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Linux CFS调度器之pick_next_task_fair选择下一个被调度的进程...
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gatieme
@@ -3392,6 +3392,7 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
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}
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/* 新sched_entity中的exec_start字段为当前clock_task */
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update_stats_curr_start(cfs_rq, se);
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/* 将se设置为curr进程 */
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cfs_rq->curr = se;
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#ifdef CONFIG_SCHEDSTATS
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/*
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@@ -498,9 +498,69 @@ wakeup_gran(struct sched_entity *curr, struct sched_entity *se)
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* 当进程yield操作的时候，进程主动放弃了调度机会，那么如果next，last指针指向了这个sched_entity，那么需要清除相应指针。
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**pick_next_entity流程总结**
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pick_next_entity函数选择出下一个最渴望被公平调度器调度的进程, 函数的执行流程其实很简单
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## set_next_entity
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-------
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set_next_entity()函数会调用__dequeue_entity(cfs_rq, se)把选中的下一个进程移出红黑树
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```c
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static void
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set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
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{
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/* 'current' is not kept within the tree. */
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if (se->on_rq) /* 如果se尚在rq队列上 */
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{
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/*
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* Any task has to be enqueued before it get to execute on
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* a CPU. So account for the time it spent waiting on the
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* runqueue.
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*/
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if (schedstat_enabled())
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update_stats_wait_end(cfs_rq, se);
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/* 将se从rq队列中删除 */
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__dequeue_entity(cfs_rq, se);
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update_load_avg(se, 1);
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}
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/* 新sched_entity中的exec_start字段为当前clock_task */
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update_stats_curr_start(cfs_rq, se);
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/* 将se设置为curr进程 */
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cfs_rq->curr = se;
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#ifdef CONFIG_SCHEDSTATS
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/*
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* Track our maximum slice length, if the CPU's load is at
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* least twice that of our own weight (i.e. dont track it
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* when there are only lesser-weight tasks around):
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*/
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if (schedstat_enabled() && rq_of(cfs_rq)->load.weight >= 2*se->load.weight) {
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se->statistics.slice_max = max(se->statistics.slice_max,
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se->sum_exec_runtime - se->prev_sum_exec_runtime);
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}
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#endif
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/* //更新task上一次投入运行的从时间 */
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se->prev_sum_exec_runtime = se->sum_exec_runtime;
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}
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```
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##总结
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-------
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pick_next_task_fair的基本流程如下
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其基本流程如下
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| 流程 | 描述 |
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|:-------:|:-------:|
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| !cfs_rq->nr_running -=> goto idle; | 如果nr_running计数器为0, 当前队列上没有可运行进程, 则需要调度idle进程 |
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| put_prev_task(rq, prev); | 将当前进程放入运行队列的合适位置, 每次当进程被调度后都会使用set_next_entity从红黑树中移除, 因此被抢占时需要重新加如红黑树中等待被调度 |
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| se = pick_next_entity(cfs_rq, NULL); | 选出下一个可执行调度实体 |
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| set_next_entity(cfs_rq, se); | set_next_entity会调用__dequeue_entity把选中的进程从红黑树移除，并更新红黑树 |
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其中最关键的pick_next_entity函数选择出下一个最渴望被公平调度器调度的进程, 函数的执行流程其实很简单
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1. 先从最左节点left和当前节点curr中选择出最渴望被调度(即虚拟运行vruntime最小)的那个调度实体色
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@@ -529,8 +589,150 @@ pick_next_entity函数选择出下一个最渴望被公平调度器调度的进
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以及cfs_rq的调度实体curr, last和next, curr是当前正在运行的进程, 它虽然已经运行, 但是可能仍然很饥渴, 那么我们应该继续补偿它, 而last表示最后一个执行wakeup的sched_entity, next表示最后一个被wakeup的sched_entity, 刚被唤醒的进程可能更希望得到CPU, 因此在pick新sched_entity的时候，会优先选择这些last或者next指针的sched_entity,有利于提高缓存的命中率
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## set_next_entity
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#idle进程的调度
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-------
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```c
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/* 如果nr_running计数器为0,
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* 当前队列上没有可运行进程,
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* 则需要调度idle进程 */
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if (!cfs_rq->nr_running)
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goto idle;
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```
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http://blog.csdn.net/sunnybeike/article/details/6918586
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如果系统中当前运行队列上没有可调度的进程, 那么会调到idle标签去调度idle进程.
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idle标签如下所示
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```c
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idle:
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/*
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* This is OK, because current is on_cpu, which avoids it being picked
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* for load-balance and preemption/IRQs are still disabled avoiding
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* further scheduler activity on it and we're being very careful to
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* re-start the picking loop.
