linux-kernel - [PATCH MANUALSEL 4.9 1/3] time/sched_clock: Round the frequency reported to nearest rather than down

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Message-Id: <20220605135547.61902-1-sashal@kernel.org>
Date:   Sun,  5 Jun 2022 09:55:43 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     "Maciej W. Rozycki" <macro@...am.me.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Stultz <jstultz@...gle.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH MANUALSEL 4.9 1/3] time/sched_clock: Round the frequency reported to nearest rather than down

From: "Maciej W. Rozycki" <macro@...am.me.uk>

[ Upstream commit 92067440f1311dfa4d77b57a9da6b3706f5da32e ]

The frequency reported for clock sources are rounded down, which gives
misleading figures, e.g.:

 I/O ASIC clock frequency 24999480Hz
 sched_clock: 32 bits at 24MHz, resolution 40ns, wraps every 85901132779ns
 MIPS counter frequency 59998512Hz
 sched_clock: 32 bits at 59MHz, resolution 16ns, wraps every 35792281591ns

Rounding to nearest is more adequate:

 I/O ASIC clock frequency 24999664Hz
 sched_clock: 32 bits at 25MHz, resolution 40ns, wraps every 85900499947ns
 MIPS counter frequency 59999728Hz
 sched_clock: 32 bits at 60MHz, resolution 16ns, wraps every 35791556599ns

Signed-off-by: Maciej W. Rozycki <macro@...am.me.uk>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Acked-by: John Stultz <jstultz@...gle.com>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2204240055590.9383@angie.orcam.me.uk
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 kernel/time/sched_clock.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 382b159d8592..0a828ba65c0b 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -11,6 +11,7 @@
 #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/syscore_ops.h>
@@ -212,11 +213,11 @@ sched_clock_register(u64 (*read)(void), int bits, unsigned long rate)
 
 	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 = ' ';
-- 
2.35.1