我们知道runc没有做到完全隔离/proc、/sys路径下的文件，所以容器内通过top、free等命令看到的数据都是物理机上的。对于习惯了虚机，物理机的同学来说不太友好，而且这些命令似乎也失去了本质意义。lxcfs作用就是将容器内/proc、/sys文件与物理机隔离，让top等命令显示容器内真实数据。

lxcfs是以用户空间文件系统（Filesystem in Userspace）为基础，以cgroup技术实现的用户空间的虚拟文件系统。先对fuse和cgroup有大致了解，看本文效果更好些。本文不介绍lxcfs的安装及使用，网上不乏这样的好文章。我们主要介绍下lxcfs对cpuonline、loadavg的现实，这两部分弄懂，其它也大体相同。

lxcfs程序启动时会指定一个路径（如下图是/var/lib/lxcfs）作为挂载点，以后读取这个路径的下文件（cgroup、proc、sys）vfs都会调用内核fuse，fuse回调lxcfs实现的文件操作函数。容器内读取lxcfs文件系统中的数据时，通过gblic系统调用vfs接口然后转到fuse内核模块，内核模块fuse回调lxcfs程序中实现的回调函数，获取容器的cgroup，然后去宿主机对应cgroup下读取并计算后得到容器的实际mem、cpu等信息。lxcfs将物理机的cgroups挂载到运行时环境/run/lxcfs/controllers，但直接在物理机上看不见，因为程序中用unshare做了mounts namespace隔离。lxcfs程序中所有的cgroups信息都从/run/lxcfs/controllers下获得。

因为工作中正好需要这两部分，所以主要介绍下cpuonline和loadavg的实现。nginx、java等程序根据cpu核心数启动相应个数的进程，cpuonline是相关系统调用的数据来源。没隔离导致的容器内获取到cpu核数是物理机的，本应该创建2个进程，实际却创建40个（容器2c，物理机40c），由于更多的上下文切换导致明显的性能下降。loadavg目前没看到有关的分析，这里也简单介绍下。

1、cpuonline

物理机

容器2c4g

2、loadavg

物理机

容器2c4g

可以看到cpuonline和load average都已经隔离

注：cgropu的各个controller文件在main函数执行前打开，保存在fd_hierarchies中，后面使用直接掉openat，不是每次都要open、close文件。通过c语言的attribute((constructor)) 属性，声明collect_and_mount_subsystems这个函数。

看下collect_and_mount_subsystems

static void __attribute__((constructor)) collect_and_mount_subsystems(void){    FILE *f;    char *cret, *line = NULL;    char cwd[MAXPATHLEN];    size_t len = 0;    int i, init_ns = -1;    bool found_unified = false;

    if ((f = fopen("/proc/self/cgroup", "r")) == NULL) {        lxcfs_error("Error opening /proc/self/cgroup: %s\n", strerror(errno));        return;    }    // 读取宿主机上namespaces controller保存到hierarchies    while (getline(&line, &len, f) != -1) {......        if (!store_hierarchy(line, p))            goto out;    }

    /* Preserve initial namespace. */    init_ns = preserve_mnt_ns(getpid());    if (init_ns < 0) {        lxcfs_error("%s\n", "Failed to preserve initial mount namespace.");        goto out;    }

    fd_hierarchies = malloc(sizeof(int) * num_hierarchies);    if (!fd_hierarchies) {        lxcfs_error("%s\n", strerror(errno));        goto out;    }

    for (i = 0; i < num_hierarchies; i++)        fd_hierarchies[i] = -1;

    cret = getcwd(cwd, MAXPATHLEN);    if (!cret)        lxcfs_debug("Could not retrieve current working directory: %s.\n", strerror(errno));

