sched: new task placement for vruntime

add proper new task placement for the vruntime based math too.

( note: introduces a swap() macro, but the swap token is too
  widely used in the kernel namespace for a generic version
  to be added without changing non-scheduler code - so this
  cleanup will be done separately. )

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 27 insertions, 9 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 7041dc697855..95487e3c8b06 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -203,6 +203,20 @@ static struct sched_entity *__pick_next_entity(struct cfs_rq *cfs_rq)
  * Scheduling class statistics methods:
  */
 
+static u64 __sched_period(unsigned long nr_running)
+{
+        u64 period = sysctl_sched_latency;
+        unsigned long nr_latency =
+                sysctl_sched_latency / sysctl_sched_min_granularity;
+
+        if (unlikely(nr_running > nr_latency)) {
+                period *= nr_running;
+                do_div(period, nr_latency);
+        }
+
+        return period;
+}
+
 /*
  * Calculate the preemption granularity needed to schedule every
  * runnable task once per sysctl_sched_latency amount of time.
@@ -1103,6 +1117,8 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr)
         }
 }
 
+#define swap(a,b) do { typeof(a) tmp = (a); (a) = (b); (b) = tmp; } while (0)
+
 /*
  * Share the fairness runtime between parent and child, thus the
  * total amount of pressure for CPU stays equal - new tasks
@@ -1118,14 +1134,9 @@ static void task_new_fair(struct rq *rq, struct task_struct *p)
         sched_info_queued(p);
 
         update_curr(cfs_rq);
+        se->vruntime = cfs_rq->min_vruntime;
         update_stats_enqueue(cfs_rq, se);
-        /*
-         * Child runs first: we let it run before the parent
-         * until it reschedules once. We set up the key so that
-         * it will preempt the parent:
-         */
-        se->fair_key = curr->fair_key -
-                niced_granularity(curr, sched_granularity(cfs_rq)) - 1;
+
         /*
          * The first wait is dominated by the child-runs-first logic,
          * so do not credit it with that waiting time yet:
@@ -1138,9 +1149,16 @@ static void task_new_fair(struct rq *rq, struct task_struct *p)
          * -granularity/2, so initialize the task with that:
          */
         if (sched_feat(START_DEBIT))
-                se->wait_runtime = -(sched_granularity(cfs_rq) / 2);
+                se->wait_runtime = -(__sched_period(cfs_rq->nr_running+1) / 2);
+
+        if (sysctl_sched_child_runs_first &&
+                        curr->vruntime < se->vruntime) {
+
+                dequeue_entity(cfs_rq, curr, 0);
+                swap(curr->vruntime, se->vruntime);
+                enqueue_entity(cfs_rq, curr, 0);
+        }
 
-        se->vruntime = cfs_rq->min_vruntime;
         update_stats_enqueue(cfs_rq, se);
         __enqueue_entity(cfs_rq, se);
         resched_task(rq->curr);