linux-kernel - [PATCH v2 4/7] MIPS: csrc-r4k: Don't register as sched

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Message-Id: <20240612-mips-clks-v2-4-a57e6f49f3db@flygoat.com>
Date: Wed, 12 Jun 2024 09:54:31 +0100
From: Jiaxun Yang <jiaxun.yang@...goat.com>
To: Thomas Bogendoerfer <tsbogend@...ha.franken.de>, 
 Serge Semin <fancer.lancer@...il.com>, 
 Daniel Lezcano <daniel.lezcano@...aro.org>, 
 Thomas Gleixner <tglx@...utronix.de>
Cc: "Maciej W. Rozycki" <macro@...am.me.uk>, linux-mips@...r.kernel.org, 
 linux-kernel@...r.kernel.org, Jiaxun Yang <jiaxun.yang@...goat.com>
Subject: [PATCH v2 4/7] MIPS: csrc-r4k: Don't register as sched_clock if
 unfit

When we have more than one CPU in system, counter synchronisation
overhead can lead to a scenario that sched_clock goes backward when
being read from different CPUs.

This is accommodated by CONFIG_HAVE_UNSTABLE_SCHED_CLOCK, but it's
unavailable on 32bit kernel.

We don't want to risk sched_clock correctness, so if we have multiple
CPU in system and CONFIG_HAVE_UNSTABLE_SCHED_CLOCK is not set, we just
don't use counter as sched_clock source.

Signed-off-by: Jiaxun Yang <jiaxun.yang@...goat.com>
---
 arch/mips/kernel/csrc-r4k.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/arch/mips/kernel/csrc-r4k.c b/arch/mips/kernel/csrc-r4k.c
index 055747a7417d..bdb1fa8931f4 100644
--- a/arch/mips/kernel/csrc-r4k.c
+++ b/arch/mips/kernel/csrc-r4k.c
@@ -68,6 +68,18 @@ static bool rdhwr_count_usable(void)
 	return false;
 }

+static inline __init bool count_can_be_sched_clock(void)
+{
+	if (IS_ENABLED(CONFIG_CPU_FREQ))
+		return false;
+
+	if (num_possible_cpus() > 1 &&
+			!IS_ENABLED(CONFIG_HAVE_UNSTABLE_SCHED_CLOCK))
+		return false;
+
+	return true;
+}
+
 #ifdef CONFIG_CPU_FREQ

 static bool __read_mostly r4k_clock_unstable;
@@ -125,9 +137,8 @@ int __init init_r4k_clocksource(void)

 	clocksource_register_hz(&clocksource_mips, mips_hpt_frequency);

-#ifndef CONFIG_CPU_FREQ
-	sched_clock_register(r4k_read_sched_clock, 32, mips_hpt_frequency);
-#endif
+	if (count_can_be_sched_clock())
+		sched_clock_register(r4k_read_sched_clock, 32, mips_hpt_frequency);

 	return 0;
 }

-- 
2.43.0