    /* This function calls unshare(CLONE_NEWNS) our initial mount namespace     * to privately mount lxcfs cgroups. */    // 关键是这里，将cgroup下各个控制模块，挂载到lxcfs进程的自由的mount ns下（/run/lxcfs/container）    if (!cgfs_setup_controllers()) {        lxcfs_error("%s\n", "Failed to setup private cgroup mounts for lxcfs.");        goto out;    }......}static bool cgfs_setup_controllers(void){    // 主要调用unshare 创建私有的mount ns    if (!cgfs_prepare_mounts())        return false;

    if (!cgfs_mount_hierarchies()) {        lxcfs_error("%s\n", "Failed to set up private lxcfs cgroup mounts.");        return false;    }

    if (!permute_root())        return false;

    return true;}static bool cgfs_mount_hierarchies(void){    char *target;    size_t clen, len;    int i, ret;

    for (i = 0; i < num_hierarchies; i++) {        char *controller = hierarchies[i];

        clen = strlen(controller);        len = strlen(BASEDIR) + clen + 2;        target = malloc(len);        if (!target)            return false;

        ret = snprintf(target, len, "%s/%s", BASEDIR, controller);        if (ret < 0 || ret >= len) {            free(target);            return false;        }        if (mkdir(target, 0755) < 0 && errno != EEXIST) {            free(target);            return false;        }        if (!strcmp(controller, "unified"))            ret = mount("none", target, "cgroup2", 0, NULL);        else            ret = mount(controller, target, "cgroup", 0, controller);        if (ret < 0) {            lxcfs_error("Failed mounting cgroup %s: %s\n", controller, strerror(errno));            free(target);            return false;        }                // 将所有cgroup controller 文件打开，保存文件描述符        fd_hierarchies[i] = open(target, O_DIRECTORY);        if (fd_hierarchies[i] < 0) {            free(target);            return false;        }        free(target);    }    return true;}

lxcfs.c main中主要是解析命令行参数，并调用fuse提供的fuse_main函数将lxcfs相关的文件操作注册，并传入挂载点。

......if (!fuse_main(nargs, newargv, &lxcfs_ops, opts))......const struct fuse_operations lxcfs_ops = {    .getattr = lxcfs_getattr,    .readlink = NULL,    .getdir = NULL,    .mknod = NULL,    .mkdir = lxcfs_mkdir,    .unlink = NULL,    .rmdir = lxcfs_rmdir,    .symlink = NULL,    .rename = NULL,    .link = NULL,    .chmod = lxcfs_chmod,    .chown = lxcfs_chown,    .truncate = lxcfs_truncate,    .utime = NULL,

    .open = lxcfs_open,    .read = lxcfs_read,    .release = lxcfs_release,    .write = lxcfs_write,

    .statfs = NULL,    .flush = lxcfs_flush,    .fsync = lxcfs_fsync,

    .setxattr = NULL,    .getxattr = NULL,    .listxattr = NULL,    .removexattr = NULL,

    .opendir = lxcfs_opendir,    .readdir = lxcfs_readdir,    .releasedir = lxcfs_releasedir,

    .fsyncdir = NULL,    .init = NULL,    .destroy = NULL,    .access = lxcfs_access,    .create = NULL,    .ftruncate = NULL,    .fgetattr = NULL,};

cpuonline

1、cpuonline信息在/sys/devices/system/cpu/路径下，lxcfs将对/sys（当然这里使用任何路径都可以）的操作注册到fuse

lxcfs.c：

const struct fuse_operations lxcfs_ops = {......    .open = lxcfs_open,    .read = lxcfs_read,    .release = lxcfs_release,    .write = lxcfs_write,......}

static int lxcfs_read(const char *path, char *buf, size_t size, off_t offset,        struct fuse_file_info *fi){    int ret;    if (strncmp(path, "/cgroup", 7) == 0) {        up_users();        ret = do_cg_read(path, buf, size, offset, fi);        down_users();        return ret;    }    if (strncmp(path, "/proc", 5) == 0) {        up_users();        ret = do_proc_read(path, buf, size, offset, fi);        down_users();        return ret;    }    if (strncmp(path, "/sys", 4) == 0) {        up_users();        ret = do_sys_read(path, buf, size, offset, fi);        down_users();        return ret;    }

