Re: [PATCH] sched_clock: Add option to use absolute time against hardware clock reset

[Feng Tang]

On Sat, Sep 05, 2026 at 10:39:26PM +0200, Thomas Gleixner wrote:
> On Wed, Sep 02 2026 at 16:21, Feng Tang wrote:
> > Locally, it did help on chasing some RAS issues which needed cooperation
> > between kernel, SCP firmware and ATF, by mapping the actions from each
> > players into one timeline based on the timestamps in their logs.
> 
> I told you before that we don't care about your bug chasing war stories
> at all. Educate your firmware people and stop pestering us with your
> firmware debug hacks.

Hi Thomas,

Yes, this patch is hacky.

>From the reply of Marc in this thread:

"
I really have to ask: why isn't this just a one-off sampling of the
counter, kept in some user accessible location (debugfs or something
else), and ultimately post-processed to align your logs? People have
been doing this... forever, and that has been "good enough" so far.
"

IIUC, getting an absolute timestamp for logs is still practical in some
cases. How about adding the offset-since-reset info into the message
like Marc suggested:

---
diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index f3aaef695b8c..d5a54276b49d 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -176,7 +176,7 @@ static enum hrtimer_restart sched_clock_poll(struct hrtimer *hrt)
 
 void sched_clock_register(u64 (*read)(void), int bits, unsigned long rate)
 {
-	u64 res, wrap, new_mask, new_epoch, cyc, ns;
+	u64 res, wrap, new_mask, new_epoch, cyc, ns, reset_ns;
 	u32 new_mult, new_shift;
 	unsigned long r, flags;
 	char r_unit;
@@ -235,8 +235,11 @@ void sched_clock_register(u64 (*read)(void), int bits, unsigned long rate)
 	/* Calculate the ns resolution of this counter */
 	res = cyc_to_ns(1ULL, new_mult, new_shift);
 
-	pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lluns\n",
-		bits, r, r_unit, res, wrap);
+	/* Calculate the time since last counter resetting to 0 */
+	reset_ns = mul_u64_u64_div_u64(new_epoch, NSEC_PER_SEC, rate);
+
+	pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lluns, has run %lluns since counter reset\n",
+		bits, r, r_unit, res, wrap, reset_ns);
 
 	/* Enable IRQ time accounting if we have a fast enough sched_clock() */
 	if (irqtime > 0 || (irqtime == -1 && rate >= 1000000))

Thanks,
Feng