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CFS调度器之虚拟时钟...
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Linux进程调度器的设计
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Linux进程调度CFS调度器
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=======
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Linux进程调度器的设计
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Linux CFS调度器之虚拟时钟与调度延迟
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=======
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Linux CFS调度器之队列操作
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=======
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| 日期 | 内核版本 | 架构| 作者 | GitHub| CSDN |
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| ------- |:-------:|:-------:|:-------:|:-------:|:-------:|
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| 2016-06-29 | [Linux-4.6](http://lxr.free-electrons.com/source/?v=4.6) | X86 & arm | [gatieme](http://blog.csdn.net/gatieme) | [LinuxDeviceDrivers](https://github.com/gatieme/LDD-LinuxDeviceDrivers) | [Linux进程管理与调度](http://blog.csdn.net/gatieme/article/details/51456569) |
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CFS负责处理普通非实时进程, 这类进程是我们linux中最普遍的进程
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#前景回顾
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-------
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## CFS调度算法
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-------
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**CFS调度算法的思想**
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理想状态下每个进程都能获得相同的时间片，并且同时运行在CPU上，但实际上一个CPU同一时刻运行的进程只能有一个。也就是说，当一个进程占用CPU时，其他进程就必须等待。CFS为了实现公平，必须惩罚当前正在运行的进程，以使那些正在等待的进程下次被调度.
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## 负荷权重和虚拟时钟
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**虚拟时钟是红黑树排序的依据**
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具体实现时，CFS通过每个进程的**虚拟运行时间(vruntime)**来衡量哪个进程最值得被调度. CFS中的就绪队列是一棵以vruntime为键值的红黑树，虚拟时间越小的进程越靠近整个红黑树的最左端。因此，调度器每次选择位于红黑树最左端的那个进程，该进程的vruntime最小.
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**优先级计算负荷权重, 负荷权重和当前时间计算出虚拟运行时间**
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虚拟运行时间是通过进程的实际运行时间和进程的权重(weight)计算出来的。在CFS调度器中，将进程优先级这个概念弱化，而是强调进程的权重。一个进程的权重越大，则说明这个进程更需要运行，因此它的虚拟运行时间就越小，这样被调度的机会就越大。而，CFS调度器中的权重在内核是对用户态进程的优先级nice值, 通过prio_to_weight数组进行nice值和权重的转换而计算出来的
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**虚拟时钟相关公式**
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linux内核采用了计算公式：
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| 属性 | 公式 | 描述 |
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|:-------:|:-------:|
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| ideal_time | sum_runtime *se.weight/cfs_rq.weight | 每个进程应该运行的时间 |
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| sum_exec_runtime | | 运行队列中所有任务运行完一遍的时间 |
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| se.weight | | 当前进程的权重 |
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| cfs.weight | | 整个cfs_rq的总权重 |
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这里se.weight和cfs.weight根据上面讲解我们可以算出, sum_runtime是怎们计算的呢，linux内核中这是个经验值，其经验公式是
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| 条件 | 公式 |
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|:-------:|:-------:|
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| 进程数 > sched_nr_latency | sum_runtime=sysctl_sched_min_granularity *nr_running |
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| 进程数 <=sched_nr_latency | sum_runtime=sysctl_sched_latency = 20ms |
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>注：sysctl_sched_min_granularity =4ms
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>
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>sched_nr_latency是内核在一个延迟周期中处理的最大活动进程数目
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linux内核代码中是通过一个叫vruntime的变量来实现上面的原理的，即：
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每一个进程拥有一个vruntime,每次需要调度的时候就选运行队列中拥有最小vruntime的那个进程来运行，vruntime在时钟中断里面被维护，每次时钟中断都要更新当前进程的vruntime,即vruntime以如下公式逐渐增长：
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| 条件 | 公式 |
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|:-------:|:-------:|
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| curr.nice!=NICE_0_LOAD | vruntime += delta* NICE_0_LOAD/se.weight; |
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| curr.nice=NICE_0_LOAD | vruntime += delta; |
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##今日内容--CFS进程入队和出队
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-------
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完全公平调度器CFS中有两个函数可用来增删队列的成员:enqueue_task_fair和dequeue_task_fair分别用来向CFS就绪队列中添加或者删除进程
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# enqueue_task_fair入队操作
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-------
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## enque_task_fair函数
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-------
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向就绪队列中放置新进程的工作由函数enqueue_task_fair函数完成, 该函数定义在[kernel/sched/fair.c, line 5442](http://lxr.free-electrons.com/source/kernel/sched/fair.c?v=4.6#L5442), 其函数原型如下
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该函数将task_struct *p所指向的进程插入到rq所在的就绪队列中, 除了指向所述的就绪队列rq和task_struct的指针外, 该函数还有另外一个参数wakeup. 这使得可以指定入队的进程是否最近才被唤醒并转换为运行状态(此时需指定wakeup = 1), 还是此前就是可运行的(那么wakeup = 0).
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```c
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static void
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enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
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```
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enqueue_task_fair的执行流程如下
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* 如果通过struct sched_entity的on_rq成员判断进程已经在就绪队列上, 则无事可做.
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* 否则, 具体的工作委托给enqueue_entity完成, 其中内核会借机用update_curr更新统计量.
