[PATCH v3 2/4] sched: track the next-highest priority on each runqueue

Gregory Haskins
2008-12-04T15:41:26Z

We will use this later in the series to reduce the amount of rq-lock
contention during a pull operation

Signed-off-by: Gregory Haskins <ghaskins@novell.com>
---

kernel/sched.c | 8 ++++-
kernel/sched_rt.c | 81 ++++++++++++++++++++++++++++++++++++++++-------------
2 files changed, 67 insertions(+), 22 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 6237b9b..24b11eb 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -463,7 +463,10 @@ struct rt_rq {
struct rt_prio_array active;
unsigned long rt_nr_running;
#if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
- int highest_prio; /* highest queued rt task prio */
+ struct {
+ int curr; /* highest queued rt task prio */
+ int next; /* next highest */
+ } highest_prio;
#endif
#ifdef CONFIG_SMP
unsigned long rt_nr_migratory;
@@ -8073,7 +8076,8 @@ static void init_rt_rq(struct rt_rq *rt_rq, struct rq *rq)
__set_bit(MAX_RT_PRIO, array->bitmap);

#if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
- rt_rq->highest_prio = MAX_RT_PRIO;
+ rt_rq->highest_prio.curr = MAX_RT_PRIO;
+ rt_rq->highest_prio.next = MAX_RT_PRIO;
#endif
#ifdef CONFIG_SMP
rt_rq->rt_nr_migratory = 0;
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index fb1d4d7..7dad033 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -108,7 +108,7 @@ static void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
if (rt_rq->rt_nr_running) {
if (rt_se && !on_rt_rq(rt_se))
enqueue_rt_entity(rt_se);
- if (rt_rq->highest_prio < curr->prio)
+ if (rt_rq->highest_prio.curr < curr->prio)
resched_task(curr);
}
}
@@ -473,7 +473,7 @@ static inline int rt_se_prio(struct sched_rt_entity *rt_se)
struct rt_rq *rt_rq = group_rt_rq(rt_se);

if (rt_rq)
- return rt_rq->highest_prio;
+ return rt_rq->highest_prio.curr;
#endif

return rt_task_of(rt_se)->prio;
@@ -547,6 +547,21 @@ static void update_curr_rt(struct rq *rq)
}
}

+#if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
+
+static struct task_struct *pick_next_highest_task_rt(struct rq *rq, int cpu);
+
+static inline int next_prio(struct rq *rq)
+{
+ struct task_struct *next = pick_next_highest_task_rt(rq, rq->cpu);
+
+ if (next && rt_prio(next->prio))
+ return next->prio;
+ else
+ return MAX_RT_PRIO;
+}
+#endif
+
static inline
void inc_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
{
@@ -560,14 +575,32 @@ void inc_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
WARN_ON(!rt_prio(prio));
rt_rq->rt_nr_running++;
#if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
- if (prio < rt_rq->highest_prio) {
+ if (prio < rt_rq->highest_prio.curr) {

- rt_rq->highest_prio = prio;
+ /*
+ * If the new task is higher in priority than anything on the
+ * run-queue, we have a new high that must be published to
+ * the world. We also know that the previous high becomes
+ * our next-highest.
+ */
+ rt_rq->highest_prio.next = rt_rq->highest_prio.curr;
+ rt_rq->highest_prio.curr = prio;
#ifdef CONFIG_SMP
if (rq->online)
cpupri_set(&rq->rd->cpupri, rq->cpu, prio);
#endif
- }
+ } else if (prio == rt_rq->highest_prio.curr)
+ /*
+ * If the next task is equal in priority to the highest on
+ * the run-queue, then we implicitly know that the next highest
+ * task cannot be any lower than current
+ */
+ rt_rq->highest_prio.next = prio;
+ else if (prio < rt_rq->highest_prio.next)
+ /*
+ * Otherwise, we need to recompute next-highest
+ */
+ rt_rq->highest_prio.next = next_prio(rq);
#endif
#ifdef CONFIG_SMP
if (rt_se->nr_cpus_allowed > 1)
@@ -591,7 +624,7 @@ void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
{
#ifdef CONFIG_SMP
struct rq *rq = rq_of_rt_rq(rt_rq);
- int highest_prio = rt_rq->highest_prio;
+ int highest_prio = rt_rq->highest_prio.curr;
#endif

