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

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 95f0d10..7b4df6d 100644 (file)
--- 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 (file)
--- 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)