ceph源码分析： Log实现

每一个大型的项目，都会必须要设计log，log是重要的调试手段，也是很好的学习入口。跟踪log可以让一个新手快速的理解代码，分析log可以帮助工程师很好的定位问题。


下面通过跟踪ceph-mon这个可执行文件，了解ceph中的log实现。

ceph_mon 初始化中 会调用global_init, global_init一开始就会调用 common_preinit函数

创建一个重要的数据结构CephContext cct。

CephContext::CephContext(uint32_t module_type_)
  : nref(1),
    _conf(new md_config_t()),
    _log(NULL),
    _module_type(module_type_),
    _service_thread(NULL),
    _log_obs(NULL),
    _admin_socket(NULL),
    _perf_counters_collection(NULL),
    _perf_counters_conf_obs(NULL),
    _heartbeat_map(NULL),
    _crypto_none(NULL),
    _crypto_aes(NULL)
{
  pthread_spin_init(&_service_thread_lock, PTHREAD_PROCESS_SHARED);

  _log = new ceph::log::Log(&_conf->subsys);
  _log->start();

  _log_obs = new LogObs(_log);
  _conf->add_observer(_log_obs);

  _perf_counters_collection = new PerfCountersCollection(this);
  _admin_socket = new AdminSocket(this);
  _heartbeat_map = new HeartbeatMap(this);

  _admin_hook = new CephContextHook(this);
  _admin_socket->register_command("perfcounters_dump", "perfcounters_dump", _admin_hook, "");
  _admin_socket->register_command("1", "1", _admin_hook, "");
  _admin_socket->register_command("perf dump", "perf dump", _admin_hook, "dump perfcounters value");
  _admin_socket->register_command("perfcounters_schema", "perfcounters_schema", _admin_hook, "");
  _admin_socket->register_command("2", "2", _admin_hook, "");
  _admin_socket->register_command("perf schema", "perf schema", _admin_hook, "dump perfcounters schema");
  _admin_socket->register_command("config show", "config show", _admin_hook, "dump current config settings");
  _admin_socket->register_command("config set", "config set name=var,type=CephString name=val,type=CephString,n=N", _admin_hook, "config set   [ ...]: set a config variable");
  _admin_socket->register_command("config get", "config get name=var,type=CephString", _admin_hook, "config get : get the config value");
  _admin_socket->register_command("log flush", "log flush", _admin_hook, "flush log entries to log file");
  _admin_socket->register_command("log dump", "log dump", _admin_hook, "dump recent log entries to log file");
  _admin_socket->register_command("log reopen", "log reopen", _admin_hook, "reopen log file");

  _crypto_none = new CryptoNone;
  _crypto_aes = new CryptoAES;
}

这个函数并不长，但是绝不简单，本文就是介绍Log线程
 

  _log = new ceph::log::Log(&_conf->subsys);
  _log->start();

    首先是创建了成员变量_log

    第一句 new 是创建了Log实例：基本就是初始化

   1 初始化自旋锁  m_lock

   2 初始化互斥量  m_flush_mutex 和m_queue_mutex

   3 初始化条件变量 m_con_logger 和m_cond_flusher

Log::Log(SubsystemMap *s)
  : m_indirect_this(NULL),
    m_subs(s),
    m_new(), m_recent(),
    m_fd(-1),
    m_syslog_log(-2), m_syslog_crash(-2),
    m_stderr_log(1), m_stderr_crash(-1),
    m_stop(false),
    m_max_new(DEFAULT_MAX_NEW),
    m_max_recent(DEFAULT_MAX_RECENT)
{
  int ret;

  ret = pthread_spin_init(&m_lock, PTHREAD_PROCESS_SHARED);
  assert(ret == 0);

  ret = pthread_mutex_init(&m_flush_mutex, NULL);
  assert(ret == 0);

  ret = pthread_mutex_init(&m_queue_mutex, NULL);
  assert(ret == 0);

  ret = pthread_cond_init(&m_cond_loggers, NULL);
  assert(ret == 0);

  ret = pthread_cond_init(&m_cond_flusher, NULL);
  assert(ret == 0);

