从cgroup_init_early函数学习cgroup——初始化代码

前面分析了cgroup机制的框架及数据结构之间的关系，现在看代码就比较见简单了。
代码部分
在init/main.c的start_kernel函数中会调用cgroup_init_early函数进行cgroup的初始化。其实对cgroup的初始化分两次，分别是cgroup_init_early和cgroup_init。这么做的原因是，系统初始阶段需要使用一些 subsystem，先对这一部分进行初始化。
来看一下cgroup_init_early

/** * cgroup_init_early - cgroup initialization at system boot * * Initialize cgroups at system boot, and initialize any * subsystems that request early init. */int __init cgroup_init_early(void){    struct cgroup_subsys *ss;    int i;    atomic_set(&init_css_set.refcount, 1);    INIT_LIST_HEAD(&init_css_set.cgrp_links);    INIT_LIST_HEAD(&init_css_set.tasks);    INIT_HLIST_NODE(&init_css_set.hlist);    css_set_count = 1;    init_cgroup_root(&cgroup_dummy_root);    cgroup_root_count = 1;    RCU_INIT_POINTER(init_task.cgroups, &init_css_set);    init_cgrp_cset_link.cset = &init_css_set;    init_cgrp_cset_link.cgrp = cgroup_dummy_top;    list_add(&init_cgrp_cset_link.cset_link, &cgroup_dummy_top->cset_links);    list_add(&init_cgrp_cset_link.cgrp_link, &init_css_set.cgrp_links);    /* at bootup time, we don't worry about modular subsystems */    for_each_builtin_subsys(ss, i) {        BUG_ON(!ss->name);        BUG_ON(strlen(ss->name) > MAX_CGROUP_TYPE_NAMELEN);        BUG_ON(!ss->css_alloc);        BUG_ON(!ss->css_free);        if (ss->subsys_id != i) {            printk(KERN_ERR "cgroup: Subsys %s id == %d\n",                   ss->name, ss->subsys_id);            BUG();        }        if (ss->early_init)            cgroup_init_subsys(ss);    }    return 0;}

注释的说的很清楚了，initialize any subsystems that request early init.
init_css_set是init进程使用的css_set结构体，先进行初始化：
设置init_css_set的引用计数为1.
初始化init_css_set.cgrp_links链表，该链表与cgrp_cset_link的cgrp_link相关联.
初始化init_css_set.tasks链表，该链表与所有应用init_css_set的task相关联.
初始化init_css_set.hlist链表，哈希表相关，没有深入研究.
css_set_count用来记录系统css_set结构体变量的个数.
初始化cgroup_dummy_root，定义为

static struct cgroupfs_root cgroup_dummy_root;/* * A cgroupfs_root represents the root of a cgroup hierarchy, and may be * associated with a superblock to form an active hierarchy.  This is * internal to cgroup core.  Don't access directly from controllers. */struct cgroupfs_root {    struct super_block *sb;    /* The bitmask of subsystems attached to this hierarchy */    unsigned long subsys_mask;    /* Unique id for this hierarchy. */    int hierarchy_id;    /* The root cgroup for this hierarchy */    struct cgroup top_cgroup;    /* Tracks how many cgroups are currently defined in hierarchy.*/    int number_of_cgroups;    /* A list running through the active hierarchies */    struct list_head root_list;    /* Hierarchy-specific flags */    unsigned long flags;    /* IDs for cgroups in this hierarchy */    struct idr cgroup_idr;    /* The path to use for release notifications. */    char release_agent_path[PATH_MAX];    /* The name for this hierarchy - may be empty */    char name[MAX_CGROUP_ROOT_NAMELEN];};static void init_cgroup_root(struct cgroupfs_root *root){    struct cgroup *cgrp = &root->top_cgroup;    INIT_LIST_HEAD(&root->root_list);    root->number_of_cgroups = 1;    cgrp->root = root;    RCU_INIT_POINTER(cgrp->name, &root_cgroup_name);    init_cgroup_housekeeping(cgrp);    idr_init(&root->cgroup_idr);}

初始化cgroup_dummy_root的root_list链表
将层次图中的cgroup个数设置为1
将自身设置为层次图的顶端cgroup，即top_cgroup->root指向cgroup_dummy_root
init_cgroup_housekeeping用于构建层次图的list变量和互斥变量
初始化cgroup_dummy_root->cgroup_idr链表
回到cgroup_init_early：
设置cgroup_root_count = 1;
接下来几行代码：

    init_cgrp_cset_link.cset = &init_css_set;    init_cgrp_cset_link.cgrp = cgroup_dummy_top;    list_add(&init_cgrp_cset_link.cset_link, &cgroup_dummy_top->cset_links);    list_add(&init_cgrp_cset_link.cgrp_link, &init_css_set.cgrp_links);

内核定义

static struct cgrp_cset_link init_cgrp_cset_link;

/* dummy_top is a shorthand for the dummy hierarchy's top cgroup */static struct cgroup * const cgroup_dummy_top = &cgroup_dummy_root.top_cgroup;