    return -EINVAL;}

static int do_sys_read(const char *path, char *buf, size_t size, off_t offset,        struct fuse_file_info *fi){    int (*sys_read)(const char *path, char *buf, size_t size, off_t offset,        struct fuse_file_info *fi);    char *error;

    dlerror();    /* Clear any existing error */    sys_read = (int (*)(const char *, char *, size_t, off_t, struct fuse_file_info *)) dlsym(dlopen_handle, "sys_read");    error = dlerror();    if (error != NULL) {        lxcfs_error("%s\n", error);        return -1;    }

    return sys_read(path, buf, size, offset, fi);}

文件操作相关代码（bindings.c、sysfs_fuse.c，cpuset.c）被封装成liblxcfs.so动态库，供lxcfs.c调用。上面do_sys_read通过dlsym获取liblxcfs.so动态库中的sys_read函数。

2、接着看下读cpuonline的过程
sysfs_fuse.c：

int sys_read(const char *path, char *buf, size_t size, off_t offset,         struct fuse_file_info *fi){    struct file_info *f = (struct file_info *)fi->fh;

    switch (f->type) {    //cpuonline 模块，type在open时设置，这里不做过多介绍，主要看下    //sys_devices_system_cpu_online_read函数的实现    case LXC_TYPE_SYS_DEVICES_SYSTEM_CPU_ONLINE:          return sys_devices_system_cpu_online_read(buf, size, offset, fi);    case LXC_TYPE_SYS_DEVICES:    case LXC_TYPE_SYS_DEVICES_SYSTEM:    case LXC_TYPE_SYS_DEVICES_SYSTEM_CPU:    default:        return -EINVAL;    }}

static int sys_devices_system_cpu_online_read(char *buf, size_t size,                          off_t offset,                          struct fuse_file_info *fi){    //获取上下文信息，主要是读取cuponline进程（例如cat /sys/devices/system/cpu/online的cat进程，以下简称“调用进程”）的进程id    struct fuse_context *fc = fuse_get_context();    struct file_info *d = (struct file_info *)fi->fh;    char *cache = d->buf;    char *cg;    char *cpuset = NULL;    bool use_view;

    int max_cpus = 0;    pid_t initpid;    ssize_t total_len = 0;

    if (offset) {        if (!d->cached)            return 0;        if (offset > d->size)            return -EINVAL;        int left = d->size - offset;        total_len = left > size ? size : left;        memcpy(buf, cache + offset, total_len);        return total_len;    }

    //获取容器中1号进程在物理机上的进程id；initpid返回为0时，说明调用进程是物理上的进程    initpid = lookup_initpid_in_store(fc->pid);    if (initpid <= 0)        initpid = fc->pid;    //获取容器1号进程的cgroup    //例如：docker/368adedeb87172d68388cee9818e873d73503a5b1d1d2a6b47fbd053f6d68601    cg = get_pid_cgroup(initpid, "cpuset");    if (!cg)        return read_file("/sys/devices/system/cpu/online", buf, size, d);    prune_init_slice(cg);

    cpuset = get_cpuset(cg);    if (!cpuset)        goto err;    // 检查cpu、 cpuacct 控制器是否存在，不存在直接返回物理机cpuonine信息    use_view = use_cpuview(cg);

    if (use_view)        // 获取容器真正可使用的cpu个数，如果容器没配置cpu quota（默认-1），则直接返回物理信息        max_cpus = max_cpu_count(cg);

    if (max_cpus == 0)        return read_file("/sys/devices/system/cpu/online", buf, size, d);    if (max_cpus > 1)        total_len = snprintf(d->buf, d->buflen, "0-%d\n", max_cpus - 1);    else        total_len = snprintf(d->buf, d->buflen, "0\n");    if (total_len < 0 || total_len >= d->buflen) {        lxcfs_error("%s\n", "failed to write to cache");        return 0;    }

    d->size = (int)total_len;    d->cached = 1;

    if (total_len > size)        total_len = size;

