svn commit: r352059 - head/sys/kern

[Konstantin Belousov]

On Mon, Sep 09, 2019 at 11:31:32AM -0700, Conrad Meyer wrote:
> What’s the motivation for using more timehands?
See the thread titled 'Is it a good idea to use a usb-serial adapter
for PPS input? Yes, it is.' on arm@ and usb at . In short, there are some
reports of timecounters misbehaving on single-core systems when only two
timehands are enabled.

The change eases the experimentation and provides a workaround which does
not require modifying the sources to apply.

> 
> 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);
> >
> >