  // kludge for prealloc testing
  if (false)
    for (int i=0; i < PREALLOC; i++)
      m_recent.enqueue(new Entry);
}

 接下来启动了log线程：
 

void Log::start()
{
  assert(!is_started());
  pthread_mutex_lock(&m_queue_mutex);
  m_stop = false;
  pthread_mutex_unlock(&m_queue_mutex);
  create();
}

 关键一句是create，这个create从哪里冒出来的？ 这个调用的是 Thread里面的create 方法：

int Thread::try_create(size_t stacksize)
{
  pthread_attr_t *thread_attr = NULL;
  stacksize &= CEPH_PAGE_MASK; // must be multiple of page
  if (stacksize) {
    thread_attr = (pthread_attr_t*) malloc(sizeof(pthread_attr_t));
    if (!thread_attr)
      return -ENOMEM;
    pthread_attr_init(thread_attr);
    pthread_attr_setstacksize(thread_attr, stacksize);
  }
  int r;
  // The child thread will inherit our signal mask. Set our signal mask to
  // the set of signals we want to block. (It's ok to block signals more
  // signals than usual for a little while-- they will just be delivered to
  // another thread or delieverd to this thread later.)
  sigset_t old_sigset;
  if (g_code_env == CODE_ENVIRONMENT_LIBRARY) {
    block_signals(NULL, &old_sigset);
  }
  else {
    int to_block[] = { SIGPIPE , 0 };
    block_signals(to_block, &old_sigset);
  }
  r = pthread_create(&thread_id, thread_attr, _entry_func, (void*)this);
  restore_sigset(&old_sigset);
  if (thread_attr)
    free(thread_attr);
  return r;
}
void Thread::create(size_t stacksize)
{
  int ret = try_create(stacksize);
  if (ret != 0) {
    char buf[256];
    snprintf(buf, sizeof(buf), "Thread::try_create(): pthread_create "
     "failed with error %d", ret);
    dout_emergency(buf);
    assert(ret == 0);
  }
}

创建线程，该方法做了下面的事情：

1  可以设置线程栈的size 

2  对于daemon进程，会创建线程前会阻塞SIGPIPE

3  调用pthread_create函数创建线程

等一下，线程执行什么函数？ _entry_func到底是什么？

void *Thread::_entry_func(void *arg) {
  void *r = ((Thread*)arg)->entry();
  return r;
}

也就是说线程执行的函数，记录在pthread_create的第四个变量的里面：

namespace ceph {
namespace log {
class Log : private Thread
{
  Log **m_indirect_this;
  SubsystemMap *m_subs;

  pthread_spinlock_t m_lock;
  pthread_mutex_t m_queue_mutex;
  pthread_mutex_t m_flush_mutex;
  pthread_cond_t m_cond_loggers;
  pthread_cond_t m_cond_flusher;
  EntryQueue m_new; ///< new entries
  EntryQueue m_recent; ///< recent (less new) entries we've already written at low detail
  std::string m_log_file;
  int m_fd;
  int m_syslog_log, m_syslog_crash;
  int m_stderr_log, m_stderr_crash;
  bool m_stop;
  int m_max_new, m_max_recent;
  void *entry();
  void _flush(EntryQueue *q, EntryQueue *requeue, bool crash);
  void _log_message(const char *s, bool crash);
public:
  Log(SubsystemMap *s);
  virtual ~Log();
  void set_flush_on_exit();
  void set_max_new(int n);
  void set_max_recent(int n);
  void set_log_file(std::string fn);
  void reopen_log_file();
  void flush();
  void dump_recent();
  void set_syslog_level(int log, int crash);
  void set_stderr_level(int log, int crash);
  Entry *create_entry(int level, int subsys);
  void submit_entry(Entry *e);
  void start();
  void stop();
};
}
}

