sched: re-instate vruntime based wakeup preemption

The advantage is that vruntime based wakeup preemption has a better
conceptual model. Here wakeup_gran = 0 means: preempt when 'fair'.
Therefore wakeup_gran is the granularity of unfairness we allow in order
to make progress.

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

1 files changed, 92 insertions, 6 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index b71ee2c62297..7af17e04a6db 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -143,6 +143,49 @@ static inline struct sched_entity *parent_entity(struct sched_entity *se)
         return se->parent;
 }
 
+/* return depth at which a sched entity is present in the hierarchy */
+static inline int depth_se(struct sched_entity *se)
+{
+        int depth = 0;
+
+        for_each_sched_entity(se)
+                depth++;
+
+        return depth;
+}
+
+static void
+find_matching_se(struct sched_entity **se, struct sched_entity **pse)
+{
+        int se_depth, pse_depth;
+
+        /*
+         * preemption test can be made between sibling entities who are in the
+         * same cfs_rq i.e who have a common parent. Walk up the hierarchy of
+         * both tasks until we find their ancestors who are siblings of common
+         * parent.
+         */
+
+        /* First walk up until both entities are at same depth */
+        se_depth = depth_se(*se);
+        pse_depth = depth_se(*pse);
+
+        while (se_depth > pse_depth) {
+                se_depth--;
+                *se = parent_entity(*se);
+        }
+
+        while (pse_depth > se_depth) {
+                pse_depth--;
+                *pse = parent_entity(*pse);
+        }
+
+        while (!is_same_group(*se, *pse)) {
+                *se = parent_entity(*se);
+                *pse = parent_entity(*pse);
+        }
+}
+
 #else   /* CONFIG_FAIR_GROUP_SCHED */
 
 static inline struct rq *rq_of(struct cfs_rq *cfs_rq)
@@ -193,6 +236,11 @@ static inline struct sched_entity *parent_entity(struct sched_entity *se)
         return NULL;
 }
 
+static inline void
+find_matching_se(struct sched_entity **se, struct sched_entity **pse)
+{
+}
+
 #endif  /* CONFIG_FAIR_GROUP_SCHED */
 
 
@@ -1244,13 +1292,42 @@ static unsigned long wakeup_gran(struct sched_entity *se)
          * More easily preempt - nice tasks, while not making it harder for
          * + nice tasks.
          */
-        if (sched_feat(ASYM_GRAN))
-                gran = calc_delta_mine(gran, NICE_0_LOAD, &se->load);
+        if (!sched_feat(ASYM_GRAN) || se->load.weight > NICE_0_LOAD)
+                gran = calc_delta_fair(sysctl_sched_wakeup_granularity, se);
 
         return gran;
 }
 
 /*
+ * Should 'se' preempt 'curr'.
+ *
+ *             |s1
+ *        |s2
+ *   |s3
+ *         g
+ *      |<--->|c
+ *
+ *  w(c, s1) = -1
+ *  w(c, s2) =  0
+ *  w(c, s3) =  1
+ *
+ */
+static int
+wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
+{
+        s64 gran, vdiff = curr->vruntime - se->vruntime;
+
+        if (vdiff <= 0)
+                return -1;
+
+        gran = wakeup_gran(curr);
+        if (vdiff > gran)
+                return 1;
+
+        return 0;
+}
+
+/*
  * Preempt the current task with a newly woken task if needed:
  */
 static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int sync)
@@ -1258,7 +1335,6 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int sync)
         struct task_struct *curr = rq->curr;
         struct cfs_rq *cfs_rq = task_cfs_rq(curr);
         struct sched_entity *se = &curr->se, *pse = &p->se;
-        s64 delta_exec;
 
         if (unlikely(rt_prio(p->prio))) {
                 update_rq_clock(rq);
@@ -1296,9 +1372,19 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int sync)
                 return;
         }
 
-        delta_exec = se->sum_exec_runtime - se->prev_sum_exec_runtime;
-        if (delta_exec > wakeup_gran(pse))
-                resched_task(curr);
+        find_matching_se(&se, &pse);
+
+        while (se) {
+                BUG_ON(!pse);
+
+                if (wakeup_preempt_entity(se, pse) == 1) {
+                        resched_task(curr);
+                        break;
+                }
+
+                se = parent_entity(se);
+                pse = parent_entity(pse);
+        }
 }
 
 static struct task_struct *pick_next_task_fair(struct rq *rq)