sched: clear buddies more aggressively

It was noticed that a task could get re-elected past its run quota due to buddy
affinities. This could increase latency a little. Cure it by more aggresively
clearing buddy state.

We do so in two situations:
 - when we force preempt
 - when we select a buddy to run

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

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index fdc4175..75248b9 100644 (file)
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -768,8 +768,14 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 
        ideal_runtime = sched_slice(cfs_rq, curr);
        delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
-       if (delta_exec > ideal_runtime)
+       if (delta_exec > ideal_runtime) {
                resched_task(rq_of(cfs_rq)->curr);
+               /*
+                * The current task ran long enough, ensure it doesn't get
+                * re-elected due to buddy favours.
+                */
+               clear_buddies(cfs_rq, curr);
+       }
 }
 
 static void
@@ -1445,6 +1451,11 @@ static struct task_struct *pick_next_task_fair(struct rq *rq)
 
        do {
                se = pick_next_entity(cfs_rq);
+               /*
+                * If se was a buddy, clear it so that it will have to earn
+                * the favour again.
+                */
+               clear_buddies(cfs_rq, se);
                set_next_entity(cfs_rq, se);
                cfs_rq = group_cfs_rq(se);
        } while (cfs_rq);