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From:   Arun KS <arunks.linux@gmail.com>
Date:   Thu, 20 Jul 2023 15:54:56 +0530
Message-ID: <CAKZGPAOY9uxvSTuta+4a41=GGqTxus8YDcv54T7iAL9P-fj_+w@mail.gmail.com>
Subject: Re: Question on sched_clock
To:     linux-kernel@vger.kernel.org
Cc:     linux-arm-kernel@lists.infradead.org,
        Arun KS <getarunks@gmail.com>, peterz@infradead.org,
        pmladek@suse.com, tglx@linutronix.de
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CCing maintainers

On Wed, Jul 19, 2023 at 10:36=E2=80=AFAM Arun KS <arunks.linux@gmail.com> w=
rote:
>
> Hi,
>
> Kernel=E2=80=99s printk uses local_clock() for timestamps and it is mappe=
d to
> sched_clock(). Two problems/requirements I see,
>
> One, Kernel=E2=80=99s printk timestamps start from 0, I want to change th=
is to
> match with actual time since boot.
> Two, sched_clock() doesn=E2=80=99t 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 co=
unter*/
>         new_epoch =3D read();
>         cyc =3D cd.actual_read_sched_clock();
> -       ns =3D rd.epoch_ns + cyc_to_ns((cyc - rd.epoch_cyc) &
> rd.sched_clock_mask, rd.mult, rd.shift);
> +       if (!cyc)
> +               ns =3D cyc_to_ns(new_epoch, new_mult, new_shift)
> +       else
> +               ns =3D rd.epoch_ns + cyc_to_ns((cyc - rd.epoch_cyc) &
> rd.sched_clock_mask, rd.mult, rd.shift);
>         cd.actual_read_sched_clock =3D read;
>
>         rd.read_sched_clock     =3D read;
>
> @@ -287,7 +290,6 @@ void sched_clock_resume(void)
>  {
>         struct clock_read_data *rd =3D &cd.read_data[0];
>
> -       rd->epoch_cyc =3D cd.actual_read_sched_clock();
>         hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL_HA=
RD);
>         rd->read_sched_clock =3D cd.actual_read_sched_clock;
>  }
>
> Regards,
> Arun