[PATCH] watchdog: nohz: don't run watchdog on nohz_full cores

[cmetcalf]

From: Chris Metcalf <cmetcalf@xxxxxxxxxx>

Running watchdog can be a helpful debugging feature on regular
cores, but it's incompatible with nohz_full, since it forces
regular scheduling events.  Accordingly, just exit out immediately
from any nohz_full core.

An alternate approach would be to add a flags field or function to
smp_hotplug_thread to control on which cores the percpu threads
are created, but it wasn't clear that much mechanism was useful.

Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxx>
---
 kernel/watchdog.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 3174bf8e3538..8a46d9d8a66f 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -19,6 +19,7 @@
 #include <linux/sysctl.h>
 #include <linux/smpboot.h>
 #include <linux/sched/rt.h>
+#include <linux/tick.h>
 
 #include <asm/irq_regs.h>
 #include <linux/kvm_para.h>
@@ -431,6 +432,10 @@ static void watchdog_enable(unsigned int cpu)
 	hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
 	hrtimer->function = watchdog_timer_fn;
 
+	/* nohz_full cpus do not do watchdog checking. */
+	if (tick_nohz_full_cpu(cpu))
+		do_exit(0);
+
 	/* Enable the perf event */
 	watchdog_nmi_enable(cpu);
 
-- 
2.1.2

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