watchdog: Boot-disable by default on full dynticks

When the watchdog runs, it prevents the full dynticks
CPUs from stopping their tick because the hard lockup
detector uses perf events internally, which in turn
rely on the periodic tick.

Since this is a rather confusing behaviour that is not
easy to track down and identify for those who want to
test CONFIG_NO_HZ_FULL, let's default disable the
watchdog on boot time when full dynticks is enabled.

The user can still enable it later on runtime using
proc or sysctl.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Anish Singh <anish198519851985@gmail.com>

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 51c4f34d258ea397266e0dd1a96a16436415a38f..1241d8c91d5e75efb3ba08ff18109a0a4f84e12c 100644 (file)
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -553,6 +553,14 @@ void __init lockup_detector_init(void)
 {
        set_sample_period();
 
+#ifdef CONFIG_NO_HZ_FULL
+       if (watchdog_user_enabled) {
+               watchdog_user_enabled = 0;
+               pr_warning("Disabled lockup detectors by default for full dynticks\n");
+               pr_warning("You can reactivate it with 'sysctl -w kernel.watchdog=1'\n");
+       }
+#endif
+
        if (watchdog_user_enabled)
                watchdog_enable_all_cpus();
 }