stop_machine: reimplement using cpu_stop

Reimplement stop_machine using cpu_stop.  As cpu stoppers are
guaranteed to be available for all online cpus,
stop_machine_create/destroy() are no longer necessary and removed.

With resource management and synchronization handled by cpu_stop, the
new implementation is much simpler.  Asking the cpu_stop to execute
the stop_cpu() state machine on all online cpus with cpu hotplug
disabled is enough.

stop_machine itself doesn't need to manage any global resources
anymore, so all per-instance information is rolled into struct
stop_machine_data and the mutex and all static data variables are
removed.

The previous implementation created and destroyed RT workqueues as
necessary which made stop_machine() calls highly expensive on very
large machines.  According to Dimitri Sivanich, preventing the dynamic
creation/destruction makes booting faster more than twice on very
large machines.  cpu_stop resources are preallocated for all online
cpus and should have the same effect.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Dimitri Sivanich <sivanich@sgi.com>

1 files changed, 38 insertions, 120 deletions

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 7e3f9182aef3..884c7a1afeed 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -388,174 +388,92 @@ enum stopmachine_state {
         /* Exit */
         STOPMACHINE_EXIT,
 };
-static enum stopmachine_state state;
 
 struct stop_machine_data {
-        int (*fn)(void *);
-        void *data;
-        int fnret;
+        int                     (*fn)(void *);
+        void                    *data;
+        /* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
+        unsigned int            num_threads;
+        const struct cpumask    *active_cpus;
+
+        enum stopmachine_state  state;
+        atomic_t                thread_ack;
 };
 
-/* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
-static unsigned int num_threads;
-static atomic_t thread_ack;
-static DEFINE_MUTEX(lock);
-/* setup_lock protects refcount, stop_machine_wq and stop_machine_work. */
-static DEFINE_MUTEX(setup_lock);
-/* Users of stop_machine. */
-static int refcount;
-static struct workqueue_struct *stop_machine_wq;
-static struct stop_machine_data active, idle;
-static const struct cpumask *active_cpus;
-static void __percpu *stop_machine_work;
-
-static void set_state(enum stopmachine_state newstate)
+static void set_state(struct stop_machine_data *smdata,
+                      enum stopmachine_state newstate)
 {
         /* Reset ack counter. */
-        atomic_set(&thread_ack, num_threads);
+        atomic_set(&smdata->thread_ack, smdata->num_threads);
         smp_wmb();
-        state = newstate;
+        smdata->state = newstate;
 }
 
 /* Last one to ack a state moves to the next state. */
-static void ack_state(void)
+static void ack_state(struct stop_machine_data *smdata)
 {
-        if (atomic_dec_and_test(&thread_ack))
-                set_state(state + 1);
+        if (atomic_dec_and_test(&smdata->thread_ack))
+                set_state(smdata, smdata->state + 1);
 }
 
-/* This is the actual function which stops the CPU. It runs
- * in the context of a dedicated stopmachine workqueue. */
-static void stop_cpu(struct work_struct *unused)
+/* This is the cpu_stop function which stops the CPU. */
+static int stop_machine_cpu_stop(void *data)
 {
+        struct stop_machine_data *smdata = data;
         enum stopmachine_state curstate = STOPMACHINE_NONE;
-        struct stop_machine_data *smdata = &idle;
-        int cpu = smp_processor_id();
-        int err;
+        int cpu = smp_processor_id(), err = 0;
+        bool is_active;
+
+        if (!smdata->active_cpus)
+                is_active = cpu == cpumask_first(cpu_online_mask);
+        else
+                is_active = cpumask_test_cpu(cpu, smdata->active_cpus);
 
