sched: Add yield_to(task, preempt) functionality

Currently only implemented for fair class tasks.

Add a yield_to_task method() to the fair scheduling class. allowing the
caller of yield_to() to accelerate another thread in it's thread group,
task group.

Implemented via a scheduler hint, using cfs_rq->next to encourage the
target being selected.  We can rely on pick_next_entity to keep things
fair, so noone can accelerate a thread that has already used its fair
share of CPU time.

This also means callers should only call yield_to when they really
mean it.  Calling it too often can result in the scheduler just
ignoring the hint.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20110201095051.4ddb7738@annuminas.surriel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 20 insertions, 0 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index c0fbeb992833..027024694043 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1975,6 +1975,25 @@ static void yield_task_fair(struct rq *rq)
         set_skip_buddy(se);
 }
 
+static bool yield_to_task_fair(struct rq *rq, struct task_struct *p, bool preempt)
+{
+        struct sched_entity *se = &p->se;
+
+        if (!se->on_rq)
+                return false;
+
+        /* Tell the scheduler that we'd really like pse to run next. */
+        set_next_buddy(se);
+
+        /* Make p's CPU reschedule; pick_next_entity takes care of fairness. */
+        if (preempt)
+                resched_task(rq->curr);
+
+        yield_task_fair(rq);
+
+        return true;
+}
+
 #ifdef CONFIG_SMP
 /**************************************************
  * Fair scheduling class load-balancing methods:
@@ -4243,6 +4262,7 @@ static const struct sched_class fair_sched_class = {
         .enqueue_task           = enqueue_task_fair,
         .dequeue_task           = dequeue_task_fair,
         .yield_task             = yield_task_fair,
+        .yield_to_task          = yield_to_task_fair,
 
         .check_preempt_curr     = check_preempt_wakeup,