writeback机制源码分析

writeback相关数据结构

与writeback相关的数据结构主要有：

1，backing_dev_info，该数据结构描述了backing_dev的所有信息，通常块设备的request queue中会包含backing_dev对象。

2，bdi_writeback，该数据结构封装了writeback的内核线程以及需要操作的inode队列。

3，wb_writeback_work，该数据结构封装了writeback的工作任务。

各数据结构之间的关系如下图所示：

下面对各个数据结构做简要介绍。

bdi information

bdi对象在块设备添加的时候需要注册到系统的bdi队列中。对于ext3而言，在mount的时候需要将底层块设备的bdi对象联系到ext3 root_inode中。bdi对象数据结构定义如下：

 

struct backing_dev_info {      struct list_head bdi_list;      unsigned long ra_pages; /* max readahead in PAGE_CACHE_SIZE units */    unsigned long state;    /* Always use atomic bitops on this */    unsigned int capabilities; /* Device capabilities */    congested_fn *congested_fn; /* Function pointer if device is md/dm */    void *congested_data;   /* Pointer to aux data for congested func */         char *name;           struct percpu_counter bdi_stat[NR_BDI_STAT_ITEMS];           unsigned long bw_time_stamp;    /* last time write bw is updated */    unsigned long dirtied_stamp;      unsigned long written_stamp;    /* pages written at bw_time_stamp */    unsigned long write_bandwidth;  /* the estimated write bandwidth */    unsigned long avg_write_bandwidth; /* further smoothed write bw */         /*       * The base dirty throttle rate, re-calculated on every 200ms.       * All the bdi tasks' dirty rate will be curbed under it.       * @dirty_ratelimit tracks the estimated @balanced_dirty_ratelimit       * in small steps and is much more smooth/stable than the latter.       */    unsigned long dirty_ratelimit;      unsigned long balanced_dirty_ratelimit;           struct prop_local_percpu completions;      int dirty_exceeded;           unsigned int min_ratio;      unsigned int max_ratio, max_prop_frac;           struct bdi_writeback wb;  /* default writeback info for this bdi，writeback对象 */    spinlock_t wb_lock;   /* protects work_list */         /* 任务链表 */    struct list_head work_list;           struct device *dev;      /* 在laptop模式下应用的定时器 */    struct timer_list laptop_mode_wb_timer;       #ifdef CONFIG_DEBUG_FS      struct dentry *debug_dir;      struct dentry *debug_stats;  #endif  };

在bdi数据结构中定义了一个writeback对象，该对象是对writeback内核线程的描述，并且封装了需要处理的inode队列。在bdi数据结构中有一条work_list，该work队列维护了writeback内核线程需要处理的任务。如果该队列上没有work可以处理，那么writeback内核线程将会睡眠等待。

writeback

writeback对象封装了内核线程task以及需要处理的inode队列。当page cache/buffer cache需要刷新radix tree上的inode时，可以将该inode挂载到writeback对象的b_dirty队列上，然后唤醒writeback线程。在处理过程中，inode会被移到b_io队列上进行处理。多条链表的方式可以降低多线程之间的资源共享。writeback数据结构具体定义如下：

 

struct bdi_writeback {      struct backing_dev_info *bdi;   /* our parent bdi */    unsigned int nr;           unsigned long last_old_flush;   /* last old data flush */    unsigned long last_active;  /* last time bdi thread was active */         struct task_struct *task;   /* writeback thread */    struct timer_list wakeup_timer; /* used for delayed bdi thread wakeup */    struct list_head b_dirty;   /* dirty inodes */    struct list_head b_io;      /* parked for writeback */    struct list_head b_more_io; /* parked for more writeback */    spinlock_t list_lock;       /* protects the b_* lists */};

 

writeback work

wb_writeback_work数据结构是对writeback任务的封装，不同的任务可以采用不同的刷新策略。writeback线程的处理对象就是writeback_work。如果writeback_work队列为空，那么内核线程就可以睡眠了。Writeback_work的数据结构定义如下：

