[PATCH v3 05/17] pull RT tasks

Steven Rostedt
2007-11-17T06:31:21Z

[-- Attachment #1: rt-balance-pull-tasks.patch --]
[-- Type: text/plain, Size: 8059 bytes --]

This patch adds the algorithm to pull tasks from RT overloaded runqueues.

When a pull RT is initiated, all overloaded runqueues are examined for
a RT task that is higher in prio than the highest prio task queued on the
target runqueue. If another runqueue holds a RT task that is of higher
prio than the highest prio task on the target runqueue is found it is pulled
to the target runqueue.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>

---
kernel/sched.c | 2
kernel/sched_rt.c | 196 ++++++++++++++++++++++++++++++++++++++++++++++++++----
2 files changed, 187 insertions(+), 11 deletions(-)

Index: linux-compile.git/kernel/sched.c
===================================================================
--- linux-compile.git.orig/kernel/sched.c 2007-11-16 22:11:37.000000000 -0500
+++ linux-compile.git/kernel/sched.c 2007-11-16 22:12:13.000000000 -0500
@@ -3646,6 +3646,8 @@ need_resched_nonpreemptible:
switch_count = &prev->nvcsw;
}

+ schedule_balance_rt(rq, prev);
+
if (unlikely(!rq->nr_running))
idle_balance(cpu, rq);

Index: linux-compile.git/kernel/sched_rt.c
===================================================================
--- linux-compile.git.orig/kernel/sched_rt.c 2007-11-16 22:12:07.000000000 -0500
+++ linux-compile.git/kernel/sched_rt.c 2007-11-16 22:12:13.000000000 -0500
@@ -176,8 +176,17 @@ static void put_prev_task_rt(struct rq *
static int double_lock_balance(struct rq *this_rq, struct rq *busiest);
static void deactivate_task(struct rq *rq, struct task_struct *p, int sleep);

+static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
+{
+ if (!task_running(rq, p) &&
+ (cpu < 0 || cpu_isset(cpu, p->cpus_allowed)))
+ return 1;
+ return 0;
+}
+
/* Return the second highest RT task, NULL otherwise */
-static struct task_struct *pick_next_highest_task_rt(struct rq *rq)
+static struct task_struct *pick_next_highest_task_rt(struct rq *rq,
+ int cpu)
{
struct rt_prio_array *array = &rq->rt.active;
struct task_struct *next;
@@ -196,26 +205,36 @@ static struct task_struct *pick_next_hig
}

queue = array->queue + idx;
+ BUG_ON(list_empty(queue));
+
next = list_entry(queue->next, struct task_struct, run_list);
- if (unlikely(next != rq->curr))
- return next;
+ if (unlikely(pick_rt_task(rq, next, cpu)))
+ goto out;

if (queue->next->next != queue) {
/* same prio task */
next = list_entry(queue->next->next, struct task_struct, run_list);
- return next;
+ if (pick_rt_task(rq, next, cpu))
+ goto out;
}

+ retry:
/* slower, but more flexible */
idx = find_next_bit(array->bitmap, MAX_RT_PRIO, idx+1);
- if (unlikely(idx >= MAX_RT_PRIO)) {
- WARN_ON(1); /* rt_nr_running was 2 and above! */
+ if (unlikely(idx >= MAX_RT_PRIO))
return NULL;
- }

queue = array->queue + idx;
- next = list_entry(queue->next, struct task_struct, run_list);
+ BUG_ON(list_empty(queue));

+ list_for_each_entry(next, queue, run_list) {
+ if (pick_rt_task(rq, next, cpu))
+ goto out;
+ }
+
+ goto retry;
+
+ out:
return next;
}

@@ -300,13 +319,15 @@ static int push_rt_task(struct rq *this_

assert_spin_locked(&this_rq->lock);

- next_task = pick_next_highest_task_rt(this_rq);
+ next_task = pick_next_highest_task_rt(this_rq, -1);
if (!next_task)
return 0;

retry:
- if (unlikely(next_task == this_rq->curr))
+ if (unlikely(next_task == this_rq->curr)) {
+ WARN_ON(1);
return 0;
+ }

/*
* It's possible that the next_task slipped in of
@@ -330,7 +351,7 @@ static int push_rt_task(struct rq *this_
* so it is possible that next_task has changed.
* If it has, then try again.
*/
- task = pick_next_highest_task_rt(this_rq);
+ task = pick_next_highest_task_rt(this_rq, -1);
if (unlikely(task != next_task) && task && paranoid--) {
put_task_struct(next_task);
next_task = task;
@@ -373,6 +394,158 @@ static void push_rt_tasks(struct rq *rq)
;
}