    memcpy(buf, d->buf, total_len);err:    free(cpuset);    free(cg);    return total_len;}/* * Return the maximum number of visible CPUs based on CPU quotas. * If there is no quota set, zero is returned. */int max_cpu_count(const char *cg){    int rv, nprocs;    int64_t cfs_quota, cfs_period;    int nr_cpus_in_cpuset = 0;    char *cpuset = NULL;    // 读取物理机上容器cpu的quota值    if (!read_cpu_cfs_param(cg, "quota", &cfs_quota))        return 0;    // 读取物理机上容器cpu的period值    if (!read_cpu_cfs_param(cg, "period", &cfs_period))        return 0;

    cpuset = get_cpuset(cg);    if (cpuset)        nr_cpus_in_cpuset = cpu_number_in_cpuset(cpuset);

    if (cfs_quota <= 0 || cfs_period <= 0){        if (nr_cpus_in_cpuset > 0)            return nr_cpus_in_cpuset;

        return 0;    }

    // 容器何用的cpu计算    rv = cfs_quota / cfs_period;

    /* In case quota/period does not yield a whole number, add one CPU for     * the remainder.这里的意思是限制cpu为0.5和，视图效果为1核。1.5 即 2     */    if ((cfs_quota % cfs_period) > 0)        rv += 1;

    /*获取可用的cpu核数sysconf(_SC_NPROCESSORS_ONLN)*/    nprocs = get_nprocs();

    if (rv > nprocs)        rv = nprocs;

    /* use min value in cpu quota and cpuset */    if (nr_cpus_in_cpuset > 0 && nr_cpus_in_cpuset < rv)        rv = nr_cpus_in_cpuset;

    return rv;}// 看下quota是怎么获取的/* * Read cgroup CPU quota parameters from `cpu.cfs_quota_us` or `cpu.cfs_period_us`, * depending on `param`. Parameter value is returned throuh `value`. */static bool read_cpu_cfs_param(const char *cg, const char *param, int64_t *value){    bool rv = false;    char file[11 + 6 + 1]; // cpu.cfs__us + quota/period + \0    char *str = NULL;

    sprintf(file, "cpu.cfs_%s_us", param);

    // 重点是这里    if (!cgfs_get_value("cpu", cg, file, &str))        goto err;......}bool cgfs_get_value(const char *controller, const char *cgroup, const char *file, char **value){    int ret, fd, cfd;    size_t len;    char *fnam, *tmpc;    // 获取cpu controller文件描述符，到之前说过fd_hierarchies中查    tmpc = find_mounted_controller(controller, &cfd);    if (!tmpc)        return false;

    /* Make sure we pass a relative path to *at() family of functions.     * . + /cgroup + / + file + \0     */    len = strlen(cgroup) + strlen(file) + 3;    fnam = alloca(len);    ret = snprintf(fnam, len, "%s%s/%s", *cgroup == '/' ? "." : "", cgroup, file);    if (ret < 0 || (size_t)ret >= len)        return false;    // fd也就是 /run/lxcfs/controllers/cpu/docker/dockerid/cpu.cfs_quota_us    fd = openat(cfd, fnam, O_RDONLY);    if (fd < 0)        return false;    // 读值cfs_quota_us    *value = slurp_file(fnam, fd);    return *value != NULL;}

loadavg

• 平均负载的概念：平均负载是一段时间内活跃task队列的平均值，活跃进程指的是TASK_RUNNING, TASK_UNINTERRUPTIBLE状态的进程。内核计算loadavg的方式，感兴趣的同学可以看看源码。

• loadavg和其他部分不太一样的是，lxcfs需要用daemon进程计算平均负载，因为我们需要的容器（也就是特定进程的cgroup）的平均负载，宿主机没有这部分数据。lxcfs用与内核完全相同的方式计算负载，所以loadavg的值还是相当准足准确的。宿主机计算的平均负载是根据所有的task（进程、线程）计算得到，容器的平均负载是根据容器内的进程计算而得。

