sched: Remove double_rq_lock() from __migrate_task()

Avoid double_rq_lock() and use TASK_ON_RQ_MIGRATING for
__migrate_task(). The advantage is (obviously) not holding two
rq->lock's at the same time and thereby increasing parallelism.

The important point to note is that because we acquire dst->lock
immediately after releasing src->lock the potential wait time of
task_rq_lock() callers on TASK_ON_RQ_MIGRATING is not longer
than it would have been in the double rq lock scenario.

Signed-off-by: Kirill Tkhai <ktkhai@parallels.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul Turner <pjt@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Kirill Tkhai <tkhai@yandex.ru>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1408528070.23412.89.camel@tkhai
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 15 insertions, 8 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 71b836034912..a773c919d88d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4679,20 +4679,20 @@ EXPORT_SYMBOL_GPL(set_cpus_allowed_ptr);
  */
 static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
 {
-        struct rq *rq_dest, *rq_src;
+        struct rq *rq;
         int ret = 0;
 
         if (unlikely(!cpu_active(dest_cpu)))
                 return ret;
 
-        rq_src = cpu_rq(src_cpu);
-        rq_dest = cpu_rq(dest_cpu);
+        rq = cpu_rq(src_cpu);
 
         raw_spin_lock(&p->pi_lock);
-        double_rq_lock(rq_src, rq_dest);
+        raw_spin_lock(&rq->lock);
         /* Already moved. */
         if (task_cpu(p) != src_cpu)
                 goto done;
+
         /* Affinity changed (again). */
         if (!cpumask_test_cpu(dest_cpu, tsk_cpus_allowed(p)))
                 goto fail;
@@ -4702,15 +4702,22 @@ static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
          * placed properly.
          */
         if (task_on_rq_queued(p)) {
-                dequeue_task(rq_src, p, 0);
+                dequeue_task(rq, p, 0);
+                p->on_rq = TASK_ON_RQ_MIGRATING;
                 set_task_cpu(p, dest_cpu);
-                enqueue_task(rq_dest, p, 0);
-                check_preempt_curr(rq_dest, p, 0);
+                raw_spin_unlock(&rq->lock);
+
+                rq = cpu_rq(dest_cpu);
+                raw_spin_lock(&rq->lock);
+                BUG_ON(task_rq(p) != rq);
+                p->on_rq = TASK_ON_RQ_QUEUED;
+                enqueue_task(rq, p, 0);
+                check_preempt_curr(rq, p, 0);
         }
 done:
         ret = 1;
 fail:
-        double_rq_unlock(rq_src, rq_dest);
+        raw_spin_unlock(&rq->lock);
         raw_spin_unlock(&p->pi_lock);
         return ret;
 }