watchdog/hardlockup/perf: Implement init time detection of perf

Use the init time detection of the perf NMI watchdog to determine whether
the perf NMI watchdog is functional. If not disable it permanentely. It
won't come back magically at runtime.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Don Zickus <dzickus@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Ulrich Obergfell <uobergfe@redhat.com>
Link: http://lkml.kernel.org/r/20170912194148.099799541@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/watchdog.c

@@ -44,6 +44,7 @@ int __read_mostly watchdog_user_enabled = 1;
 int __read_mostly nmi_watchdog_user_enabled = NMI_WATCHDOG_DEFAULT;
 int __read_mostly soft_watchdog_user_enabled = 1;
 int __read_mostly watchdog_thresh = 10;
+int __read_mostly nmi_watchdog_available;
 
 struct cpumask watchdog_allowed_mask __read_mostly;
 static bool softlockup_threads_initialized __read_mostly;
@@ -114,6 +115,12 @@ void __weak watchdog_nmi_disable(unsigned int cpu)
 	hardlockup_detector_perf_disable();
 }
 
+/* Return 0, if a NMI watchdog is available. Error code otherwise */
+int __weak __init watchdog_nmi_probe(void)
+{
+	return hardlockup_detector_perf_init();
+}
+
 /**
  * watchdog_nmi_reconfigure - Optional function to reconfigure NMI watchdogs
  * @run:	If false stop the watchdogs on all enabled CPUs
@@ -145,7 +152,7 @@ static void lockup_detector_update_enable(void)
 	watchdog_enabled = 0;
 	if (!watchdog_user_enabled)
 		return;
-	if (nmi_watchdog_user_enabled)
+	if (nmi_watchdog_available && nmi_watchdog_user_enabled)
 		watchdog_enabled |= NMI_WATCHDOG_ENABLED;
 	if (soft_watchdog_user_enabled)
 		watchdog_enabled |= SOFT_WATCHDOG_ENABLED;
@@ -692,6 +699,8 @@ int proc_watchdog(struct ctl_table *table, int write,
 int proc_nmi_watchdog(struct ctl_table *table, int write,
 		      void __user *buffer, size_t *lenp, loff_t *ppos)
 {
+	if (!nmi_watchdog_available && write)
+		return -ENOTSUPP;
 	return proc_watchdog_common(NMI_WATCHDOG_ENABLED,
 				    table, write, buffer, lenp, ppos);
 }
@@ -764,5 +773,7 @@ void __init lockup_detector_init(void)
 	cpumask_copy(&watchdog_cpumask, cpu_possible_mask);
 #endif
 
+	if (!watchdog_nmi_probe())
+		nmi_watchdog_available = true;
 	softlockup_init_threads();
 }