[PATCH v8 2/3] watchdog: add watchdog_cpumask sysctl to assist nohz

[Chris Metcalf]

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.

If we allowed the watchdog to run on nohz_full cores, the timer
interrupts and scheduler work would prevent the desired tickless
operation on those cores.  But 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.

Acked-by: Don Zickus <dzickus@xxxxxxxxxx>
Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxx>
---
v8: use new semantics of smpboot_update_cpumask_percpu_thread() [Frederic]
    improve documentation in "Documentation/" and in changelong [akpm]

v7: use cpumask field instead of valid_cpu() callback

v6: use alloc_cpumask_var() [Sasha Levin]
    switch from watchdog_exclude to watchdog_cpumask [Frederic]
    simplify the smp_hotplug_thread API to watchdog [Frederic]
    add Don's Acked-by

 Documentation/lockup-watchdogs.txt | 18 ++++++++++++++
 Documentation/sysctl/kernel.txt    | 15 ++++++++++++
 include/linux/nmi.h                |  3 +++
 kernel/sysctl.c                    |  7 ++++++
 kernel/watchdog.c                  | 49 ++++++++++++++++++++++++++++++++++++++
 5 files changed, 92 insertions(+)

diff --git a/Documentation/lockup-watchdogs.txt b/Documentation/lockup-watchdogs.txt
index ab0baa692c13..22dd6af2e4bd 100644
--- a/Documentation/lockup-watchdogs.txt
+++ b/Documentation/lockup-watchdogs.txt
@@ -61,3 +61,21 @@ As explained above, a kernel knob is provided that allows
 administrators to configure the period of the hrtimer and the perf
 event. The right value for a particular environment is a trade-off
 between fast response to lockups and detection overhead.
+
+By default, the watchdog runs on all online cores.  However, on a
+kernel configured with NO_HZ_FULL, by default the watchdog runs only
+on the housekeeping cores, not the cores specified in the "nohz_full"
+boot argument.  If we allowed the watchdog to run by default on
+the "nohz_full" cores, we would have to run timer ticks to activate
+the scheduler, which would prevent the "nohz_full" functionality
+from protecting the user code on those cores from the kernel.
+Of course, disabling it by default on the nohz_full cores means that
+when those cores do enter the kernel, by default we will not be
+able to detect if they lock up.  However, allowing the watchdog
+to continue to run on the housekeeping (non-tickless) cores means
+that we will continue to detect lockups properly on those cores.
+
+In either case, the set of cores excluded from running the watchdog
+may be adjusted via the kernel.watchdog_cpumask sysctl.  For
+nohz_full cores, this may be useful for debugging a case where the
+kernel seems to be hanging on the nohz_full cores.
diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index c831001c45f1..f1697858d71c 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -923,6 +923,21 @@ and nmi_watchdog.
 
 ==============================================================
 
+watchdog_cpumask:
+
+This value can be used to control on which cpus the watchdog may run.
+The default cpumask is all possible cores, but if NO_HZ_FULL is
+enabled in the kernel config, and cores are specified with the
+nohz_full= boot argument, those cores are excluded by default.
+Offline cores can be included in this mask, and if the core is later
+brought online, the watchdog will be started based on the mask value.
+
+Typically this value would only be touched in the nohz_full case
+to re-enable cores that by default were not running the watchdog,
+if a kernel lockup was suspected on those cores.
+
+==============================================================
+
 watchdog_thresh:
 
 This value can be used to control the frequency of hrtimer and NMI
diff --git a/include/linux/nmi.h b/include/linux/nmi.h
index 3d46fb4708e0..f94da0e65dea 100644
--- a/include/linux/nmi.h
+++ b/include/linux/nmi.h
@@ -67,6 +67,7 @@ extern int nmi_watchdog_enabled;
 extern int soft_watchdog_enabled;
 extern int watchdog_user_enabled;
 extern int watchdog_thresh;
+extern unsigned long *watchdog_cpumask_bits;
 extern int sysctl_softlockup_all_cpu_backtrace;
 struct ctl_table;
 extern int proc_watchdog(struct ctl_table *, int ,
@@ -77,6 +78,8 @@ extern int proc_soft_watchdog(struct ctl_table *, int ,
 			      void __user *, size_t *, loff_t *);
 extern int proc_watchdog_thresh(struct ctl_table *, int ,
 				void __user *, size_t *, loff_t *);
+extern int proc_watchdog_cpumask(struct ctl_table *, int,
+				 void __user *, size_t *, loff_t *);
 #endif
 
 #ifdef CONFIG_HAVE_ACPI_APEI_NMI
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 2082b1a88fb9..699571a74e3b 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -881,6 +881,13 @@ static struct ctl_table kern_table[] = {
 		.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 2316f50b07a4..5bd80a212486 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>
@@ -56,6 +57,9 @@ int __read_mostly sysctl_softlockup_all_cpu_backtrace;
 #else
 #define sysctl_softlockup_all_cpu_backtrace 0
 #endif
+static cpumask_var_t watchdog_cpumask_for_smpboot;
+static cpumask_var_t watchdog_cpumask;
+unsigned long *watchdog_cpumask_bits;
 
 static int __read_mostly watchdog_running;
 static u64 __read_mostly sample_period;
@@ -694,6 +698,7 @@ static int watchdog_enable_all_cpus(void)
 	int err = 0;
 
 	if (!watchdog_running) {
+		cpumask_copy(watchdog_cpumask_for_smpboot, watchdog_cpumask);
 		err = smpboot_register_percpu_thread(&watchdog_threads);
 		if (err)
 			pr_err("Failed to create watchdog threads, disabled\n");
@@ -869,12 +874,56 @@ out:
 	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_enabled && watchdog_thresh)
+			smpboot_update_cpumask_percpu_thread(&watchdog_threads,
+							     watchdog_cpumask);
+	}
+	mutex_unlock(&watchdog_proc_mutex);
+	return err;
+}
+
 #endif /* CONFIG_SYSCTL */
 
 void __init lockup_detector_init(void)
 {
 	set_sample_period();
 
+	/* One cpumask is allocated for smpboot to own. */
+	alloc_cpumask_var(&watchdog_cpumask_for_smpboot, GFP_KERNEL);
+	watchdog_threads.cpumask = watchdog_cpumask_for_smpboot;
+
+	/* Another cpumask is allocated for /proc to use. */
+	alloc_cpumask_var(&watchdog_cpumask, GFP_KERNEL);
+	watchdog_cpumask_bits = cpumask_bits(watchdog_cpumask);
+
+#ifdef CONFIG_NO_HZ_FULL
+	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);
+#endif
+
 	if (watchdog_enabled)
 		watchdog_enable_all_cpus();
 }
-- 
2.1.2

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