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*
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## for_each_sched_entity
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-------
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首先内核查找到待天机进程p所在的调度实体信息, 然后通过for_each_sched_entity循环所有调度实体,
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```c
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// enqueue_task_fair函数
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struct cfs_rq *cfs_rq;
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struct sched_entity *se = &p->se;
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for_each_sched_entity(se)
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{
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/* ...... */
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}
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````
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但是有个疑问是, 进程p所在的调度时提就一个为嘛要循环才能遍历啊, 这是因为为了支持组调度.组调度下调度实体是有层次结构的, 我们将进程加入的时候, 同时要更新其父调度实体的调度信息, 而非组调度情况下, 就不需要调度实体的层次结构
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linux对组调度的支持可以通过CONFIG_FAIR_GROUP_SCHED来启用, 在启用和不启用的条件下, 内核对很多函数的实现也会因条件而异, 这点对for_each_sched_entity函数尤为明显, 参见[启用CONFIG_FAIR_GROUP_SCHED](http://lxr.free-electrons.com/source/kernel/sched/fair.c?v=4.6#L246)和[不启用CONFIG_FAIR_GROUP_SCHED](http://lxr.free-electrons.com/source/kernel/sched/fair.c?v=4.6#L367)
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```c
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#ifdef CONFIG_FAIR_GROUP_SCHED
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/* An entity is a task if it doesn't "own" a runqueue */
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#define entity_is_task(se) (!se->my_q)
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// http://lxr.free-electrons.com/source/kernel/sched/fair.c?v=4.6#L266
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/* Walk up scheduling entities hierarchy */
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#define for_each_sched_entity(se) \
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for (; se; se = se->parent)
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#else /* !CONFIG_FAIR_GROUP_SCHED */
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#define entity_is_task(se) 1
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// http://lxr.free-electrons.com/source/kernel/sched/fair.c?v=4.6#L381
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#define for_each_sched_entity(se) \
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for (; se; se = NULL)
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```
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* 如果通过struct sched_entity的on_rq成员判断进程已经在就绪队列上, 则无事可做.
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* 否则, 具体的工作委托给enqueue_entity完成, 其中内核会借机用update_curr更新统计量.
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```c
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// enqueue_task_fair函数
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/* 如果当前进程已经在就绪队列上 */
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if (se->on_rq)
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break;
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/* 获取到当前进程所在的cfs_rq就绪队列 */
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cfs_rq = cfs_rq_of(se);
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/* 内核委托enqueue_entity完成真正的插入工作 */
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enqueue_entity(cfs_rq, se, flags);
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````
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##enqueue_entity插入进程
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-------
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首先如果进程最近正在运行, 其虚拟时间时间仍然有效, 那么(除非它当前在执行中)他可以
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```c
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````
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/*
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* The dequeue_task method is called before nr_running is
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* decreased. We remove the task from the rbtree and
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* update the fair scheduling stats:
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*/
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static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
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{
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struct cfs_rq *cfs_rq;
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struct sched_entity *se = &p->se;
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int task_sleep = flags & DEQUEUE_SLEEP;
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for_each_sched_entity(se) {
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cfs_rq = cfs_rq_of(se);
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dequeue_entity(cfs_rq, se, flags);
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/*
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* end evaluation on encountering a throttled cfs_rq
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*
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* note: in the case of encountering a throttled cfs_rq we will
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* post the final h_nr_running decrement below.
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*/
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if (cfs_rq_throttled(cfs_rq))
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break;
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cfs_rq->h_nr_running--;
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/* Don't dequeue parent if it has other entities besides us */
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if (cfs_rq->load.weight) {
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/*
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* Bias pick_next to pick a task from this cfs_rq, as
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* p is sleeping when it is within its sched_slice.
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*/
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if (task_sleep && parent_entity(se))
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set_next_buddy(parent_entity(se));
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/* avoid re-evaluating load for this entity */
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se = parent_entity(se);
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break;
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}
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flags |= DEQUEUE_SLEEP;
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}
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for_each_sched_entity(se) {
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cfs_rq = cfs_rq_of(se);
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cfs_rq->h_nr_running--;
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if (cfs_rq_throttled(cfs_rq))
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break;
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update_load_avg(se, 1);
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update_cfs_shares(cfs_rq);
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}
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if (!se)
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sub_nr_running(rq, 1);
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hrtick_update(rq);
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}
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/*
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* The enqueue_task method is called before nr_running is
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* increased. Here we update the fair scheduling stats and
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* then put the task into the rbtree:
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*/
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static void
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enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
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{
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struct cfs_rq *cfs_rq;
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struct sched_entity *se = &p->se;
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for_each_sched_entity(se)
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{
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/* 如果当前进程已经在就绪队列上 */
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if (se->on_rq)
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break;
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/* 获取到当前进程所在的cfs_rq就绪队列 */
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cfs_rq = cfs_rq_of(se);
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/* 内核委托enqueue_entity完成真正的插入工作 */
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enqueue_entity(cfs_rq, se, flags);
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/*
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* end evaluation on encountering a throttled cfs_rq
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*
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* note: in the case of encountering a throttled cfs_rq we will
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* post the final h_nr_running increment below.
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*/
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if (cfs_rq_throttled(cfs_rq))
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break;
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cfs_rq->h_nr_running++;
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flags = ENQUEUE_WAKEUP;
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}
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for_each_sched_entity(se) {
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cfs_rq = cfs_rq_of(se);
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cfs_rq->h_nr_running++;
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if (cfs_rq_throttled(cfs_rq))
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break;
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update_load_avg(se, 1);
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update_cfs_shares(cfs_rq);
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}
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if (!se)
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add_nr_running(rq, 1);
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hrtick_update(rq);
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}
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