WARN_ON(!rt_prio(rt_se_prio(rt_se)));
@@ -599,24 +632,32 @@ void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
rt_rq->rt_nr_running--;
#if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
if (rt_rq->rt_nr_running) {
- struct rt_prio_array *array;
+ int prio = rt_se_prio(rt_se);
+
+ WARN_ON(prio < rt_rq->highest_prio.curr);

- WARN_ON(rt_se_prio(rt_se) < rt_rq->highest_prio);
- if (rt_se_prio(rt_se) == rt_rq->highest_prio) {
- /* recalculate */
- array = &rt_rq->active;
- rt_rq->highest_prio =
+ /*
+ * This may have been our highest or next-highest priority
+ * task and therefore we may have some recomputation to do
+ */
+ if (prio == rt_rq->highest_prio.curr) {
+ struct rt_prio_array *array = &rt_rq->active;
+
+ rt_rq->highest_prio.curr =
sched_find_first_bit(array->bitmap);
- } /* otherwise leave rq->highest prio alone */
+ }
+
+ if (prio <= rt_rq->highest_prio.next)
+ rt_rq->highest_prio.next = next_prio(rq);
} else
- rt_rq->highest_prio = MAX_RT_PRIO;
+ rt_rq->highest_prio.curr = MAX_RT_PRIO;
#endif
#ifdef CONFIG_SMP
if (rt_se->nr_cpus_allowed > 1)
rq->rt.rt_nr_migratory--;

- if (rq->online && rt_rq->highest_prio != highest_prio)
- cpupri_set(&rq->rd->cpupri, rq->cpu, rt_rq->highest_prio);
+ if (rq->online && rt_rq->highest_prio.curr != highest_prio)
+ cpupri_set(&rq->rd->cpupri, rq->cpu, rt_rq->highest_prio.curr);

update_rt_migration(rq);
#endif /* CONFIG_SMP */
@@ -1066,7 +1107,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
}

/* If this rq is still suitable use it. */
- if (lowest_rq->rt.highest_prio > task->prio)
+ if (lowest_rq->rt.highest_prio.curr > task->prio)
break;

/* try again */
@@ -1254,7 +1295,7 @@ static int pull_rt_task(struct rq *this_rq)
static void pre_schedule_rt(struct rq *rq, struct task_struct *prev)
{
/* Try to pull RT tasks here if we lower this rq's prio */
- if (unlikely(rt_task(prev)) && rq->rt.highest_prio > prev->prio)
+ if (unlikely(rt_task(prev)) && rq->rt.highest_prio.curr > prev->prio)
pull_rt_task(rq);
}

@@ -1340,7 +1381,7 @@ static void rq_online_rt(struct rq *rq)

__enable_runtime(rq);

- cpupri_set(&rq->rd->cpupri, rq->cpu, rq->rt.highest_prio);
+ cpupri_set(&rq->rd->cpupri, rq->cpu, rq->rt.highest_prio.curr);
}

/* Assumes rq->lock is held */
@@ -1431,7 +1472,7 @@ static void prio_changed_rt(struct rq *rq, struct task_struct *p,
* can release the rq lock and p could migrate.
* Only reschedule if p is still on the same runqueue.
*/
- if (p->prio > rq->rt.highest_prio && rq->curr == p)
+ if (p->prio > rq->rt.highest_prio.curr && rq->curr == p)
resched_task(p);
#else
/* For UP simply resched on drop of prio */