[tip: timers/core] clocksource: Enable TSC watchdog checking of HPET and PMTMR only when requested

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     0051293c533017e2a860e0a0a33517bc40240fff
Gitweb:        https://git.kernel.org/tip/0051293c533017e2a860e0a0a33517bc40240fff
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Wed, 01 Feb 2023 13:53:07 -08:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 06 Feb 2023 16:38:30 -08:00

clocksource: Enable TSC watchdog checking of HPET and PMTMR only when requested

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@xxxxxxxxxxxxx>
Reported-by: Waiman Long <longman@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Acked-by: Waiman Long <longman@xxxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt |  6 +++++-
 arch/x86/kernel/tsc.c                           | 18 ++++++++++++++--
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 95f0d10..7b4df6d 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 a5371c6..306c233 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)