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From:   Arun KS <arunks.linux@gmail.com>
Date:   Wed, 19 Jul 2023 10:36:08 +0530
Message-ID: <CAKZGPAOYPp3ANWfBWxcsT3TJdPt8jH-f2ZJzpin=UZ=-b_-QFg@mail.gmail.com>
Subject: Question on sched_clock
To:     linux-kernel@vger.kernel.org
Cc:     linux-arm-kernel@lists.infradead.org, Arun KS <getarunks@gmail.com>
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Hi,

Kernel=E2=80=99s printk uses local_clock() for timestamps and it is mapped =
to
sched_clock(). Two problems/requirements I see,

One, Kernel=E2=80=99s printk timestamps start from 0, I want to change this=
 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 coun=
ter*/
        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_HARD=
);
        rd->read_sched_clock =3D cd.actual_read_sched_clock;
 }

Regards,
Arun