![Gregory Haskins](/assets/img/avatar_default.png)
In the original patch series that Steven Rostedt and I worked on together, we both took different approaches to low-priority wakeup path. I utilized "pre-routing" (push the task away to a less important RQ before activating) approach, while Steve utilized a "post-routing" approach. The advantage of my approach is that you avoid the overhead of a wasted activate/deactivate cycle and peripherally related burdens. The advantage of Steve's method is that it neatly solves an issue preventing a "pull" optimization from being deployed. In the end, we ended up deploying Steve's idea. But it later dawned on me that we could get the best of both worlds by deploying both ideas together, albeit slightly modified. The idea is simple: Use a "light-weight" lookup for pre-routing, since we only need to approximate a good home for the task. And we also retain the post-routing push logic to clean up any inaccuracies caused by a condition of "priority mistargeting" caused by the lightweight lookup. Most of the time, the pre-routing should work and yield lower overhead. In the cases where it doesnt, the post-router will bat cleanup. Signed-off-by: Gregory Haskins <ghaskins@novell.com> Signed-off-by: Steven Rostedt <srostedt@redhat.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
723 lines
17 KiB
C
723 lines
17 KiB
C
/*
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* Real-Time Scheduling Class (mapped to the SCHED_FIFO and SCHED_RR
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* policies)
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*/
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#ifdef CONFIG_SMP
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static cpumask_t rt_overload_mask;
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static atomic_t rto_count;
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static inline int rt_overloaded(void)
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{
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return atomic_read(&rto_count);
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}
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static inline cpumask_t *rt_overload(void)
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{
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return &rt_overload_mask;
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}
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static inline void rt_set_overload(struct rq *rq)
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{
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cpu_set(rq->cpu, rt_overload_mask);
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/*
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* Make sure the mask is visible before we set
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* the overload count. That is checked to determine
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* if we should look at the mask. It would be a shame
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* if we looked at the mask, but the mask was not
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* updated yet.
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*/
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wmb();
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atomic_inc(&rto_count);
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}
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static inline void rt_clear_overload(struct rq *rq)
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{
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/* the order here really doesn't matter */
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atomic_dec(&rto_count);
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cpu_clear(rq->cpu, rt_overload_mask);
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}
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static void update_rt_migration(struct rq *rq)
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{
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if (rq->rt.rt_nr_migratory && (rq->rt.rt_nr_running > 1))
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rt_set_overload(rq);
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else
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rt_clear_overload(rq);
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}
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#endif /* CONFIG_SMP */
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/*
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* Update the current task's runtime statistics. Skip current tasks that
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* are not in our scheduling class.
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*/
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static void update_curr_rt(struct rq *rq)
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{
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struct task_struct *curr = rq->curr;
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u64 delta_exec;
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if (!task_has_rt_policy(curr))
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return;
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delta_exec = rq->clock - curr->se.exec_start;
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if (unlikely((s64)delta_exec < 0))
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delta_exec = 0;
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schedstat_set(curr->se.exec_max, max(curr->se.exec_max, delta_exec));
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curr->se.sum_exec_runtime += delta_exec;
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curr->se.exec_start = rq->clock;
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cpuacct_charge(curr, delta_exec);
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}
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static inline void inc_rt_tasks(struct task_struct *p, struct rq *rq)
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{
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WARN_ON(!rt_task(p));
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rq->rt.rt_nr_running++;
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#ifdef CONFIG_SMP
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if (p->prio < rq->rt.highest_prio)
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rq->rt.highest_prio = p->prio;
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if (p->nr_cpus_allowed > 1)
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rq->rt.rt_nr_migratory++;
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update_rt_migration(rq);
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#endif /* CONFIG_SMP */
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}
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static inline void dec_rt_tasks(struct task_struct *p, struct rq *rq)
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{
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WARN_ON(!rt_task(p));
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WARN_ON(!rq->rt.rt_nr_running);
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rq->rt.rt_nr_running--;
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#ifdef CONFIG_SMP
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if (rq->rt.rt_nr_running) {
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struct rt_prio_array *array;
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WARN_ON(p->prio < rq->rt.highest_prio);
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if (p->prio == rq->rt.highest_prio) {
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/* recalculate */
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array = &rq->rt.active;
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rq->rt.highest_prio =
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sched_find_first_bit(array->bitmap);
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} /* otherwise leave rq->highest prio alone */
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} else
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rq->rt.highest_prio = MAX_RT_PRIO;
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if (p->nr_cpus_allowed > 1)
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rq->rt.rt_nr_migratory--;
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update_rt_migration(rq);
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#endif /* CONFIG_SMP */
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}
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static void enqueue_task_rt(struct rq *rq, struct task_struct *p, int wakeup)
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{
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struct rt_prio_array *array = &rq->rt.active;
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list_add_tail(&p->run_list, array->queue + p->prio);
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__set_bit(p->prio, array->bitmap);
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inc_cpu_load(rq, p->se.load.weight);
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inc_rt_tasks(p, rq);
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}
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/*
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* Adding/removing a task to/from a priority array:
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*/
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static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int sleep)
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{
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struct rt_prio_array *array = &rq->rt.active;
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update_curr_rt(rq);
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list_del(&p->run_list);
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if (list_empty(array->queue + p->prio))
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__clear_bit(p->prio, array->bitmap);
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dec_cpu_load(rq, p->se.load.weight);
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dec_rt_tasks(p, rq);
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}
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/*
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* Put task to the end of the run list without the overhead of dequeue
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* followed by enqueue.
