Question on sched_clock

[Arun KS]

On Thu, Jul 20, 2023 at 4:07 PM Peter Zijlstra <peterz at infradead.org> wrote:
>
> On Thu, Jul 20, 2023 at 03:54:56PM +0530, Arun KS wrote:
> > CCing maintainers
> >
> > On Wed, Jul 19, 2023 at 10:36 AM Arun KS <arunks.linux at gmail.com> wrote:
> > >
> > > Hi,
> > >
> > > Kernel’s printk uses local_clock() for timestamps and it is mapped to
> > > sched_clock(). Two problems/requirements I see,
> > >
> > > One, Kernel’s printk timestamps start from 0, I want to change this to
> > > match with actual time since boot.
>
> You can fundamentally only consistently tell time since the clock gets
> initialized. Starting at 0 is what you get.
>
> > > Two, sched_clock() doesn’t account for time spend in low power
> > > state(suspend to ram)
>
> Why would we do that? The next person will complain that they don't want
> this. Then another person complains they also want time spend in
> suspend-to-disk, and another person wants a pony.

Thanks Peter, Got your point. I was trying to understand if there are
any other side effects of doing this downstream.

>
> > >
> > > Could workout patches to modify these behaviours and found working in
> > > my system. But need to hear expert opinion on why this is not done in
> > > the upstream.
> > >
> > > diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
> > > index 68d6c1190ac7..b63b2ded5727 100644
> > > --- a/kernel/time/sched_clock.c
> > > +++ b/kernel/time/sched_clock.c
>
> This is only one of many sched_clock implementations...
>
> > > @@ -190,7 +190,10 @@ sched_clock_register(u64 (*read)(void), int bits,
> > > unsigned long rate)
> > >         /* Update epoch for new counter and update 'epoch_ns' from old counter*/
> > >         new_epoch = read();
> > >         cyc = cd.actual_read_sched_clock();
> > > -       ns = rd.epoch_ns + cyc_to_ns((cyc - rd.epoch_cyc) &
> > > rd.sched_clock_mask, rd.mult, rd.shift);
> > > +       if (!cyc)
> > > +               ns = cyc_to_ns(new_epoch, new_mult, new_shift)
> > > +       else
> > > +               ns = rd.epoch_ns + cyc_to_ns((cyc - rd.epoch_cyc) &
> > > rd.sched_clock_mask, rd.mult, rd.shift);
> > >         cd.actual_read_sched_clock = read;
> > >
> > >         rd.read_sched_clock     = read;
> > >
> > > @@ -287,7 +290,6 @@ void sched_clock_resume(void)
> > >  {
> > >         struct clock_read_data *rd = &cd.read_data[0];
> > >
> > > -       rd->epoch_cyc = cd.actual_read_sched_clock();
>
> And what if you've been suspended long enough to wrap the clock ?!?
Fair point. Will take a note.

>
> > >         hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL_HARD);
> > >         rd->read_sched_clock = cd.actual_read_sched_clock;
> > >  }
> > >
> > > Regards,
> > > Arun