即Log的类中，有成员函数 entry，即线程应该执行的函数：

void *Log::entry()
{
  pthread_mutex_lock(&m_queue_mutex);
  while (!m_stop) {
    if (!m_new.empty()) {
      pthread_mutex_unlock(&m_queue_mutex);
      flush();
      pthread_mutex_lock(&m_queue_mutex);
      continue;
    }
    pthread_cond_wait(&m_cond_flusher, &m_queue_mutex);
  }
  pthread_mutex_unlock(&m_queue_mutex);
  flush();
  return NULL;
}

这就是log线程执行的动作。

1  m_stop来控制线程是否终止

2 如果m_new 这个队列不空，就调用flush，负责写入log

3 如果队列空了，条件等待，有新的log出现在队列上，会通知到这个线程

暂且不管如果控制线程，谁来通知log线程有新的log，直接看下线程的主要工作，flush主要工作如下

void Log::flush()
{
  pthread_mutex_lock(&m_flush_mutex);
  pthread_mutex_lock(&m_queue_mutex);
  EntryQueue t;
  t.swap(m_new);
  pthread_cond_broadcast(&m_cond_loggers);
  pthread_mutex_unlock(&m_queue_mutex);
  _flush(&t, &m_recent, false);
  // trim
  //m_recent有一个最大值，超出了最大值，就从队列中删除最老的log，内存也就释放了
  while (m_recent.m_len > m_max_recent) {
    delete m_recent.dequeue();
  }
  pthread_mutex_unlock(&m_flush_mutex);
}

首先，创建一个临时队列，t，执行swap将m_new里面的log都接了过去。所谓接过去，不过是将指针指向队列内容的事情交给临时队列，m_new 头指针和尾指针置成NULL

做了这件事之后，m_new又变成了空的队列，好心地给其他线程发了广播之后，就可以解锁m_queue_mutex互斥量了。

swap操作比较简单，就是交换指针的彼此指向：

void swap(EntryQueue& other) {
    int len = m_len;
    struct Entry *h = m_head, *t = m_tail;
    m_len = other.m_len;
    m_head = other.m_head;
    m_tail = other.m_tail;
    other.m_len = len;
    other.m_head = h;
    other.m_tail = t;
  }

然后将主要的写日志的工作就委托给了__flush函数。
可以看出，有的log需要写入日志文件，有的需要写入syslog，有些日志需要写入stderr
同时，日志还有一个优先级的概念，因此会根据should_log 来控制。

void Log::_flush(EntryQueue *t, EntryQueue *requeue, bool crash)
{
  Entry *e;
  char buf[80];
  while ((e = t->dequeue()) != NULL) {
    unsigned sub = e->m_subsys;
    bool should_log = crash || m_subs->get_log_level(sub) >= e->m_prio;
    bool do_fd = m_fd >= 0 && should_log;
    bool do_syslog = m_syslog_crash >= e->m_prio && should_log;
    bool do_stderr = m_stderr_crash >= e->m_prio && should_log;
    if (do_fd || do_syslog || do_stderr) {
      int buflen = 0;
      if (crash)
    buflen += snprintf(buf, sizeof(buf), "%6d> ", -t->m_len);
      buflen += e->m_stamp.sprintf(buf + buflen, sizeof(buf)-buflen);
      buflen += snprintf(buf + buflen, sizeof(buf)-buflen, " %lx %2d ",
            (unsigned long)e->m_thread, e->m_prio);
      // FIXME: this is slow
      string s = e->get_str();
      if (do_fd) {
    int r = safe_write(m_fd, buf, buflen);
    if (r >= 0)
      r = safe_write(m_fd, s.data(), s.size());
    if (r >= 0)
      r = write(m_fd, "\n", 1);
    if (r < 0)
      cerr << "problem writing to " << m_log_file << ": " << cpp_strerror(r) << std::endl;
      }
      if (do_syslog) {
    syslog(LOG_USER, "%s%s", buf, s.c_str());
      }
      if (do_stderr) {
    cerr << buf << s << std::endl;
      }
    }
    requeue->enqueue(e);
  }
}