1、loadavg daemon分析

load daemon的调用流程：main-> start_loadavg-> load_daemon-> load_begin

load_begin就像注释写的一样，每5s遍历一次load哈希表，并更新负载值

/* * Traverse the hash table and update it. */void *load_begin(void *arg){

......    while (1) {        if (loadavg_stop == 1)            return NULL;

        time1 = clock();        for (i = 0; i < LOAD_SIZE; i++) {            pthread_mutex_lock(&load_hash[i].lock);            if (load_hash[i].next == NULL) {                pthread_mutex_unlock(&load_hash[i].lock);                continue;            }            f = load_hash[i].next;            first_node = 1;            while (f) {......                // 更新负载                sum = refresh_load(f, path);                if (sum == 0) {                    f = del_node(f, i);                } else {out:                    f = f->next;                }                free(path);......        }

        if (loadavg_stop == 1)            return NULL;

        time2 = clock();        usleep(FLUSH_TIME * 1000000 - (int)((time2 - time1) * 1000000 / CLOCKS_PER_SEC));    }}

主要分析下refresh_load

/* * Return 0 means that container p->cg is closed. * Return -1 means that error occurred in refresh. * Positive num equals the total number of pid. */static int refresh_load(struct load_node *p, char *path){    FILE *f = NULL;    char **idbuf;    char proc_path[256];    int i, ret, run_pid = 0, total_pid = 0, last_pid = 0;    char *line = NULL;    size_t linelen = 0;    int sum, length;    DIR *dp;    struct dirent *file;

    do {        idbuf = malloc(sizeof(char *));    } while (!idbuf);        // 这里从/sys/fs/cgroup/cpu/docker/containerid/cgroup.procs to find the process pid.那容器内进程的pid    sum = calc_pid(&idbuf, path, DEPTH_DIR, 0, p->cfd);    /*  normal exit  */    if (sum == 0)        goto out;

    for (i = 0; i < sum; i++) {        /*clean up '\n' */        length = strlen(idbuf[i])-1;        idbuf[i][length] = '\0';        ret = snprintf(proc_path, 256, "/proc/%s/task", idbuf[i]);        if (ret < 0 || ret > 255) {            lxcfs_error("%s\n", "snprintf() failed in refresh_load.");            i = sum;            sum = -1;            goto err_out;        }

        dp = opendir(proc_path);        if (!dp) {            lxcfs_error("%s\n", "Open proc_path failed in refresh_load.");            continue;        }                // 遍历/proc/<pid>/task 目录（一个进程中创建的每个线程，/proc/<pid>/task 中会创建一个相应的目录），查找状态为R或者D的task        while ((file = readdir(dp)) != NULL) {            if (strncmp(file->d_name, ".", 1) == 0)                continue;            if (strncmp(file->d_name, "..", 1) == 0)                continue;            total_pid++;            /* We make the biggest pid become last_pid.*/            ret = atof(file->d_name);            last_pid = (ret > last_pid) ? ret : last_pid;

            ret = snprintf(proc_path, 256, "/proc/%s/task/%s/status", idbuf[i], file->d_name);            if (ret < 0 || ret > 255) {                lxcfs_error("%s\n", "snprintf() failed in refresh_load.");                i = sum;                sum = -1;                closedir(dp);                goto err_out;            }            f = fopen(proc_path, "r");            if (f != NULL) {                while (getline(&line, &linelen, f) != -1) {                    /* Find State */                    if ((line[0] == 'S') && (line[1] == 't'))                        break;                }            if ((line[7] == 'R') || (line[7] == 'D'))                run_pid++;            fclose(f);            }        }        closedir(dp);    }    /*Calculate the loadavg.*/    // 获取到活跃的task数量后，是时候表演真正的技术了（计算平均负载）。计算公式与内核一致：load(t) = load(t-1) e-5/60 + n (1 - e-5/60)    // 具体含义可以参考：[https://www.helpsystems.com/resources/guideshow-it-works](https://w/unix-load-average-part-1-ww.helpsystems.com/resources/guides/unix-load-average-part-1-how-it-works)

    p->avenrun[0] = calc_load(p->avenrun[0], EXP_1, run_pid);    p->avenrun[1] = calc_load(p->avenrun[1], EXP_5, run_pid);    p->avenrun[2] = calc_load(p->avenrun[2], EXP_15, run_pid);    p->run_pid = run_pid;    p->total_pid = total_pid;    p->last_pid = last_pid;

    free(line);err_out:    for (; i > 0; i--)        free(idbuf[i-1]);out:    free(idbuf);    return sum;}