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*/
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static void requeue_task_rt(struct rq *rq, struct task_struct *p)
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{
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struct rt_prio_array *array = &rq->rt.active;
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list_move_tail(&p->run_list, array->queue + p->prio);
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}
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static void
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yield_task_rt(struct rq *rq)
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{
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requeue_task_rt(rq, rq->curr);
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}
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#ifdef CONFIG_SMP
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static int find_lowest_rq(struct task_struct *task);
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static int select_task_rq_rt(struct task_struct *p, int sync)
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{
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struct rq *rq = task_rq(p);
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/*
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* If the task will not preempt the RQ, try to find a better RQ
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* before we even activate the task
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*/
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if ((p->prio >= rq->rt.highest_prio)
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&& (p->nr_cpus_allowed > 1)) {
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int cpu = find_lowest_rq(p);
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return (cpu == -1) ? task_cpu(p) : cpu;
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}
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/*
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* Otherwise, just let it ride on the affined RQ and the
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* post-schedule router will push the preempted task away
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*/
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return task_cpu(p);
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}
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#endif /* CONFIG_SMP */
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/*
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* Preempt the current task with a newly woken task if needed:
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*/
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static void check_preempt_curr_rt(struct rq *rq, struct task_struct *p)
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{
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if (p->prio < rq->curr->prio)
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resched_task(rq->curr);
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}
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static struct task_struct *pick_next_task_rt(struct rq *rq)
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{
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struct rt_prio_array *array = &rq->rt.active;
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struct task_struct *next;
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struct list_head *queue;
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int idx;
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idx = sched_find_first_bit(array->bitmap);
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if (idx >= MAX_RT_PRIO)
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return NULL;
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queue = array->queue + idx;
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next = list_entry(queue->next, struct task_struct, run_list);
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next->se.exec_start = rq->clock;
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return next;
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}
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static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
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{
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update_curr_rt(rq);
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p->se.exec_start = 0;
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}
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#ifdef CONFIG_SMP
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/* Only try algorithms three times */
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#define RT_MAX_TRIES 3
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static int double_lock_balance(struct rq *this_rq, struct rq *busiest);
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static void deactivate_task(struct rq *rq, struct task_struct *p, int sleep);
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static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
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{
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if (!task_running(rq, p) &&
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(cpu < 0 || cpu_isset(cpu, p->cpus_allowed)) &&
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(p->nr_cpus_allowed > 1))
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return 1;
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return 0;
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}
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/* Return the second highest RT task, NULL otherwise */
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static struct task_struct *pick_next_highest_task_rt(struct rq *rq,
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int cpu)
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{
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struct rt_prio_array *array = &rq->rt.active;
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struct task_struct *next;
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struct list_head *queue;
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int idx;
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assert_spin_locked(&rq->lock);
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if (likely(rq->rt.rt_nr_running < 2))
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return NULL;
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idx = sched_find_first_bit(array->bitmap);
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if (unlikely(idx >= MAX_RT_PRIO)) {
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WARN_ON(1); /* rt_nr_running is bad */
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return NULL;
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}
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queue = array->queue + idx;
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BUG_ON(list_empty(queue));
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next = list_entry(queue->next, struct task_struct, run_list);
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if (unlikely(pick_rt_task(rq, next, cpu)))
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goto out;
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if (queue->next->next != queue) {
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/* same prio task */
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next = list_entry(queue->next->next, struct task_struct, run_list);
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if (pick_rt_task(rq, next, cpu))
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goto out;
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}
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retry:
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/* slower, but more flexible */
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idx = find_next_bit(array->bitmap, MAX_RT_PRIO, idx+1);
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if (unlikely(idx >= MAX_RT_PRIO))
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return NULL;
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queue = array->queue + idx;
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BUG_ON(list_empty(queue));
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list_for_each_entry(next, queue, run_list) {
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if (pick_rt_task(rq, next, cpu))
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goto out;
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}
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goto retry;
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out:
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return next;
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}
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static DEFINE_PER_CPU(cpumask_t, local_cpu_mask);
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static int find_lowest_rq(struct task_struct *task)
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{
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int cpu;
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cpumask_t *cpu_mask = &__get_cpu_var(local_cpu_mask);
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struct rq *lowest_rq = NULL;
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cpus_and(*cpu_mask, cpu_online_map, task->cpus_allowed);
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/*
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* Scan each rq for the lowest prio.
