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

2 files changed, 8 insertions, 1 deletions

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))