writeback: trace event writeback_queue_io

Note that it adds a little overheads to account the moved/enqueued
inodes from b_dirty to b_io. The "moved" accounting may be later used to
limit the number of inodes that can be moved in one shot, in order to
keep spinlock hold time under control.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 5185fad..6caa982 100644 (file)
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -248,15 +248,16 @@ static bool inode_dirtied_after(struct inode *inode, unsigned long t)
 /*
  * Move expired dirty inodes from @delaying_queue to @dispatch_queue.
  */
-static void move_expired_inodes(struct list_head *delaying_queue,
+static int move_expired_inodes(struct list_head *delaying_queue,
                               struct list_head *dispatch_queue,
-                               unsigned long *older_than_this)
+                              unsigned long *older_than_this)
 {
        LIST_HEAD(tmp);
        struct list_head *pos, *node;
        struct super_block *sb = NULL;
        struct inode *inode;
        int do_sb_sort = 0;
+       int moved = 0;
 
        while (!list_empty(delaying_queue)) {
                inode = wb_inode(delaying_queue->prev);
@@ -267,12 +268,13 @@ static void move_expired_inodes(struct list_head *delaying_queue,
                        do_sb_sort = 1;
                sb = inode->i_sb;
                list_move(&inode->i_wb_list, &tmp);
+               moved++;
        }
 
        /* just one sb in list, splice to dispatch_queue and we're done */
        if (!do_sb_sort) {
                list_splice(&tmp, dispatch_queue);
-               return;
+               goto out;
        }
 
        /* Move inodes from one superblock together */
@@ -284,6 +286,8 @@ static void move_expired_inodes(struct list_head *delaying_queue,
                                list_move(&inode->i_wb_list, dispatch_queue);
                }
        }
+out:
+       return moved;
 }
 
 /*
@@ -299,9 +303,11 @@ static void move_expired_inodes(struct list_head *delaying_queue,
  */
 static void queue_io(struct bdi_writeback *wb, unsigned long *older_than_this)
 {
+       int moved;
        assert_spin_locked(&wb->list_lock);
        list_splice_init(&wb->b_more_io, &wb->b_io);
-       move_expired_inodes(&wb->b_dirty, &wb->b_io, older_than_this);
+       moved = move_expired_inodes(&wb->b_dirty, &wb->b_io, older_than_this);
+       trace_writeback_queue_io(wb, older_than_this, moved);
 }
 
 static int write_inode(struct inode *inode, struct writeback_control *wbc)

diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 898277b..205d149 100644 (file)
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -162,6 +162,31 @@ DEFINE_WBC_EVENT(wbc_balance_dirty_written);
 DEFINE_WBC_EVENT(wbc_balance_dirty_wait);
 DEFINE_WBC_EVENT(wbc_writepage);
 
+TRACE_EVENT(writeback_queue_io,
+       TP_PROTO(struct bdi_writeback *wb,
+                unsigned long *older_than_this,
+                int moved),
+       TP_ARGS(wb, older_than_this, moved),
+       TP_STRUCT__entry(
+               __array(char,           name, 32)
+               __field(unsigned long,  older)
+               __field(long,           age)
+               __field(int,            moved)
+       ),
+       TP_fast_assign(
+               strncpy(__entry->name, dev_name(wb->bdi->dev), 32);
+               __entry->older  = older_than_this ?  *older_than_this : 0;
+               __entry->age    = older_than_this ?
+                                 (jiffies - *older_than_this) * 1000 / HZ : -1;
+               __entry->moved  = moved;
+       ),
+       TP_printk("bdi %s: older=%lu age=%ld enqueue=%d",
+               __entry->name,
+               __entry->older, /* older_than_this in jiffies */
+               __entry->age,   /* older_than_this in relative milliseconds */
+               __entry->moved)
+);
+
 DECLARE_EVENT_CLASS(writeback_congest_waited_template,
 
        TP_PROTO(unsigned int usec_timeout, unsigned int usec_delayed),