linux-kernel - [PATCH 1/3] sched_clock: Round the frequency reported to nearest rather than down

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Date:   Sun, 24 Apr 2022 12:47:20 +0100 (BST)
From:   "Maciej W. Rozycki" <macro@...am.me.uk>
To:     John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Ingo Molnar <mingo@...nel.org>
cc:     linux-kernel@...r.kernel.org
Subject: [PATCH 1/3] sched_clock: Round the frequency reported to nearest
 rather than down

We currently round the frequency reported for clock sources down, which 
gives misleading figures, e.g.:

I/O ASIC clock frequency 24999480Hz
clocksource: dec-ioasic: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 76452008078 ns
sched_clock: 32 bits at 24MHz, resolution 40ns, wraps every 85901132779ns
MIPS counter frequency 59998512Hz
clocksource: MIPS: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 31855130776 ns
sched_clock: 32 bits at 59MHz, resolution 16ns, wraps every 35792281591ns

Rounding to nearest seems more adequate:

I/O ASIC clock frequency 24999664Hz
clocksource: dec-ioasic: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 76451445358 ns
sched_clock: 32 bits at 25MHz, resolution 40ns, wraps every 85900499947ns
MIPS counter frequency 59999728Hz
clocksource: MIPS: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 31854485176 ns
sched_clock: 32 bits at 60MHz, resolution 16ns, wraps every 35791556599ns

Signed-off-by: Maciej W. Rozycki <macro@...am.me.uk>
Fixes: 112f38a4a316 ("ARM: sched_clock: provide common infrastructure for sched_clock()")
---
 kernel/time/sched_clock.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

linux-sched-clock-rate-round.diff
Index: linux-macro/kernel/time/sched_clock.c
===================================================================
--- linux-macro.orig/kernel/time/sched_clock.c
+++ linux-macro/kernel/time/sched_clock.c
@@ -8,6 +8,7 @@
 #include <linux/jiffies.h>
 #include <linux/ktime.h>
 #include <linux/kernel.h>
+#include <linux/math.h>
 #include <linux/moduleparam.h>
 #include <linux/sched.h>
 #include <linux/sched/clock.h>
@@ -199,11 +200,11 @@ sched_clock_register(u64 (*read)(void),
 
 	r = rate;
 	if (r >= 4000000) {
-		r /= 1000000;
+		r = DIV_ROUND_CLOSEST(r, 1000000);
 		r_unit = 'M';
 	} else {
 		if (r >= 1000) {
-			r /= 1000;
+			r = DIV_ROUND_CLOSEST(r, 1000);
 			r_unit = 'k';
 		} else {
 			r_unit = ' ';