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*/
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for_each_cpu_mask(cpu, *cpu_mask) {
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struct rq *rq = cpu_rq(cpu);
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/* We look for lowest RT prio or non-rt CPU */
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if (rq->rt.highest_prio >= MAX_RT_PRIO) {
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lowest_rq = rq;
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break;
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}
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/* no locking for now */
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if (rq->rt.highest_prio > task->prio &&
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(!lowest_rq || rq->rt.highest_prio > lowest_rq->rt.highest_prio)) {
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lowest_rq = rq;
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}
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}
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return lowest_rq ? lowest_rq->cpu : -1;
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}
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/* Will lock the rq it finds */
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static struct rq *find_lock_lowest_rq(struct task_struct *task,
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struct rq *rq)
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{
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struct rq *lowest_rq = NULL;
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int cpu;
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int tries;
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for (tries = 0; tries < RT_MAX_TRIES; tries++) {
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cpu = find_lowest_rq(task);
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if ((cpu == -1) || (cpu == rq->cpu))
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break;
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lowest_rq = cpu_rq(cpu);
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/* if the prio of this runqueue changed, try again */
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if (double_lock_balance(rq, lowest_rq)) {
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/*
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* We had to unlock the run queue. In
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* the mean time, task could have
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* migrated already or had its affinity changed.
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* Also make sure that it wasn't scheduled on its rq.
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*/
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if (unlikely(task_rq(task) != rq ||
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!cpu_isset(lowest_rq->cpu, task->cpus_allowed) ||
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task_running(rq, task) ||
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!task->se.on_rq)) {
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spin_unlock(&lowest_rq->lock);
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lowest_rq = NULL;
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break;
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}
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}
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/* If this rq is still suitable use it. */
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if (lowest_rq->rt.highest_prio > task->prio)
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break;
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/* try again */
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spin_unlock(&lowest_rq->lock);
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lowest_rq = NULL;
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}
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return lowest_rq;
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}
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/*
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* If the current CPU has more than one RT task, see if the non
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* running task can migrate over to a CPU that is running a task
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* of lesser priority.
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*/
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static int push_rt_task(struct rq *rq)
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{
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struct task_struct *next_task;
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struct rq *lowest_rq;
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int ret = 0;
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int paranoid = RT_MAX_TRIES;
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assert_spin_locked(&rq->lock);
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next_task = pick_next_highest_task_rt(rq, -1);
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if (!next_task)
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return 0;
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retry:
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if (unlikely(next_task == rq->curr)) {
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WARN_ON(1);
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return 0;
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}
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/*
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* It's possible that the next_task slipped in of
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* higher priority than current. If that's the case
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* just reschedule current.
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*/
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if (unlikely(next_task->prio < rq->curr->prio)) {
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resched_task(rq->curr);
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return 0;
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}
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/* We might release rq lock */
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get_task_struct(next_task);
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/* find_lock_lowest_rq locks the rq if found */
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lowest_rq = find_lock_lowest_rq(next_task, rq);
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if (!lowest_rq) {
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struct task_struct *task;
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/*
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* find lock_lowest_rq releases rq->lock
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* so it is possible that next_task has changed.
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* If it has, then try again.
