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>

1 files changed, 21 insertions, 5 deletions

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;
 }