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@xxxxxxxxxx>
---
Sending this out as an RFC after yesterday discussion with Thomas on IRC.
AFAICT, CLOCK_SOURCE_MUST_VERIFY only controls enqueue\dequeue of the
watchdog. TSC sync checks are still done if tsc_clocksource_reliable is
set (booting w/o tsc=reliable).
Thomas, is this anywhere close to what you had in mind?
---
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 f5acf35c712f..ea7c4967aa80 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4619,6 +4619,10 @@
[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 6d5dc5dabfd7..4b753d057aee 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;
}
@@ -1347,7 +1350,7 @@ 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))
--
2.17.2