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*/
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task = pick_next_highest_task_rt(rq, -1);
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if (unlikely(task != next_task) && task && paranoid--) {
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put_task_struct(next_task);
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next_task = task;
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goto retry;
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}
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goto out;
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}
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assert_spin_locked(&lowest_rq->lock);
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deactivate_task(rq, next_task, 0);
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set_task_cpu(next_task, lowest_rq->cpu);
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activate_task(lowest_rq, next_task, 0);
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resched_task(lowest_rq->curr);
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spin_unlock(&lowest_rq->lock);
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ret = 1;
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out:
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put_task_struct(next_task);
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return ret;
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}
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|
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/*
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* TODO: Currently we just use the second highest prio task on
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* the queue, and stop when it can't migrate (or there's
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* no more RT tasks). There may be a case where a lower
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* priority RT task has a different affinity than the
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* higher RT task. In this case the lower RT task could
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* possibly be able to migrate where as the higher priority
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* RT task could not. We currently ignore this issue.
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* Enhancements are welcome!
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*/
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static void push_rt_tasks(struct rq *rq)
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{
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/* push_rt_task will return true if it moved an RT */
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while (push_rt_task(rq))
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;
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}
|
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|
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static int pull_rt_task(struct rq *this_rq)
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{
|
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struct task_struct *next;
|
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struct task_struct *p;
|
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struct rq *src_rq;
|
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cpumask_t *rto_cpumask;
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int this_cpu = this_rq->cpu;
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int cpu;
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int ret = 0;
|
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|
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assert_spin_locked(&this_rq->lock);
|
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|
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/*
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* If cpusets are used, and we have overlapping
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* run queue cpusets, then this algorithm may not catch all.
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* This is just the price you pay on trying to keep
|
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* dirtying caches down on large SMP machines.
|
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*/
|
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if (likely(!rt_overloaded()))
|
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return 0;
|
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|
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next = pick_next_task_rt(this_rq);
|
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|
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rto_cpumask = rt_overload();
|
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|
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for_each_cpu_mask(cpu, *rto_cpumask) {
|
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if (this_cpu == cpu)
|
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continue;
|
|
|
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src_rq = cpu_rq(cpu);
|
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if (unlikely(src_rq->rt.rt_nr_running <= 1)) {
|
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/*
|
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* It is possible that overlapping cpusets
|
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* will miss clearing a non overloaded runqueue.
|
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* Clear it now.
|
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*/
|
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if (double_lock_balance(this_rq, src_rq)) {
|
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/* unlocked our runqueue lock */
|
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struct task_struct *old_next = next;
|
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next = pick_next_task_rt(this_rq);
|
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if (next != old_next)
|
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ret = 1;
|
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}
|
|
if (likely(src_rq->rt.rt_nr_running <= 1))
|
|
/*
|
|
* Small chance that this_rq->curr changed
|
|
* but it's really harmless here.
|
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*/
|
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rt_clear_overload(this_rq);
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else
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/*
|
|
* Heh, the src_rq is now overloaded, since
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* we already have the src_rq lock, go straight
|
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* to pulling tasks from it.
|
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*/
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goto try_pulling;
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spin_unlock(&src_rq->lock);
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continue;
|
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}
|
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|
|
/*
|
|
* We can potentially drop this_rq's lock in
|
|
* double_lock_balance, and another CPU could
|
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* 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)
|
|
{
|
|
/* Try to pull RT tasks here if we lower this rq's prio */
|
|
if (unlikely(rt_task(prev)) &&
|
|
rq->rt.highest_prio > prev->prio)
|
|
pull_rt_task(rq);
|
|
}
|
|
|
|
static void schedule_tail_balance_rt(struct rq *rq)
|
|
{
|
|
/*
|
|
* If we have more than one rt_task queued, then
|
|
* see if we can push the other rt_tasks off to other CPUS.
|
|
* Note we may release the rq lock, and since
|
|
* the lock was owned by prev, we need to release it
|
|
* first via finish_lock_switch and then reaquire it here.
