[PATCH RFC v3 1/3] reboot: support hotplug CPUs before reboot

[Hsin-Yi Wang <hsinyi@chromium.org>]

Currently system reboots uses architecture specific codes (smp_send_stop)
to offline non reboot CPUs. Most architecture's implementation is looping
through all non reboot online CPUs and call ipi function to each of them. Some
architecture like arm64, arm, and x86... would set offline masks to cpu without
really offline them. This causes some race condition and kernel warning comes
out sometimes when system reboots.

This patch adds a config REBOOT_HOTPLUG_CPU, which would hotplug cpus in
migrate_to_reboot_cpu(). If non reboot cpus are all offlined here, the loop for
checking online cpus would be an empty loop. If architecture don't enable this
config, or some cpus somehow fails to offline, it would fallback to ipi
function.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
---
 arch/Kconfig        |  6 ++++++
 include/linux/cpu.h |  3 +++
 kernel/cpu.c        | 19 +++++++++++++++++++
 kernel/reboot.c     |  8 ++++++++
 4 files changed, 36 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index 48b5e103bdb0..a043b9be1499 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -52,6 +52,12 @@ config OPROFILE_EVENT_MULTIPLEX
 
 	  If unsure, say N.
 
+config REBOOT_HOTPLUG_CPU
+	bool "Support for hotplug CPUs before reboot"
+	depends on HOTPLUG_CPU
+	help
+	  Say Y to do a full hotplug on secondary CPUs before reboot.
+
 config HAVE_OPROFILE
 	bool
 
diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 1ca2baf817ed..3bf5ab289954 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -118,6 +118,9 @@ extern void cpu_hotplug_disable(void);
 extern void cpu_hotplug_enable(void);
 void clear_tasks_mm_cpumask(int cpu);
 int cpu_down(unsigned int cpu);
+#if IS_ENABLED(CONFIG_REBOOT_HOTPLUG_CPU)
+extern void offline_secondary_cpus(int primary);
+#endif
 
 #else /* CONFIG_HOTPLUG_CPU */
 
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 9c706af713fb..52afc47dd56a 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1057,6 +1057,25 @@ int cpu_down(unsigned int cpu)
 }
 EXPORT_SYMBOL(cpu_down);
 
+#if IS_ENABLED(CONFIG_REBOOT_HOTPLUG_CPU)
+void offline_secondary_cpus(int primary)
+{
+	int i, err;
+
+	cpu_maps_update_begin();
+
+	for_each_online_cpu(i) {
+		if (i == primary)
+			continue;
+		err = _cpu_down(i, 0, CPUHP_OFFLINE);
+		if (err)
+			pr_warn("Failed to offline cpu %d\n", i);
+	}
+	cpu_hotplug_disabled++;
+
+	cpu_maps_update_done();
+}
+#endif
 #else
 #define takedown_cpu		NULL
 #endif /*CONFIG_HOTPLUG_CPU*/
diff --git a/kernel/reboot.c b/kernel/reboot.c
index c4d472b7f1b4..fda84794ce46 100644
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -7,6 +7,7 @@
 
 #define pr_fmt(fmt)	"reboot: " fmt
 
+#include <linux/cpu.h>
 #include <linux/ctype.h>
 #include <linux/export.h>
 #include <linux/kexec.h>
@@ -220,7 +221,9 @@ void migrate_to_reboot_cpu(void)
 	/* The boot cpu is always logical cpu 0 */
 	int cpu = reboot_cpu;
 
+#if !IS_ENABLED(CONFIG_REBOOT_HOTPLUG_CPU)
 	cpu_hotplug_disable();
+#endif
 
 	/* Make certain the cpu I'm about to reboot on is online */
 	if (!cpu_online(cpu))
@@ -231,6 +234,11 @@ void migrate_to_reboot_cpu(void)
 
 	/* Make certain I only run on the appropriate processor */
 	set_cpus_allowed_ptr(current, cpumask_of(cpu));
+
+	/* Hotplug other cpus if possible */
+#if IS_ENABLED(CONFIG_REBOOT_HOTPLUG_CPU)
+	offline_secondary_cpus(cpu);
+#endif
 }
 
 /**
-- 
2.25.0.rc1.283.g88dfdc4193-goog