watchdog: add watchdog_cpumask sysctl to assist nohz
Change the default behavior of watchdog so it only runs on the
housekeeping cores when nohz_full is enabled at build and boot time.
Allow modifying the set of cores the watchdog is currently running on
with a new kernel.watchdog_cpumask sysctl.
In the current system, the watchdog subsystem runs a periodic timer that
schedules the watchdog kthread to run. However, nohz_full cores are
designed to allow userspace application code running on those cores to
have 100% access to the CPU. So the watchdog system prevents the
nohz_full application code from being able to run the way it wants to,
thus the motivation to suppress the watchdog on nohz_full cores, which
this patchset provides by default.
However, if we disable the watchdog globally, then the housekeeping
cores can't benefit from the watchdog functionality. So we allow
disabling it only on some cores. See Documentation/lockup-watchdogs.txt
for more information.
[jhubbard@nvidia.com: fix a watchdog crash in some configurations]
Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Ulrich Obergfell <uobergfe@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 5e46c2a7..7c434c3 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -338,6 +338,7 @@
*
* The cpumask field in the smp_hotplug_thread must not be updated directly
* by the client, but only by calling this function.
+ * This function can only be called on a registered smp_hotplug_thread.
*/
int smpboot_update_cpumask_percpu_thread(struct smp_hotplug_thread *plug_thread,
const struct cpumask *new)
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index b13e9d2..812fcc3 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -872,6 +872,13 @@
.extra2 = &one,
},
{
+ .procname = "watchdog_cpumask",
+ .data = &watchdog_cpumask_bits,
+ .maxlen = NR_CPUS,
+ .mode = 0644,
+ .proc_handler = proc_watchdog_cpumask,
+ },
+ {
.procname = "softlockup_panic",
.data = &softlockup_panic,
.maxlen = sizeof(int),
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 581a68a..a6ffa43 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>
@@ -58,6 +59,12 @@
#else
#define sysctl_softlockup_all_cpu_backtrace 0
#endif
+static struct cpumask watchdog_cpumask __read_mostly;
+unsigned long *watchdog_cpumask_bits = cpumask_bits(&watchdog_cpumask);
+
+/* Helper for online, unparked cpus. */
+#define for_each_watchdog_cpu(cpu) \
+ for_each_cpu_and((cpu), cpu_online_mask, &watchdog_cpumask)
static int __read_mostly watchdog_running;
static u64 __read_mostly sample_period;
@@ -207,7 +214,7 @@
* do we care if a 0 races with a timestamp?
* all it means is the softlock check starts one cycle later
*/
- for_each_online_cpu(cpu)
+ for_each_watchdog_cpu(cpu)
per_cpu(watchdog_touch_ts, cpu) = 0;
}
@@ -616,7 +623,7 @@
goto unlock;
get_online_cpus();
- for_each_online_cpu(cpu)
+ for_each_watchdog_cpu(cpu)
watchdog_nmi_enable(cpu);
put_online_cpus();
@@ -634,7 +641,7 @@
goto unlock;
get_online_cpus();
- for_each_online_cpu(cpu)
+ for_each_watchdog_cpu(cpu)
watchdog_nmi_disable(cpu);
put_online_cpus();
@@ -696,7 +703,7 @@
int cpu;
get_online_cpus();
- for_each_online_cpu(cpu)
+ for_each_watchdog_cpu(cpu)
update_watchdog(cpu);
put_online_cpus();
}
@@ -709,8 +716,12 @@
err = smpboot_register_percpu_thread(&watchdog_threads);
if (err)
pr_err("Failed to create watchdog threads, disabled\n");
- else
+ else {
+ if (smpboot_update_cpumask_percpu_thread(
+ &watchdog_threads, &watchdog_cpumask))
+ pr_err("Failed to set cpumask for watchdog threads\n");
watchdog_running = 1;
+ }
} else {
/*
* Enable/disable the lockup detectors or
@@ -879,12 +890,58 @@
mutex_unlock(&watchdog_proc_mutex);
return err;
}
+
+/*
+ * The cpumask is the mask of possible cpus that the watchdog can run
+ * on, not the mask of cpus it is actually running on. This allows the
+ * user to specify a mask that will include cpus that have not yet
+ * been brought online, if desired.
+ */
+int proc_watchdog_cpumask(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+ int err;
+
+ mutex_lock(&watchdog_proc_mutex);
+ err = proc_do_large_bitmap(table, write, buffer, lenp, ppos);
+ if (!err && write) {
+ /* Remove impossible cpus to keep sysctl output cleaner. */
+ cpumask_and(&watchdog_cpumask, &watchdog_cpumask,
+ cpu_possible_mask);
+
+ if (watchdog_running) {
+ /*
+ * Failure would be due to being unable to allocate
+ * a temporary cpumask, so we are likely not in a
+ * position to do much else to make things better.
+ */
+ if (smpboot_update_cpumask_percpu_thread(
+ &watchdog_threads, &watchdog_cpumask) != 0)
+ pr_err("cpumask update failed\n");
+ }
+ }
+ mutex_unlock(&watchdog_proc_mutex);
+ return err;
+}
+
#endif /* CONFIG_SYSCTL */
void __init lockup_detector_init(void)
{
set_sample_period();
+#ifdef CONFIG_NO_HZ_FULL
+ if (tick_nohz_full_enabled()) {
+ if (!cpumask_empty(tick_nohz_full_mask))
+ pr_info("Disabling watchdog on nohz_full cores by default\n");
+ cpumask_andnot(&watchdog_cpumask, cpu_possible_mask,
+ tick_nohz_full_mask);
+ } else
+ cpumask_copy(&watchdog_cpumask, cpu_possible_mask);
+#else
+ cpumask_copy(&watchdog_cpumask, cpu_possible_mask);
+#endif
+
if (watchdog_enabled)
watchdog_enable_all_cpus();
}