PATCH v2 clocksource 8/7] clocksource: Enable TSC watchdog checking of HPET and PMTMR only when requested

Paul E. McKenney
2023-02-03T04:37:06Z

Unconditionally enabling TSC watchdog checking of the HPET and PMTMR
clocksources can degrade latency and performance. Therefore, provide
a new "watchdog" option to the tsc= boot parameter that opts into such
checking. Note that tsc=watchdog is overridden by a tsc=nowatchdog
regardless of their relative positions in the list of boot parameters.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Reported-by: Waiman Long <longman@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 95f0d104c2322..7b4df6d89d3c3 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6373,6 +6373,12 @@
(HPET or PM timer) on systems whose TSC frequency was
obtained from HW or FW using either an MSR or CPUID(0x15).
Warn if the difference is more than 500 ppm.
+ [x86] watchdog: Use TSC as the watchdog clocksource with
+ which to check other HW timers (HPET or PM timer), but
+ only on systems where TSC has been deemed trustworthy.
+ This will be suppressed by an earlier tsc=nowatchdog and
+ can be overridden by a later tsc=nowatchdog. A console
+ message will flag any such suppression or overriding.

tsc_early_khz= [X86] Skip early TSC calibration and use the given
value instead. Useful when the early TSC frequency discovery
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index a5371c6d4b64b..306c233c98d84 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -294,6 +294,7 @@ __setup("notsc", notsc_setup);

static int no_sched_irq_time;
static int no_tsc_watchdog;
+static int tsc_as_watchdog;

static int __init tsc_setup(char *str)
{
@@ -303,10 +304,22 @@ 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"))
+ if (!strcmp(str, "nowatchdog")) {
no_tsc_watchdog = 1;
+ if (tsc_as_watchdog)
+ pr_alert("%s: Overriding earlier tsc=watchdog with tsc=nowatchdog\n",
+ __func__);
+ tsc_as_watchdog = 0;
+ }
if (!strcmp(str, "recalibrate"))
tsc_force_recalibrate = 1;
+ if (!strcmp(str, "watchdog")) {
+ if (no_tsc_watchdog)
+ pr_alert("%s: tsc=watchdog overridden by earlier tsc=nowatchdog\n",
+ __func__);
+ else
+ tsc_as_watchdog = 1;
+ }
return 1;
}

@@ -1192,7 +1205,8 @@ static void __init tsc_disable_clocksource_watchdog(void)

bool tsc_clocksource_watchdog_disabled(void)
{
- return !(clocksource_tsc.flags & CLOCK_SOURCE_MUST_VERIFY);
+ return !(clocksource_tsc.flags & CLOCK_SOURCE_MUST_VERIFY) &&
+ tsc_as_watchdog && !no_tsc_watchdog;
}

static void __init check_system_tsc_reliable(void)