cpu/hotplug: Make target state writeable

Make it possible to write a target state to the per cpu state file, so we can
switch between states.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org
Cc: Rik van Riel <riel@redhat.com>
Cc: Rafael Wysocki <rafael.j.wysocki@intel.com>
Cc: "Srivatsa S. Bhat" <srivatsa@mit.edu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul Turner <pjt@google.com>
Link: http://lkml.kernel.org/r/20160226182341.022814799@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 78 insertions, 8 deletions

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 1979b8927b86..be9335da82f1 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -48,12 +48,14 @@ static DEFINE_PER_CPU(struct cpuhp_cpu_state, cpuhp_state);
  * @teardown:   Teardown function of the step
  * @skip_onerr: Do not invoke the functions on error rollback
  *              Will go away once the notifiers are gone
+ * @cant_stop:  Bringup/teardown can't be stopped at this step
  */
 struct cpuhp_step {
         const char      *name;
         int             (*startup)(unsigned int cpu);
         int             (*teardown)(unsigned int cpu);
         bool            skip_onerr;
+        bool            cant_stop;
 };
 
 static DEFINE_MUTEX(cpuhp_state_mutex);
@@ -558,7 +560,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
         if (num_online_cpus() == 1)
                 return -EBUSY;
 
-        if (!cpu_online(cpu))
+        if (!cpu_present(cpu))
                 return -EINVAL;
 
         cpu_hotplug_begin();
@@ -683,16 +685,25 @@ static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state target)
 
         cpu_hotplug_begin();
 
-        if (cpu_online(cpu) || !cpu_present(cpu)) {
+        if (!cpu_present(cpu)) {
                 ret = -EINVAL;
                 goto out;
         }
 
-        /* Let it fail before we try to bring the cpu up */
-        idle = idle_thread_get(cpu);
-        if (IS_ERR(idle)) {
-                ret = PTR_ERR(idle);
+        /*
+         * The caller of do_cpu_up might have raced with another
+         * caller. Ignore it for now.
+         */
+        if (st->state >= target)
                 goto out;
+
+        if (st->state == CPUHP_OFFLINE) {
+                /* Let it fail before we try to bring the cpu up */
+                idle = idle_thread_get(cpu);
+                if (IS_ERR(idle)) {
+                        ret = PTR_ERR(idle);
+                        goto out;
+                }
         }
 
         cpuhp_tasks_frozen = tasks_frozen;
@@ -909,27 +920,32 @@ static struct cpuhp_step cpuhp_bp_states[] = {
                 .name                   = "threads:create",
                 .startup                = smpboot_create_threads,
                 .teardown               = NULL,
+                .cant_stop              = true,
         },
         [CPUHP_NOTIFY_PREPARE] = {
                 .name                   = "notify:prepare",
                 .startup                = notify_prepare,
                 .teardown               = notify_dead,
                 .skip_onerr             = true,
+                .cant_stop              = true,
         },
         [CPUHP_BRINGUP_CPU] = {
                 .name                   = "cpu:bringup",
                 .startup                = bringup_cpu,
                 .teardown               = NULL,
+                .cant_stop              = true,
         },
         [CPUHP_TEARDOWN_CPU] = {
                 .name                   = "cpu:teardown",
                 .startup                = NULL,
                 .teardown               = takedown_cpu,
+                .cant_stop              = true,
         },
         [CPUHP_NOTIFY_ONLINE] = {
                 .name                   = "notify:online",
                 .startup                = notify_online,
                 .teardown               = notify_down_prepare,
+                .cant_stop              = true,
         },
 #endif
         [CPUHP_ONLINE] = {
@@ -947,6 +963,7 @@ static struct cpuhp_step cpuhp_ap_states[] = {
                 .startup                = notify_starting,
                 .teardown               = notify_dying,
                 .skip_onerr             = true,
+                .cant_stop              = true,
         },
 #endif
         [CPUHP_ONLINE] = {
@@ -979,6 +996,46 @@ static ssize_t show_cpuhp_state(struct device *dev,
 }
 static DEVICE_ATTR(state, 0444, show_cpuhp_state, NULL);
 
+static ssize_t write_cpuhp_target(struct device *dev,
+                                  struct device_attribute *attr,
+                                  const char *buf, size_t count)
+{
+        struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, dev->id);
+        struct cpuhp_step *sp;
+        int target, ret;
+
+        ret = kstrtoint(buf, 10, &target);
+        if (ret)
+                return ret;
+
+#ifdef CONFIG_CPU_HOTPLUG_STATE_CONTROL
+        if (target < CPUHP_OFFLINE || target > CPUHP_ONLINE)
+                return -EINVAL;
+#else
+        if (target != CPUHP_OFFLINE && target != CPUHP_ONLINE)
+                return -EINVAL;
+#endif
+
+        ret = lock_device_hotplug_sysfs();
+        if (ret)
+                return ret;
+
+        mutex_lock(&cpuhp_state_mutex);
+        sp = cpuhp_get_step(target);
+        ret = !sp->name || sp->cant_stop ? -EINVAL : 0;
+        mutex_unlock(&cpuhp_state_mutex);
+        if (ret)
+                return ret;
+
+        if (st->state < target)
+                ret = do_cpu_up(dev->id, target);
+        else
+                ret = do_cpu_down(dev->id, target);
+
+        unlock_device_hotplug();
+        return ret ? ret : count;
+}
+
 static ssize_t show_cpuhp_target(struct device *dev,
                                  struct device_attribute *attr, char *buf)
 {
@@ -986,7 +1043,7 @@ static ssize_t show_cpuhp_target(struct device *dev,
 
         return sprintf(buf, "%d\n", st->target);
 }
-static DEVICE_ATTR(target, 0444, show_cpuhp_target, NULL);
+static DEVICE_ATTR(target, 0644, show_cpuhp_target, write_cpuhp_target);
 
 static struct attribute *cpuhp_cpu_attrs[] = {
         &dev_attr_state.attr,
@@ -1007,7 +1064,7 @@ static ssize_t show_cpuhp_states(struct device *dev,
         int i;
 
         mutex_lock(&cpuhp_state_mutex);
-        for (i = 0; i <= CPUHP_ONLINE; i++) {
+        for (i = CPUHP_OFFLINE; i <= CPUHP_ONLINE; i++) {
                 struct cpuhp_step *sp = cpuhp_get_step(i);
 
                 if (sp->name) {
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 8bfd1aca7a3d..f28f7fad452f 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1442,6 +1442,19 @@ config DEBUG_BLOCK_EXT_DEVT
 
           Say N if you are unsure.
 
+config CPU_HOTPLUG_STATE_CONTROL
+        bool "Enable CPU hotplug state control"
+        depends on DEBUG_KERNEL
+        depends on HOTPLUG_CPU
+        default n
+        help
+          Allows to write steps between "offline" and "online" to the CPUs
+          sysfs target file so states can be stepped granular. This is a debug
+          option for now as the hotplug machinery cannot be stopped and
+          restarted at arbitrary points yet.
+
+          Say N if your are unsure.
+
 config NOTIFIER_ERROR_INJECTION
         tristate "Notifier error injection"
         depends on DEBUG_KERNEL