sched: add avg_overlap decay

Impact: more precise avg_overlap metric - better load-balancing

avg_overlap is used to measure the runtime overlap of the waker and
wakee.

However, when a process changes behaviour, eg a pipe becomes
un-congested and we don't need to go to sleep after a wakeup
for a while, the avg_overlap value grows stale.

When running we use the avg runtime between preemption as a
measure for avg_overlap since the amount of runtime can be
correlated to cache footprint.

The longer we run, the less likely we'll be wanting to be
migrated to another CPU.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1236709131.25234.576.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 23 insertions, 1 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index af5cd1b2d03e..2f28351892c9 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4620,6 +4620,28 @@ static inline void schedule_debug(struct task_struct *prev)
 #endif
 }
 
+static void put_prev_task(struct rq *rq, struct task_struct *prev)
+{
+        if (prev->state == TASK_RUNNING) {
+                u64 runtime = prev->se.sum_exec_runtime;
+
+                runtime -= prev->se.prev_sum_exec_runtime;
+                runtime = min_t(u64, runtime, 2*sysctl_sched_migration_cost);
+
+                /*
+                 * In order to avoid avg_overlap growing stale when we are
+                 * indeed overlapping and hence not getting put to sleep, grow
+                 * the avg_overlap on preemption.
+                 *
+                 * We use the average preemption runtime because that
+                 * correlates to the amount of cache footprint a task can
+                 * build up.
+                 */
+                update_avg(&prev->se.avg_overlap, runtime);
+        }
+        prev->sched_class->put_prev_task(rq, prev);
+}
+
 /*
  * Pick up the highest-prio task:
  */
@@ -4698,7 +4720,7 @@ need_resched_nonpreemptible:
         if (unlikely(!rq->nr_running))
                 idle_balance(cpu, rq);
 
-        prev->sched_class->put_prev_task(rq, prev);
+        put_prev_task(rq, prev);
         next = pick_next_task(rq);
 
         if (likely(prev != next)) {