linux-kernel - [tip:x86/timers] x86/tsc: Add option to disable tsc clocksource watchdog

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Message-ID: <tip-0f0b7e1cc7abf8e1a8b301f2868379d611d05ae2@git.kernel.org>
Date:   Fri, 22 Mar 2019 06:19:19 -0700
From:   tip-bot for Juri Lelli <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        juri.lelli@...hat.com, hpa@...or.com, tglx@...utronix.de
Subject: [tip:x86/timers] x86/tsc: Add option to disable tsc clocksource
 watchdog

Commit-ID:  0f0b7e1cc7abf8e1a8b301f2868379d611d05ae2
Gitweb:     https://git.kernel.org/tip/0f0b7e1cc7abf8e1a8b301f2868379d611d05ae2
Author:     Juri Lelli <juri.lelli@...hat.com>
AuthorDate: Thu, 7 Mar 2019 13:09:13 +0100
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Fri, 22 Mar 2019 14:14:58 +0100

x86/tsc: Add option to disable tsc clocksource watchdog

Clocksource watchdog has been found responsible for generating latency
spikes (in the 10-20 us range) when woken up to check for TSC stability.

Add an option to disable it at boot.

Signed-off-by: Juri Lelli <juri.lelli@...hat.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: bigeasy@...utronix.de
Cc: linux-rt-users@...r.kernel.org
Cc: peterz@...radead.org
Cc: bristot@...hat.com
Cc: williams@...hat.com
Link: https://lkml.kernel.org/r/20190307120913.13168-1-juri.lelli@redhat.com

---
 Documentation/admin-guide/kernel-parameters.txt | 4 ++++
 arch/x86/kernel/tsc.c                           | 5 ++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 2b8ee90bb644..c4d830003b21 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4703,6 +4703,10 @@
 			[x86] unstable: mark the TSC clocksource as unstable, this
 			marks the TSC unconditionally unstable at bootup and
 			avoids any further wobbles once the TSC watchdog notices.
+			[x86] nowatchdog: disable clocksource watchdog. Used
+			in situations with strict latency requirements (where
+			interruptions from clocksource watchdog are not
+			acceptable).
 
 	turbografx.map[2|3]=	[HW,JOY]
 			TurboGraFX parallel port interface
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 3fae23834069..aab0c82e0a0d 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -283,6 +283,7 @@ int __init notsc_setup(char *str)
 __setup("notsc", notsc_setup);
 
 static int no_sched_irq_time;
+static int no_tsc_watchdog;
 
 static int __init tsc_setup(char *str)
 {
@@ -292,6 +293,8 @@ static int __init tsc_setup(char *str)
 		no_sched_irq_time = 1;
 	if (!strcmp(str, "unstable"))
 		mark_tsc_unstable("boot parameter");
+	if (!strcmp(str, "nowatchdog"))
+		no_tsc_watchdog = 1;
 	return 1;
 }
 
@@ -1349,7 +1352,7 @@ static int __init init_tsc_clocksource(void)
 	if (tsc_unstable)
 		goto unreg;
 
-	if (tsc_clocksource_reliable)
+	if (tsc_clocksource_reliable || no_tsc_watchdog)
 		clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
 
 	if (boot_cpu_has(X86_FEATURE_NONSTOP_TSC_S3))