 

struct wb_writeback_work {      long nr_pages;      struct super_block *sb; /* superblock对象 */    unsigned long *older_than_this;      enum writeback_sync_modes sync_mode;      unsigned int tagged_writepages:1;      unsigned int for_kupdate:1;      unsigned int range_cyclic:1;      unsigned int for_background:1;      enum wb_reason reason;      /* why was writeback initiated? */              struct list_head list;      /* pending work list，链入bdi-> work_list队列 */    struct completion *done;    /* set if the caller waits，work完成时通知调用者 */};

 

writeback主要函数分析

writeback机制的主要函数包括如下两个方面：

1，管理bdi对象并且fork相应的writeback内核线程处理cache数据的刷新工作。

2，writeback内核线程处理函数，实现dirty page的刷新操作

writeback线程管理

Linux中有一个内核守护线程，该线程用来管理系统bdi队列，并且负责为block device创建writeback thread。当bdi中有dirty page并且还没有为bdi分配内核线程的时候，bdi_forker_thread程序会为其分配线程资源；当一个writeback线程长时间处于空闲状态时，bdi_forker_thread程序会释放该线程资源。

writeback线程管理程序分析如下：

static int bdi_forker_thread(void *ptr)  {      struct bdi_writeback *me = ptr;           current->flags |= PF_SWAPWRITE;      set_freezable();           /*       * Our parent may run at a different priority, just set us to normal       */    set_user_nice(current, 0);           for (;;) {          struct task_struct *task = NULL;          struct backing_dev_info *bdi;          enum {              NO_ACTION,   /* Nothing to do */            FORK_THREAD, /* Fork bdi thread */            KILL_THREAD, /* Kill inactive bdi thread */        } action = NO_ACTION;               /*           * Temporary measure, we want to make sure we don't see           * dirty data on the default backing_dev_info           */        if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list)) {              del_timer(&me->wakeup_timer);              wb_do_writeback(me, 0);          }               spin_lock_bh(&bdi_lock);          /*           * In the following loop we are going to check whether we have           * some work to do without any synchronization with tasks           * waking us up to do work for them. Set the task state here           * so that we don't miss wakeups after verifying conditions.           */        set_current_state(TASK_INTERRUPTIBLE);          /* 遍历所有的bdi对象，检查这些bdi是否存在脏数据，如果有脏数据，那么需要为其fork线程，然后做writeback操作 */        list_for_each_entry(bdi, &bdi_list, bdi_list) {              bool have_dirty_io;                   if (!bdi_cap_writeback_dirty(bdi) ||                   bdi_cap_flush_forker(bdi))                  continue;                   WARN(!test_bit(BDI_registered, &bdi->state),                   "bdi %p/%s is not registered!\n", bdi, bdi->name);              /* 检查是否存在脏数据 */            have_dirty_io = !list_empty(&bdi->work_list) ||                      wb_has_dirty_io(&bdi->wb);                   /*               * If the bdi has work to do, but the thread does not               * exist - create it.               */            if (!bdi->wb.task && have_dirty_io) {                  /*                   * Set the pending bit - if someone will try to                   * unregister this bdi - it'll wait on this bit.                   */                /* 如果有脏数据，并且不存在线程，那么接下来做线程的FORK操作 */                set_bit(BDI_pending, &bdi->state);                  action = FORK_THREAD;                  break;              }                   spin_lock(&bdi->wb_lock);                   /*               * If there is no work to do and the bdi thread was               * inactive long enough - kill it. The wb_lock is taken               * to make sure no-one adds more work to this bdi and               * wakes the bdi thread up.               */            /* 如果一个bdi长时间没有脏数据，那么执行线程的KILL操作，结束掉该bdi对应的writeback线程 */            if (bdi->wb.task && !have_dirty_io &&                  time_after(jiffies, bdi->wb.last_active +                          bdi_longest_inactive())) {                  task = bdi->wb.task;                  bdi->wb.task = NULL;                  spin_unlock(&bdi->wb_lock);                  set_bit(BDI_pending, &bdi->state);                  action = KILL_THREAD;                  break;              }              spin_unlock(&bdi->wb_lock);          }          spin_unlock_bh(&bdi_lock);               /* Keep working if default bdi still has things to do */        if (!list_empty(&me->bdi->work_list))              __set_current_state(TASK_RUNNING);          /* 执行线程的FORK和KILL操作 */        switch (action) {          case FORK_THREAD:              /* FORK一个bdi_writeback_thread线程，该线程的名字为flush-major:minor */            __set_current_state(TASK_RUNNING);              task = kthread_create(bdi_writeback_thread, &bdi->wb,                            "flush-%s", dev_name(bdi->dev));              if (IS_ERR(task)) {                  /*                   * If thread creation fails, force writeout of                   * the bdi from the thread. Hopefully 1024 is                   * large enough for efficient IO.                   */                writeback_inodes_wb(&bdi->wb, 1024,                              WB_REASON_FORKER_THREAD);              } else {                  /*                   * The spinlock makes sure we do not lose                   * wake-ups when racing with 'bdi_queue_work()'.                   * And as soon as the bdi thread is visible, we                   * can start it.                   */                spin_lock_bh(&bdi->wb_lock);                  bdi->wb.task = task;                  spin_unlock_bh(&bdi->wb_lock);                  wake_up_process(task);              }              bdi_clear_pending(bdi);              break;               case KILL_THREAD:              /* KILL一个线程 */            __set_current_state(TASK_RUNNING);              kthread_stop(task);              bdi_clear_pending(bdi);              break;               case NO_ACTION:              /* 如果没有可执行的动作，那么调度本线程睡眠一段时间 */            if (!wb_has_dirty_io(me) || !dirty_writeback_interval)                  /*                   * There are no dirty data. The only thing we                   * should now care about is checking for                   * inactive bdi threads and killing them. Thus,                   * let's sleep for longer time, save energy and                   * be friendly for battery-driven devices.                   */                schedule_timeout(bdi_longest_inactive());              else                schedule_timeout(msecs_to_jiffies(dirty_writeback_interval * 10));              try_to_freeze();              break;          }      }           return 0;  }