所以这里是将init进程的css_set(即init_css_set)与cgroup_dummy_root的top_cgroup(即cgroup_dummy_top)通过init_cgrp_cset_link相关联。
继续看cgroup_init_early的剩余代码

    /* at bootup time, we don't worry about modular subsystems */    for_each_builtin_subsys(ss, i) {        BUG_ON(!ss->name);        BUG_ON(strlen(ss->name) > MAX_CGROUP_TYPE_NAMELEN);        BUG_ON(!ss->css_alloc);        BUG_ON(!ss->css_free);        if (ss->subsys_id != i) {            printk(KERN_ERR "cgroup: Subsys %s id == %d\n",                   ss->name, ss->subsys_id);            BUG();        }        if (ss->early_init)            cgroup_init_subsys(ss);    }

这里应该是遍历所有builtin的subsystems并进行初始化，从注释不难看出，linux中subsystems可以分为builtin和modular两类。
其中会做一些检查，检查子系统的默认信息是否有误，出现问题时停止初始化。检查之后调用cgroup_init_subsys进行初始化。
这些buildtin的subsystem从何而来？

/** * for_each_builtin_subsys - iterate all built-in cgroup subsystems * @ss: the iteration cursor * @i: the index of @ss, CGROUP_BUILTIN_SUBSYS_COUNT after reaching the end * * Bulit-in subsystems are always present and iteration itself doesn't * require any synchronization. */#define for_each_builtin_subsys(ss, i)                  \    for ((i) = 0; (i) < CGROUP_BUILTIN_SUBSYS_COUNT &&      \         (((ss) = cgroup_subsys[i]) || true); (i)++)

for_each_builtin_subsys是一个宏，展开并去掉BUG_ON信息后得到：

    for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT &&               (((ss) = cgroup_subsys[i]) || true); i++){        if (ss->early_init)            cgroup_init_subsys(ss);    }

cgroup_subsys数组的定义在kernel/group.c

static struct cgroup_subsys *cgroup_subsys[CGROUP_SUBSYS_COUNT] = {#include <linux/cgroup_subsys.h>};

成员变量是#include

/* * List of cgroup subsystems. * * DO NOT ADD ANY SUBSYSTEM WITHOUT EXPLICIT ACKS FROM CGROUP MAINTAINERS. */#if IS_SUBSYS_ENABLED(CONFIG_CPUSETS)SUBSYS(cpuset)#endif#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_DEBUG)SUBSYS(debug)#endif#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_SCHED)SUBSYS(cpu_cgroup)#endif#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_CPUACCT)SUBSYS(cpuacct)#endif#if IS_SUBSYS_ENABLED(CONFIG_MEMCG)SUBSYS(mem_cgroup)#endif#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_DEVICE)SUBSYS(devices)#endif#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_FREEZER)SUBSYS(freezer)#endif#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_NET_CLASSID)SUBSYS(net_cls)#endif#if IS_SUBSYS_ENABLED(CONFIG_BLK_CGROUP)SUBSYS(blkio)#endif#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_PERF)SUBSYS(perf)#endif#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_NET_PRIO)SUBSYS(net_prio)#endif#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_HUGETLB)SUBSYS(hugetlb)#endif/* * DO NOT ADD ANY SUBSYSTEM WITHOUT EXPLICIT ACKS FROM CGROUP MAINTAINERS. */

一共有CGROUP_BUILTIN_SUBSYS_COUNT个subsystem,与inclue/linux/cgroup_subsys.h中subsystem中cgroup subsystems的数目相等。这个数值的定义用了一种很巧妙的方法，有兴趣的同学可以看一下。

接下来看cgroup_init_subsys

static void __init cgroup_init_subsys(struct cgroup_subsys *ss){    struct cgroup_subsys_state *css;    printk(KERN_INFO "Initializing cgroup subsys %s\n", ss->name);    mutex_lock(&cgroup_mutex);    /* init base cftset */    cgroup_init_cftsets(ss);    /* Create the top cgroup state for this subsystem */    ss->root = &cgroup_dummy_root;    css = ss->css_alloc(cgroup_css(cgroup_dummy_top, ss));    /* We don't handle early failures gracefully */    BUG_ON(IS_ERR(css));    init_css(css, ss, cgroup_dummy_top);    /* Update the init_css_set to contain a subsys     * pointer to this state - since the subsystem is     * newly registered, all tasks and hence the     * init_css_set is in the subsystem's top cgroup. */    init_css_set.subsys[ss->subsys_id] = css;    need_forkexit_callback |= ss->fork || ss->exit;    /* At system boot, before all subsystems have been     * registered, no tasks have been forked, so we don't     * need to invoke fork callbacks here. */    BUG_ON(!list_empty(&init_task.tasks));    BUG_ON(online_css(css));    mutex_unlock(&cgroup_mutex);    /* this function shouldn't be used with modular subsystems, since they     * need to register a subsys_id, among other things */    BUG_ON(ss->module);}

这里主要是做一些初始化，由于时间问题，先不去深究了。
这里我只是分析了一下cgroup的框架及初始化，其实还有很多值得研究的地方，比如subsys_id，比如cpu等cgroup是如何运作的，这写内容以后一定会补上。