|
|
*/
|
|
if (unlikely(rq->rt.rt_nr_running > 1)) {
|
|
spin_lock_irq(&rq->lock);
|
|
push_rt_tasks(rq);
|
|
spin_unlock_irq(&rq->lock);
|
|
}
|
|
}
|
|
|
|
|
|
static void wakeup_balance_rt(struct rq *rq, struct task_struct *p)
|
|
{
|
|
if (unlikely(rt_task(p)) &&
|
|
!task_running(rq, p) &&
|
|
(p->prio >= rq->curr->prio))
|
|
push_rt_tasks(rq);
|
|
}
|
|
|
|
static unsigned long
|
|
load_balance_rt(struct rq *this_rq, int this_cpu, struct rq *busiest,
|
|
unsigned long max_load_move,
|
|
struct sched_domain *sd, enum cpu_idle_type idle,
|
|
int *all_pinned, int *this_best_prio)
|
|
{
|
|
/* don't touch RT tasks */
|
|
return 0;
|
|
}
|
|
|
|
static int
|
|
move_one_task_rt(struct rq *this_rq, int this_cpu, struct rq *busiest,
|
|
struct sched_domain *sd, enum cpu_idle_type idle)
|
|
{
|
|
/* don't touch RT tasks */
|
|
return 0;
|
|
}
|
|
static void set_cpus_allowed_rt(struct task_struct *p, cpumask_t *new_mask)
|
|
{
|
|
int weight = cpus_weight(*new_mask);
|
|
|
|
BUG_ON(!rt_task(p));
|
|
|
|
/*
|
|
* Update the migration status of the RQ if we have an RT task
|
|
* which is running AND changing its weight value.
|
|
*/
|
|
if (p->se.on_rq && (weight != p->nr_cpus_allowed)) {
|
|
struct rq *rq = task_rq(p);
|
|
|
|
if ((p->nr_cpus_allowed <= 1) && (weight > 1))
|
|
rq->rt.rt_nr_migratory++;
|
|
else if((p->nr_cpus_allowed > 1) && (weight <= 1)) {
|
|
BUG_ON(!rq->rt.rt_nr_migratory);
|
|
rq->rt.rt_nr_migratory--;
|
|
}
|
|
|
|
update_rt_migration(rq);
|
|
}
|
|
|
|
p->cpus_allowed = *new_mask;
|
|
p->nr_cpus_allowed = weight;
|
|
}
|
|
#else /* CONFIG_SMP */
|
|
# define schedule_tail_balance_rt(rq) do { } while (0)
|
|
# define schedule_balance_rt(rq, prev) do { } while (0)
|
|
# define wakeup_balance_rt(rq, p) do { } while (0)
|
|
#endif /* CONFIG_SMP */
|
|
|
|
static void task_tick_rt(struct rq *rq, struct task_struct *p)
|
|
{
|
|
update_curr_rt(rq);
|
|
|
|
/*
|
|
* RR tasks need a special form of timeslice management.
|
|
* FIFO tasks have no timeslices.
|
|
*/
|
|
if (p->policy != SCHED_RR)
|
|
return;
|
|
|
|
if (--p->time_slice)
|
|
return;
|
|
|
|
p->time_slice = DEF_TIMESLICE;
|
|
|
|
/*
|
|
* Requeue to the end of queue if we are not the only element
|
|
* on the queue:
|
|
*/
|
|
if (p->run_list.prev != p->run_list.next) {
|
|
requeue_task_rt(rq, p);
|
|
set_tsk_need_resched(p);
|
|
}
|
|
}
|
|
|
|
static void set_curr_task_rt(struct rq *rq)
|
|
{
|
|
struct task_struct *p = rq->curr;
|
|
|
|
p->se.exec_start = rq->clock;
|
|
}
|
|
|
|
const struct sched_class rt_sched_class = {
|
|
.next = &fair_sched_class,
|
|
.enqueue_task = enqueue_task_rt,
|
|
.dequeue_task = dequeue_task_rt,
|
|
.yield_task = yield_task_rt,
|
|
#ifdef CONFIG_SMP
|
|
.select_task_rq = select_task_rq_rt,
|
|
#endif /* CONFIG_SMP */
|
|
|
|
.check_preempt_curr = check_preempt_curr_rt,
|
|
|
|
.pick_next_task = pick_next_task_rt,
|
|
.put_prev_task = put_prev_task_rt,
|
|
|
|
#ifdef CONFIG_SMP
|
|
.load_balance = load_balance_rt,
|
|
.move_one_task = move_one_task_rt,
|
|
.set_cpus_allowed = set_cpus_allowed_rt,
|
|
#endif
|
|
|
|
.set_curr_task = set_curr_task_rt,
|
|
.task_tick = task_tick_rt,
|
|
};
|