+static int pull_rt_task(struct rq *this_rq)
+{
+ struct task_struct *next;
+ struct task_struct *p;
+ struct rq *src_rq;
+ cpumask_t *rto_cpumask;
+ int this_cpu = this_rq->cpu;
+ int cpu;
+ int ret = 0;
+
+ assert_spin_locked(&this_rq->lock);
+
+ /*
+ * If cpusets are used, and we have overlapping
+ * run queue cpusets, then this algorithm may not catch all.
+ * This is just the price you pay on trying to keep
+ * dirtying caches down on large SMP machines.
+ */
+ if (likely(!rt_overloaded()))
+ return 0;
+
+ next = pick_next_task_rt(this_rq);
+
+ rto_cpumask = rt_overload();
+
+ for_each_cpu_mask(cpu, *rto_cpumask) {
+ if (this_cpu == cpu)
+ continue;
+
+ src_rq = cpu_rq(cpu);
+ if (unlikely(src_rq->rt.rt_nr_running <= 1)) {
+ /*
+ * It is possible that overlapping cpusets
+ * will miss clearing a non overloaded runqueue.
+ * Clear it now.
+ */
+ if (double_lock_balance(this_rq, src_rq)) {
+ /* unlocked our runqueue lock */
+ struct task_struct *old_next = next;
+ next = pick_next_task_rt(this_rq);
+ if (next != old_next)
+ ret = 1;
+ }
+ if (likely(src_rq->rt.rt_nr_running <= 1))
+ /*
+ * Small chance that this_rq->curr changed
+ * but it's really harmless here.
+ */
+ rt_clear_overload(this_rq);
+ else
+ /*
+ * Heh, the src_rq is now overloaded, since
+ * we already have the src_rq lock, go straight
+ * to pulling tasks from it.
+ */
+ goto try_pulling;
+ spin_unlock(&src_rq->lock);
+ continue;
+ }
+
+ /*
+ * We can potentially drop this_rq's lock in
+ * double_lock_balance, and another CPU could
+ * steal our next task - hence we must cause
+ * the caller to recalculate the next task
+ * in that case:
+ */
+ if (double_lock_balance(this_rq, src_rq)) {
+ struct task_struct *old_next = next;
+ next = pick_next_task_rt(this_rq);
+ if (next != old_next)
+ ret = 1;
+ }
+
+ /*
+ * Are there still pullable RT tasks?
+ */
+ if (src_rq->rt.rt_nr_running <= 1) {
+ spin_unlock(&src_rq->lock);
+ continue;
+ }
+
+ try_pulling:
+ p = pick_next_highest_task_rt(src_rq, this_cpu);
+
+ /*
+ * Do we have an RT task that preempts
+ * the to-be-scheduled task?
+ */
+ if (p && (!next || (p->prio < next->prio))) {
+ WARN_ON(p == src_rq->curr);
+ WARN_ON(!p->se.on_rq);
+
+ /*
+ * There's a chance that p is higher in priority
+ * than what's currently running on its cpu.
+ * This is just that p is wakeing up and hasn't
+ * had a chance to schedule. We only pull
+ * p if it is lower in priority than the
+ * current task on the run queue or
+ * this_rq next task is lower in prio than
+ * the current task on that rq.
+ */
+ if (p->prio < src_rq->curr->prio ||
+ (next && next->prio < src_rq->curr->prio))
+ goto bail;
+
+ ret = 1;
+
+ deactivate_task(src_rq, p, 0);
+ set_task_cpu(p, this_cpu);
+ activate_task(this_rq, p, 0);
+ /*
+ * We continue with the search, just in
+ * case there's an even higher prio task
+ * in another runqueue. (low likelyhood
+ * but possible)
+ */
+
+ /*
+ * Update next so that we won't pick a task
+ * on another cpu with a priority lower (or equal)
+ * than the one we just picked.
+ */
+ next = p;
+
+ }
+ bail:
+ spin_unlock(&src_rq->lock);
+ }
+
+ return ret;
+}
+
+static void schedule_balance_rt(struct rq *rq,
+ struct task_struct *prev)
+{
+ struct rt_prio_array *array;
+ int next_prio;
+
+ /* Try to pull RT tasks here if we lower this rq's prio */
+ if (unlikely(rt_task(prev))) {
+ next_prio = MAX_RT_PRIO;
+ if (rq->rt.rt_nr_running) {
+ array = &rq->rt.active;
+ next_prio = sched_find_first_bit(array->bitmap);
+ }
+ if (next_prio > prev->prio)
+ pull_rt_task(rq);
+ }
+}
+
static void schedule_tail_balance_rt(struct rq *rq)
{
/*
@@ -495,6 +668,7 @@ move_one_task_rt(struct rq *this_rq, int
}
#else /* CONFIG_SMP */
# define schedule_tail_balance_rt(rq) do { } while (0)
+# define schedule_balance_rt(rq, prev) do { } while (0)
#endif /* CONFIG_SMP */

static void task_tick_rt(struct rq *rq, struct task_struct *p)

--