writeback线程

writeback线程是bdi_forker_thread 创建的，该线程的任务就是处理等待的数据回刷任务。线程处理函数为bdi_writeback_thread，其会调用wb_do_writeback函数完成具体操作，该函数分析如下：

long wb_do_writeback(struct bdi_writeback *wb, int force_wait)  {      struct backing_dev_info *bdi = wb->bdi;      struct wb_writeback_work *work;      long wrote = 0;           set_bit(BDI_writeback_running, &wb->bdi->state);      /* 处理等待的work，所有等待work pengding在bdi->work_list上 */    while ((work = get_next_work_item(bdi)) != NULL) {          /*           * Override sync mode, in case we must wait for completion           * because this thread is exiting now.           */        if (force_wait)              work->sync_mode = WB_SYNC_ALL;               trace_writeback_exec(bdi, work);          /* 调用wb_writeback函数处理相应的inode */        wrote += wb_writeback(wb, work);               /*           * Notify the caller of completion if this is a synchronous           * work item, otherwise just free it.           */        /* 通知上层软件，相应的work已经完成 */        if (work->done)              complete(work->done);          else            kfree(work);      }           /*       * Check for periodic writeback, kupdated() style       */    /* 处理周期性的dirty page刷新作业，buffer cache就会走这条路径，在下面的函数中会创建work，并且调用wb_writeback函数进行处理 */    wrote += wb_check_old_data_flush(wb);      wrote += wb_check_background_flush(wb);      clear_bit(BDI_writeback_running, &wb->bdi->state);           return wrote;  }

小结

本文在linux-3.2的基础上对writeback代码进行了浏览。整体上来讲，writeback机制是比较简单的，其核心是通过一个常驻内核线程为bdi对象分配writeback线程，实现对cache中dirty page的数据回刷。