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*/
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lockdep_unpin_lock(&rq->lock);
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new_tasks = idle_balance(rq);
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lockdep_pin_lock(&rq->lock);
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/*
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* Because idle_balance() releases (and re-acquires) rq->lock, it is
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* possible for any higher priority task to appear. In that case we
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* must re-start the pick_next_entity() loop.
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*/
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if (new_tasks < 0)
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return RETRY_TASK;
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if (new_tasks > 0)
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goto again;
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return NULL;
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```
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其关键就是调用idle_balance进行任务的迁移
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每个cpu都有自己的运行队列, 如果当前cpu上运行的任务都已经dequeue出运行队列，而且idle_balance也没有移动到当前运行队列的任务，那么schedule函数中，按照stop > idle > rt > cfs > idle这三种调度方式顺序，寻找各自的运行任务，那么如果rt和cfs都未找到运行任务，那么最后会调用idle schedule的idle进程，作为schedule函数调度的下一个任务
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如果某个cpu空闲, 而其他CPU不空闲, 即当前CPU运行队列为NULL, 而其他CPU运行队列有进程等待调度的时候, 则内核会对CPU尝试负载平衡, CPU负载均衡有两种方式: pull和push, 即空闲CPU从其他忙的CPU队列中pull拉一个进程复制到当前空闲CPU上, 或者忙的CPU队列将一个进程push推送到空闲的CPU队列中.
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idle_balance其实就是pull的工作.
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#组调度策略的支持
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-------
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组调度的情形下, 调度实体之间存在明显的层次关系, 因此在跟新子调度实体的时候, 需要更新父调度实体的信息, 同时我们为了保证同一组内的进程不能长时间占用处理机, 必须补偿其他组内的进程, 保证公平性
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```c
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#ifdef CONFIG_FAIR_GROUP_SCHED
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/* 如果nr_running计数器为0, 即当前队列上没有可运行进程,
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* 则需要调度idle进程 */
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if (!cfs_rq->nr_running)
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goto idle;
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/* 如果当前运行进程prev不是被fair调度的普通非实时进程 */
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if (prev->sched_class != &fair_sched_class)
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goto simple;
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/*
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* Because of the set_next_buddy() in dequeue_task_fair() it is rather
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* likely that a next task is from the same cgroup as the current.
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*
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* Therefore attempt to avoid putting and setting the entire cgroup
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* hierarchy, only change the part that actually changes.
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*/
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do {
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struct sched_entity *curr = cfs_rq->curr;
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/*
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* Since we got here without doing put_prev_entity() we also
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* have to consider cfs_rq->curr. If it is still a runnable
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* entity, update_curr() will update its vruntime, otherwise
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* forget we've ever seen it.
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*/
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if (curr)
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{
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/* 如果当前进程curr在队列上,
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* 则需要更新起统计量和虚拟运行时间
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* 否则设置curr为空 */
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if (curr->on_rq)
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update_curr(cfs_rq);
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else
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curr = NULL;
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/*
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* This call to check_cfs_rq_runtime() will do the
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* throttle and dequeue its entity in the parent(s).
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* Therefore the 'simple' nr_running test will indeed
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* be correct.
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*/
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if (unlikely(check_cfs_rq_runtime(cfs_rq)))
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goto simple;
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}
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/* 选择一个最优的调度实体 */
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se = pick_next_entity(cfs_rq, curr);
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cfs_rq = group_cfs_rq(se);
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} while (cfs_rq); /* 如果被调度的进程仍属于当前组，那么选取下一个可能被调度的任务，以保证组间调度的公平性 */
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/* 获取调度实体se的进程实体信息 */
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p = task_of(se);
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/*
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* Since we haven't yet done put_prev_entity and if the selected task
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* is a different task than we started out with, try and touch the
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* least amount of cfs_rqs.
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*/
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if (prev != p)
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{
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struct sched_entity *pse = &prev->se;
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while (!(cfs_rq = is_same_group(se, pse)))
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{
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int se_depth = se->depth;
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int pse_depth = pse->depth;
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if (se_depth <= pse_depth)
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{
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put_prev_entity(cfs_rq_of(pse), pse);
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pse = parent_entity(pse);
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}
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if (se_depth >= pse_depth)
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{
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set_next_entity(cfs_rq_of(se), se);
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se = parent_entity(se);
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}
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}
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put_prev_entity(cfs_rq, pse);
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set_next_entity(cfs_rq, se);
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}
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if (hrtick_enabled(rq))
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hrtick_start_fair(rq, p);
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return p;
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```
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0 http://blog.csdn.net/sunnybeike/article/details/6918586
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