2、读取loadavg

负载计算明白了，读就简单了。这里要注意下的是，load_hash，哈希表中的数据是容器第一次读/proc/loadavg时插入的（毕竟没办法事先知道容器的进程cgroup）。

static int proc_loadavg_read(char *buf, size_t size, off_t offset,        struct fuse_file_info *fi){    struct fuse_context *fc = fuse_get_context();    struct file_info *d = (struct file_info *)fi->fh;    pid_t initpid;    char *cg;    size_t total_len = 0;    char *cache = d->buf;    struct load_node *n;    int hash;    int cfd, rv = 0;    unsigned long a, b, c;

    if (offset) {        if (offset > d->size)            return -EINVAL;        if (!d->cached)            return 0;        int left = d->size - offset;        total_len = left > size ? size : left;        memcpy(buf, cache + offset, total_len);        return total_len;    }    if (!loadavg)        return read_file("/proc/loadavg", buf, size, d);

    initpid = lookup_initpid_in_store(fc->pid);    if (initpid <= 0)        initpid = fc->pid;    cg = get_pid_cgroup(initpid, "cpu");    if (!cg)        return read_file("/proc/loadavg", buf, size, d);

    prune_init_slice(cg);    hash = calc_hash(cg) % LOAD_SIZE;    // 根据cgroup在hash表查找node    n = locate_node(cg, hash);

    /* First time */    // 第一读时，先把节点信息插到hash边    if (n == NULL) {        if (!find_mounted_controller("cpu", &cfd)) {            /*             * In locate_node() above, pthread_rwlock_unlock() isn't used             * because delete is not allowed before read has ended.             */            pthread_rwlock_unlock(&load_hash[hash].rdlock);            rv = 0;            goto err;        }        do {            n = malloc(sizeof(struct load_node));        } while (!n);

        do {            n->cg = malloc(strlen(cg)+1);        } while (!n->cg);        strcpy(n->cg, cg);        n->avenrun[0] = 0;        n->avenrun[1] = 0;        n->avenrun[2] = 0;        n->run_pid = 0;        n->total_pid = 1;        n->last_pid = initpid;        n->cfd = cfd;        insert_node(&n, hash);    }    // 第二次以后开始从daemon的计算结果中读取    a = n->avenrun[0] + (FIXED_1/200);    b = n->avenrun[1] + (FIXED_1/200);    c = n->avenrun[2] + (FIXED_1/200);    total_len = snprintf(d->buf, d->buflen, "%lu.%02lu %lu.%02lu %lu.%02lu %d/%d %d\n",        LOAD_INT(a), LOAD_FRAC(a),        LOAD_INT(b), LOAD_FRAC(b),        LOAD_INT(c), LOAD_FRAC(c),        n->run_pid, n->total_pid, n->last_pid);    pthread_rwlock_unlock(&load_hash[hash].rdlock);    if (total_len < 0 || total_len >=  d->buflen) {        lxcfs_error("%s\n", "Failed to write to cache");        rv = 0;        goto err;    }    d->size = (int)total_len;    d->cached = 1;

    if (total_len > size)        total_len = size;    memcpy(buf, d->buf, total_len);    rv = total_len;

err:    free(cg);    return rv;}

• https://www.helpsystems.com/resources/guides/unix-load-average-part-1-how-it-works

• https://github.com/libfuse/libfuse

• https://github.com/lxc/lxcfs

• https://www.helpsystems.com/resources/guides/unix-load-average-part-1-how-it-works

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