接下来是common字段，日志总免不了要记录消息的发生时间。：

buflen += e->m_stamp.sprintf(buf + buflen, sizeof(buf)-buflen);

这个m_stamp是utime_t类型，他的sprintf实现如下：

int sprintf(char *out, int outlen) const {
    struct tm bdt;
    time_t tt = sec();
    localtime_r(&tt, &bdt);

    return snprintf(out, outlen,
    "%04d-%02d-%02d %02d:%02d:%02d.%06ld",
    bdt.tm_year + 1900, bdt.tm_mon + 1, bdt.tm_mday,
    bdt.tm_hour, bdt.tm_min, bdt.tm_sec, usec());
  }

因此log的前缀是

2015-05-29 10:20:45.076105

接下来的字段是，线程的线程ID以及 消息的优先级，所谓线程ID并不是调度意义上的ID，而是pthread_self的返回值那个ID

buflen += snprintf(buf + buflen, sizeof(buf)-buflen, " %lx %2d ",
            (unsigned long)e->m_thread, e->m_prio);

ceph-mon是多线程的程序：

root@test3:/var/log/ceph# pidof ceph--mon
root@test3:/var/log/ceph# pidof ceph-mon
138812
root@test3:/var/log/ceph#
root@test3:/var/log/ceph# ll /proc/138812/task
total 0
dr-xr-xr-x 23 root root 0 May 29 16:17 ./
dr-xr-xr-x  9 root root 0 May 29 16:11 ../
dr-xr-xr-x  6 root root 0 May 29 16:17 138812/
dr-xr-xr-x  6 root root 0 May 29 16:17 138813/
dr-xr-xr-x  6 root root 0 May 29 16:17 138814/
dr-xr-xr-x  6 root root 0 May 29 16:17 138815/
dr-xr-xr-x  6 root root 0 May 29 16:17 138816/
dr-xr-xr-x  6 root root 0 May 29 16:17 138817/
dr-xr-xr-x  6 root root 0 May 29 16:17 138818/
dr-xr-xr-x  6 root root 0 May 29 16:17 138819/
dr-xr-xr-x  6 root root 0 May 29 16:17 138820/
dr-xr-xr-x  6 root root 0 May 29 16:17 138821/
dr-xr-xr-x  6 root root 0 May 29 16:17 138822/
dr-xr-xr-x  6 root root 0 May 29 16:17 138823/
dr-xr-xr-x  6 root root 0 May 29 16:17 138824/
dr-xr-xr-x  6 root root 0 May 29 16:17 139643/
dr-xr-xr-x  6 root root 0 May 29 16:17 139895/
dr-xr-xr-x  6 root root 0 May 29 16:17 139896/
dr-xr-xr-x  6 root root 0 May 29 16:17 143413/
dr-xr-xr-x  6 root root 0 May 30 16:47 78042/
dr-xr-xr-x  6 root root 0 May 30 16:47 78044/
dr-xr-xr-x  6 root root 0 May 30 17:08 90496/
dr-xr-xr-x  6 root root 0 May 30 17:08 90497/

因此，我们可以取来一条log查看下ceph log的前缀：

2015-05-29 10:20:45.076105 7fa1c6a1a700  0 mon.jnqfg@2(peon).data_health(30) update_stats avail 92% total 95990980 used 2183384 avail 88908400

值得一提的是，ceph维护了一个m_recent队列，所有的消息都会存放到该队列中去，哪怕优先级比较低，不会打印到日志文件中去。
这就是_flush 函数中的 enqueue做的事情：

当然了，队列是有大小的限制，否则队列就会膨胀，导致内存耗尽。这个默认的限制是10000.

