Question on sched_clock

[Arun KS]

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.
> Two, sched_clock() doesn’t account for time spend in low power
> state(suspend to ram)
>
> 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
> @@ -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();
>         hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL_HARD);
>         rd->read_sched_clock = cd.actual_read_sched_clock;
>  }
>
> Regards,
> Arun