sched: Sanitize fork() handling

Currently we try to do task placement in wake_up_new_task() after we do
the load-balance pass in sched_fork(). This yields complicated semantics
in that we have to deal with tasks on different RQs and the
set_task_cpu() calls in copy_process() and sched_fork()

Rename ->task_new() to ->task_fork() and call it from sched_fork()
before the balancing, this gives the policy a clear point to place the
task.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 15 insertions, 13 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 613c1c749677..44ec80ccfa85 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1922,28 +1922,30 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
 }
 
 /*
- * Share the fairness runtime between parent and child, thus the
- * total amount of pressure for CPU stays equal - new tasks
- * get a chance to run but frequent forkers are not allowed to
- * monopolize the CPU. Note: the parent runqueue is locked,
- * the child is not running yet.
+ * called on fork with the child task as argument from the parent's context
+ *  - child not yet on the tasklist
+ *  - preemption disabled
  */
-static void task_new_fair(struct rq *rq, struct task_struct *p)
+static void task_fork_fair(struct task_struct *p)
 {
-        struct cfs_rq *cfs_rq = task_cfs_rq(p);
+        struct cfs_rq *cfs_rq = task_cfs_rq(current);
         struct sched_entity *se = &p->se, *curr = cfs_rq->curr;
         int this_cpu = smp_processor_id();
+        struct rq *rq = this_rq();
+        unsigned long flags;
+
+        spin_lock_irqsave(&rq->lock, flags);
 
-        sched_info_queued(p);
+        if (unlikely(task_cpu(p) != this_cpu))
+                __set_task_cpu(p, this_cpu);
 
         update_curr(cfs_rq);
+
         if (curr)
                 se->vruntime = curr->vruntime;
         place_entity(cfs_rq, se, 1);
 
-        /* 'curr' will be NULL if the child belongs to a different group */
-        if (sysctl_sched_child_runs_first && this_cpu == task_cpu(p) &&
-                        curr && entity_before(curr, se)) {
+        if (sysctl_sched_child_runs_first && curr && entity_before(curr, se)) {
                 /*
                  * Upon rescheduling, sched_class::put_prev_task() will place
                  * 'current' within the tree based on its new key value.
@@ -1952,7 +1954,7 @@ static void task_new_fair(struct rq *rq, struct task_struct *p)
                 resched_task(rq->curr);
         }
 
-        enqueue_task_fair(rq, p, 0);
+        spin_unlock_irqrestore(&rq->lock, flags);
 }
 
 /*
@@ -2052,7 +2054,7 @@ static const struct sched_class fair_sched_class = {
 
         .set_curr_task          = set_curr_task_fair,
         .task_tick              = task_tick_fair,
-        .task_new               = task_new_fair,
+        .task_fork              = task_fork_fair,
 
         .prio_changed           = prio_changed_fair,
         .switched_to            = switched_to_fair,