while (m_recent.m_len > m_max_recent) {
    delete m_recent.dequeue();
  }

ceph提供了方法来查看最近的log，这就是log dump方法。

ceph daemon /var/run/ceph/ceph-mon.*asok  log dump
{}

表面看啥也没输出，实际上将log dump到了日志文件中，比如我们例子，在log文件中出现：

v2 ==== 42+0+0 (3105867923 0 0) 0x48d6bc0 con 0x3d5c9a0
   -13> 2015-05-30 17:23:06.772212 7f5856cac700 10 mon.jnqfg@2(peon) e3 handle_subscribe mon_subscribe({monmap=4+,osdmap=0}) v2
   -12> 2015-05-30 17:23:06.772217 7f5856cac700 10 mon.jnqfg@2(peon) e3 check_sub monmap next 4 have 3
   -11> 2015-05-30 17:23:06.772222 7f5856cac700 10 mon.jnqfg@2(peon).osd e260 check_sub 0x631bdc0 next 0 (onetime)
   -10> 2015-05-30 17:23:06.773223 7f5856cac700  1 -- 10.16.20.183:6789/0 --> client.? 10.16.20.181:0/1138587 -- osd_map(260..260 src has 1..260) v3 -- ?+0 0x4230b40
    -9> 2015-05-30 17:23:06.773239 7f5856cac700  1 -- 10.16.20.183:6789/0 --> client.4442663 10.16.20.181:0/1138587 -- mon_subscribe_ack(300s) v1 -- ?+0 0x48d6680
    -8> 2015-05-30 17:23:06.773586 7f5856cac700  1 -- 10.16.20.183:6789/0 <== client.4442663 10.16.20.181:0/1138587 4 ==== mon_subscribe({monmap=4+,osdmap=0}) v2 ==== 42+0+0 (3105867923 0 0) 0x48d5180 con 0x3d5c9a0
    -7> 2015-05-30 17:23:06.773607 7f5856cac700 10 mon.jnqfg@2(peon) e3 handle_subscribe mon_subscribe({monmap=4+,osdmap=0}) v2
    -6> 2015-05-30 17:23:06.773613 7f5856cac700 10 mon.jnqfg@2(peon) e3 check_sub monmap next 4 have 3
    -5> 2015-05-30 17:23:06.773620 7f5856cac700 10 mon.jnqfg@2(peon).osd e260 check_sub 0x631a7c0 next 0 (onetime)
    -4> 2015-05-30 17:23:06.774626 7f5856cac700  1 -- 10.16.20.183:6789/0 --> client.? 10.16.20.181:0/1138587 -- osd_map(260..260 src has 1..260) v3 -- ?+0 0x4232ac0
    -3> 2015-05-30 17:23:06.774641 7f5856cac700  1 -- 10.16.20.183:6789/0 --> client.4442663 10.16.20.181:0/1138587 -- mon_subscribe_ack(300s) v1 -- ?+0 0x48d6bc0
    -2> 2015-05-30 17:23:06.775345 7f5856cac700  1 -- 10.16.20.183:6789/0 <== client.4442663 10.16.20.181:0/1138587 5 ==== mon_command({"prefix": "get_command_descriptions"} v 0) v1 ==== 80+0+0 (3374501561 0 0) 0x477f0e0 con 0x3d5c9a0
    -1> 2015-05-30 17:23:06.776789 7f5856cac700  1 -- 10.16.20.183:6789/0 --> 10.16.20.181:0/1138587 -- mon_command_ack([{"prefix": "get_command_descriptions"}]=0  v0) v1 -- ?+24689 0x3e62760 con 0x3d5c9a0
     0> 2015-05-30 17:23:06.821389 7f5859662700  1 do_command 'log dump'

很有意思的是，ceph如何做到  ceph daemon  /var/run/ceph/ceph-mon.*asok log dump 就把log dump到日志文件的。 这就牵扯到admin socket机制了。
admin socket 机制并不是一个非常新的机制，这是一个很commmon的设计。

很多进程，设计的时候，需要设计一个手段，能够运行时，接收用户的指令，不能将程序设计成一个黑匣子，相反，要提供一些手段，使是用户或者可以干预进程的运行（如改变配置项），或者可以获取到进程运行的状态信息。

admin socket机制，那是下一篇的任务。