sched: Remove get_online_cpus() usage

Remove get_online_cpus() usage from the scheduler; there's 4 sites that
use it:

 - sched_init_smp(); where its completely superfluous since we're in
   'early' boot and there simply cannot be any hotplugging.

 - sched_getaffinity(); we already take a raw spinlock to protect the
   task cpus_allowed mask, this disables preemption and therefore
   also stabilizes cpu_online_mask as that's modified using
   stop_machine. However switch to active mask for symmetry with
   sched_setaffinity()/set_cpus_allowed_ptr(). We guarantee active
   mask stability by inserting sync_rcu/sched() into _cpu_down.

 - sched_setaffinity(); we don't appear to need get_online_cpus()
   either, there's two sites where hotplug appears relevant:
    * cpuset_cpus_allowed(); for the !cpuset case we use possible_mask,
      for the cpuset case we hold task_lock, which is a spinlock and
      thus for mainline disables preemption (might cause pain on RT).
    * set_cpus_allowed_ptr(); Holds all scheduler locks and thus has
      preemption properly disabled; also it already deals with hotplug
      races explicitly where it releases them.

 - migrate_swap(); we can make stop_two_cpus() do the heavy lifting for
   us with a little trickery. By adding a sync_sched/rcu() after the
   CPU_DOWN_PREPARE notifier we can provide preempt/rcu guarantees for
   cpu_active_mask. Use these to validate that both our cpus are active
   when queueing the stop work before we queue the stop_machine works
   for take_cpu_down().

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Link: http://lkml.kernel.org/r/20131011123820.GV3081@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

3 files changed, 48 insertions, 15 deletions

diff --git a/kernel/cpu.c b/kernel/cpu.c
index d7f07a2da5a6..63aa50d7ce1e 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -308,6 +308,23 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
         }
         smpboot_park_threads(cpu);
 
+        /*
+         * By now we've cleared cpu_active_mask, wait for all preempt-disabled
+         * and RCU users of this state to go away such that all new such users
+         * will observe it.
+         *
+         * For CONFIG_PREEMPT we have preemptible RCU and its sync_rcu() might
+         * not imply sync_sched(), so explicitly call both.
+         */
+#ifdef CONFIG_PREEMPT
+        synchronize_sched();
+#endif
+        synchronize_rcu();
+
+        /*
+         * So now all preempt/rcu users must observe !cpu_active().
+         */
+
         err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
         if (err) {
                 /* CPU didn't die: tell everyone.  Can't complain. */
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a972acd468b0..c06b8d345fae 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1085,8 +1085,6 @@ int migrate_swap(struct task_struct *cur, struct task_struct *p)
         struct migration_swap_arg arg;
         int ret = -EINVAL;
 
-        get_online_cpus();
-
         arg = (struct migration_swap_arg){
                 .src_task = cur,
                 .src_cpu = task_cpu(cur),
@@ -1097,6 +1095,10 @@ int migrate_swap(struct task_struct *cur, struct task_struct *p)
         if (arg.src_cpu == arg.dst_cpu)
                 goto out;
 
+        /*
+         * These three tests are all lockless; this is OK since all of them
+         * will be re-checked with proper locks held further down the line.
+         */
         if (!cpu_active(arg.src_cpu) || !cpu_active(arg.dst_cpu))
                 goto out;
 
@@ -1109,7 +1111,6 @@ int migrate_swap(struct task_struct *cur, struct task_struct *p)
         ret = stop_two_cpus(arg.dst_cpu, arg.src_cpu, migrate_swap_stop, &arg);
 
 out:
-        put_online_cpus();
         return ret;
 }
 
@@ -3710,7 +3711,6 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
         struct task_struct *p;
         int retval;
 
-        get_online_cpus();
         rcu_read_lock();
 
         p = find_process_by_pid(pid);
@@ -3773,7 +3773,6 @@ out_free_cpus_allowed:
         free_cpumask_var(cpus_allowed);
 out_put_task:
         put_task_struct(p);
-        put_online_cpus();
         return retval;
 }
 
@@ -3818,7 +3817,6 @@ long sched_getaffinity(pid_t pid, struct cpumask *mask)
         unsigned long flags;
         int retval;
 
-        get_online_cpus();
         rcu_read_lock();
 
         retval = -ESRCH;
@@ -3831,12 +3829,11 @@ long sched_getaffinity(pid_t pid, struct cpumask *mask)
                 goto out_unlock;
 
         raw_spin_lock_irqsave(&p->pi_lock, flags);
-        cpumask_and(mask, &p->cpus_allowed, cpu_online_mask);
+        cpumask_and(mask, &p->cpus_allowed, cpu_active_mask);
         raw_spin_unlock_irqrestore(&p->pi_lock, flags);
 
 out_unlock:
         rcu_read_unlock();
-        put_online_cpus();
 
         return retval;
 }
@@ -6494,14 +6491,17 @@ void __init sched_init_smp(void)
 
         sched_init_numa();
 
-        get_online_cpus();
+        /*
+         * There's no userspace yet to cause hotplug operations; hence all the
+         * cpu masks are stable and all blatant races in the below code cannot
+         * happen.
+         */
         mutex_lock(&sched_domains_mutex);
         init_sched_domains(cpu_active_mask);
         cpumask_andnot(non_isolated_cpus, cpu_possible_mask, cpu_isolated_map);
         if (cpumask_empty(non_isolated_cpus))
                 cpumask_set_cpu(smp_processor_id(), non_isolated_cpus);
         mutex_unlock(&sched_domains_mutex);
-        put_online_cpus();
 
         hotcpu_notifier(sched_domains_numa_masks_update, CPU_PRI_SCHED_ACTIVE);
         hotcpu_notifier(cpuset_cpu_active, CPU_PRI_CPUSET_ACTIVE);
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 32a6c44d8f78..c530bc5be7cf 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -234,11 +234,13 @@ static void irq_cpu_stop_queue_work(void *arg)
  */
 int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg)
 {
-        int call_cpu;
         struct cpu_stop_done done;
         struct cpu_stop_work work1, work2;
         struct irq_cpu_stop_queue_work_info call_args;
-        struct multi_stop_data msdata = {
+        struct multi_stop_data msdata;
+
+        preempt_disable();
+        msdata = (struct multi_stop_data){
                 .fn = fn,
                 .data = arg,
                 .num_threads = 2,
@@ -262,16 +264,30 @@ int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *
         set_state(&msdata, MULTI_STOP_PREPARE);
 
         /*
+         * If we observe both CPUs active we know _cpu_down() cannot yet have
+         * queued its stop_machine works and therefore ours will get executed
+         * first. Or its not either one of our CPUs that's getting unplugged,
+         * in which case we don't care.
+         *
+         * This relies on the stopper workqueues to be FIFO.
+         */
+        if (!cpu_active(cpu1) || !cpu_active(cpu2)) {
+                preempt_enable();
+                return -ENOENT;
+        }
+
+        /*
          * Queuing needs to be done by the lowest numbered CPU, to ensure
          * that works are always queued in the same order on every CPU.
          * This prevents deadlocks.
          */
-        call_cpu = min(cpu1, cpu2);
-
-        smp_call_function_single(call_cpu, &irq_cpu_stop_queue_work,
+        smp_call_function_single(min(cpu1, cpu2),
+                                 &irq_cpu_stop_queue_work,
                                  &call_args, 0);
+        preempt_enable();
 
         wait_for_completion(&done.completion);
+
         return done.executed ? done.ret : -ENOENT;
 }