Re: git: 1d2421ad8b6d - main - Correctly measure system load averages > 1024

Yes, it can be MFCd.  The only risk I'm aware of is that the 4.4 bsd
scheduler might start acting weird - once the load average gets close to
one million.

On Fri, May 6, 2022, 6:06 PM Kubilay Kocak <koobs@freebsd.org> wrote:

> On 7/05/2022 10:04 am, Alan Somers wrote:
> > The branch main has been updated by asomers:
> >
> > URL:
> https://cgit.FreeBSD.org/src/commit/?id=1d2421ad8b6d508ef155752bdfc5948f7373bac3
> >
> > commit 1d2421ad8b6d508ef155752bdfc5948f7373bac3
> > Author:     Alan Somers <asomers@FreeBSD.org>
> > AuthorDate: 2022-05-05 21:35:23 +0000
> > Commit:     Alan Somers <asomers@FreeBSD.org>
> > CommitDate: 2022-05-06 23:25:43 +0000
> >
> >      Correctly measure system load averages > 1024
> >
> >      The old fixed-point arithmetic used for calculating load averages
> had an
> >      overflow at 1024.  So on systems with extremely high load, the
> observed
> >      load average would actually fall back to 0 and shoot up again,
> creating
> >      a kind of sawtooth graph.
> >
> >      Fix this by using 64-bit math internally, while still reporting the
> load
> >      average to userspace as a 32-bit number.
> >
> >      Sponsored by:   Axcient
> >      Reviewed by:    imp
> >      Differential Revision: https://reviews.freebsd.org/D35134
>
> Can MFC?
>
> > ---
> >   sys/kern/kern_synch.c | 9 +++++----
> >   sys/kern/tty_info.c   | 2 +-
> >   sys/sys/param.h       | 8 ++++----
> >   3 files changed, 10 insertions(+), 9 deletions(-)
> >
> > diff --git a/sys/kern/kern_synch.c b/sys/kern/kern_synch.c
> > index e78878987b57..381d6315044c 100644
> > --- a/sys/kern/kern_synch.c
> > +++ b/sys/kern/kern_synch.c
> > @@ -87,7 +87,7 @@ struct loadavg averunnable =
> >    * Constants for averages over 1, 5, and 15 minutes
> >    * when sampling at 5 second intervals.
> >    */
> > -static fixpt_t cexp[3] = {
> > +static uint64_t cexp[3] = {
> >       0.9200444146293232 * FSCALE,    /* exp(-1/12) */
> >       0.9834714538216174 * FSCALE,    /* exp(-1/60) */
> >       0.9944598480048967 * FSCALE,    /* exp(-1/180) */
> > @@ -611,14 +611,15 @@ setrunnable(struct thread *td, int srqflags)
> >   static void
> >   loadav(void *arg)
> >   {
> > -     int i, nrun;
> > +     int i;
> > +     uint64_t nrun;
> >       struct loadavg *avg;
> >
> > -     nrun = sched_load();
> > +     nrun = (uint64_t)sched_load();
> >       avg = &averunnable;
> >
> >       for (i = 0; i < 3; i++)
> > -             avg->ldavg[i] = (cexp[i] * avg->ldavg[i] +
> > +             avg->ldavg[i] = (cexp[i] * (uint64_t)avg->ldavg[i] +
> >                   nrun * FSCALE * (FSCALE - cexp[i])) >> FSHIFT;
> >
> >       /*
> > diff --git a/sys/kern/tty_info.c b/sys/kern/tty_info.c
> > index 60675557e4ed..237aa47a18da 100644
> > --- a/sys/kern/tty_info.c
> > +++ b/sys/kern/tty_info.c
> > @@ -302,7 +302,7 @@ tty_info(struct tty *tp)
> >       sbuf_set_drain(&sb, sbuf_tty_drain, tp);
> >
> >       /* Print load average. */
> > -     load = (averunnable.ldavg[0] * 100 + FSCALE / 2) >> FSHIFT;
> > +     load = ((int64_t)averunnable.ldavg[0] * 100 + FSCALE / 2) >>
> FSHIFT;
> >       sbuf_printf(&sb, "%sload: %d.%02d ", tp->t_column == 0 ? "" : "\n",
> >           load / 100, load % 100);
> >
> > diff --git a/sys/sys/param.h b/sys/sys/param.h
> > index 2d463b9ac7a2..b0b53f1a7776 100644
> > --- a/sys/sys/param.h
> > +++ b/sys/sys/param.h
> > @@ -361,12 +361,12 @@ __END_DECLS
> >    * Scale factor for scaled integers used to count %cpu time and load
> avgs.
> >    *
> >    * The number of CPU `tick's that map to a unique `%age' can be
> expressed
> > - * by the formula (1 / (2 ^ (FSHIFT - 11))).  The maximum load average
> that
> > - * can be calculated (assuming 32 bits) can be closely approximated
> using
> > - * the formula (2 ^ (2 * (16 - FSHIFT))) for (FSHIFT < 15).
> > + * by the formula (1 / (2 ^ (FSHIFT - 11))).  Since the intermediate
> > + * calculation is done with 64-bit precision, the maximum load average
> that can
> > + * be calculated is approximately 2^32 / FSCALE.
> >    *
> >    * For the scheduler to maintain a 1:1 mapping of CPU `tick' to `%age',
> > - * FSHIFT must be at least 11; this gives us a maximum load avg of
> ~1024.
> > + * FSHIFT must be at least 11.  This gives a maximum load avg of 2
> million.
> >    */
> >   #define     FSHIFT  11              /* bits to right of fixed binary
> point */
> >   #define FSCALE      (1<<FSHIFT)
> >
>
>
>