-        if (!active_cpus) {
-                if (cpu == cpumask_first(cpu_online_mask))
-                        smdata = &active;
-        } else {
-                if (cpumask_test_cpu(cpu, active_cpus))
-                        smdata = &active;
-        }
         /* Simple state machine */
         do {
                 /* Chill out and ensure we re-read stopmachine_state. */
                 cpu_relax();
-                if (state != curstate) {
-                        curstate = state;
+                if (smdata->state != curstate) {
+                        curstate = smdata->state;
                         switch (curstate) {
                         case STOPMACHINE_DISABLE_IRQ:
                                 local_irq_disable();
                                 hard_irq_disable();
                                 break;
                         case STOPMACHINE_RUN:
-                                /* On multiple CPUs only a single error code
-                                 * is needed to tell that something failed. */
-                                err = smdata->fn(smdata->data);
-                                if (err)
-                                        smdata->fnret = err;
+                                if (is_active)
+                                        err = smdata->fn(smdata->data);
                                 break;
                         default:
                                 break;
                         }
-                        ack_state();
+                        ack_state(smdata);
                 }
         } while (curstate != STOPMACHINE_EXIT);
 
         local_irq_enable();
+        return err;
 }
 
-/* Callback for CPUs which aren't supposed to do anything. */
-static int chill(void *unused)
-{
-        return 0;
-}
-
-int stop_machine_create(void)
-{
-        mutex_lock(&setup_lock);
-        if (refcount)
-                goto done;
-        stop_machine_wq = create_rt_workqueue("kstop");
-        if (!stop_machine_wq)
-                goto err_out;
-        stop_machine_work = alloc_percpu(struct work_struct);
-        if (!stop_machine_work)
-                goto err_out;
-done:
-        refcount++;
-        mutex_unlock(&setup_lock);
-        return 0;
-
-err_out:
-        if (stop_machine_wq)
-                destroy_workqueue(stop_machine_wq);
-        mutex_unlock(&setup_lock);
-        return -ENOMEM;
-}
-EXPORT_SYMBOL_GPL(stop_machine_create);
-
-void stop_machine_destroy(void)
-{
-        mutex_lock(&setup_lock);
-        refcount--;
-        if (refcount)
-                goto done;
-        destroy_workqueue(stop_machine_wq);
-        free_percpu(stop_machine_work);
-done:
-        mutex_unlock(&setup_lock);
-}
-EXPORT_SYMBOL_GPL(stop_machine_destroy);
-
 int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
 {
-        struct work_struct *sm_work;
-        int i, ret;
-
-        /* Set up initial state. */
-        mutex_lock(&lock);
-        num_threads = num_online_cpus();
-        active_cpus = cpus;
-        active.fn = fn;
-        active.data = data;
-        active.fnret = 0;
-        idle.fn = chill;
-        idle.data = NULL;
-
-        set_state(STOPMACHINE_PREPARE);
-
-        /* Schedule the stop_cpu work on all cpus: hold this CPU so one
-         * doesn't hit this CPU until we're ready. */
-        get_cpu();
-        for_each_online_cpu(i) {
-                sm_work = per_cpu_ptr(stop_machine_work, i);
-                INIT_WORK(sm_work, stop_cpu);
-                queue_work_on(i, stop_machine_wq, sm_work);
-        }
-        /* This will release the thread on our CPU. */
-        put_cpu();
-        flush_workqueue(stop_machine_wq);
-        ret = active.fnret;
-        mutex_unlock(&lock);
-        return ret;
+        struct stop_machine_data smdata = { .fn = fn, .data = data,
+                                            .num_threads = num_online_cpus(),
+                                            .active_cpus = cpus };
+
+        /* Set the initial state and stop all online cpus. */
+        set_state(&smdata, STOPMACHINE_PREPARE);
+        return stop_cpus(cpu_online_mask, stop_machine_cpu_stop, &smdata);
 }
 
 int stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
 {
         int ret;
 
-        ret = stop_machine_create();
-        if (ret)
-                return ret;
         /* No CPUs can come up or down during this. */
         get_online_cpus();
         ret = __stop_machine(fn, data, cpus);
         put_online_cpus();
-        stop_machine_destroy();
         return ret;
 }
 EXPORT_SYMBOL_GPL(stop_machine);