>  struct bintime bt_timethreshold;
>  struct bintime bt_tickthreshold;
>  sbintime_t sbt_timethreshold;
> @@ -1960,8 +1961,9 @@ done:
>  static void
>  inittimecounter(void *dummy)
>  {
> +       struct timehands *thp;
>         u_int p;
> -       int tick_rate;
> +       int i, tick_rate;
>
>         /*
>          * Set the initial timeout to
> @@ -1987,6 +1989,16 @@ inittimecounter(void *dummy)
>  #ifdef FFCLOCK
>         ffclock_init();
>  #endif
> +
> +       /* Set up the requested number of timehands. */
> +       if (timehands_count < 1)
> +               timehands_count = 1;
> +       if (timehands_count > nitems(ths))
> +               timehands_count = nitems(ths);
> +       for (i = 1, thp = &ths[0]; i < timehands_count;  thp = &ths[i++])
> +               thp->th_next = &ths[i];
> +       thp->th_next = &ths[0];
> +
>         /* warm up new timecounter (again) and get rolling. */
>         (void)timecounter->tc_get_timecount(timecounter);
>         (void)timecounter->tc_get_timecount(timecounter);
>
>