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Subject[PATCH 1/7] writeback: introduce queue b_more_io_wait
The problem is, redirty_tail() may update i_dirtied_when and result in
30s max delay. If redirty_tail() is called often enough, some inode may
even be delayed for ever.

So introduce the b_more_io_wait queue to park inodes that for some
reason cannot be synced immediately. The inodes will be sent to b_io at
the next b_io refill time, however won't be busy retried as b_more_io:
when the redirtied inodes are all in b_more_io_wait, wb_writeback() will
see empty b_more_io and hence break out of the loop.

This would be the new data flow after converting all redirty_tail()
calls to requeue_io_wait():

b_dirty --> b_io --> b_more_io/b_more_io_wait --+
^ |
| |
+----------------------------------+

Cc: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
fs/fs-writeback.c | 16 ++++++++++++++++
include/linux/backing-dev.h | 8 +++++---
mm/backing-dev.c | 10 ++++++++--
3 files changed, 29 insertions(+), 5 deletions(-)

--- linux-next.orig/fs/fs-writeback.c 2011-10-20 23:13:48.000000000 +0800
+++ linux-next/fs/fs-writeback.c 2011-10-20 23:19:12.000000000 +0800
@@ -234,6 +234,21 @@ static void requeue_io(struct inode *ino
list_move(&inode->i_wb_list, &wb->b_more_io);
}

+/*
+ * The inode should be retried in an opportunistic way.
+ *
+ * The only difference between b_more_io and b_more_io_wait is:
+ * wb_writeback() won't quit as long as b_more_io is not empty. When
+ * wb_writeback() quit on empty b_more_io and non-empty b_more_io_wait,
+ * the kupdate work will wakeup more frequently to retry the inodes in
+ * b_more_io_wait.
+ */
+static void requeue_io_wait(struct inode *inode, struct bdi_writeback *wb)
+{
+ assert_spin_locked(&wb->list_lock);
+ list_move(&inode->i_wb_list, &wb->b_more_io_wait);
+}
+
static void inode_sync_complete(struct inode *inode)
{
/*
@@ -321,6 +336,7 @@ static void queue_io(struct bdi_writebac
int moved;
assert_spin_locked(&wb->list_lock);
list_splice_init(&wb->b_more_io, &wb->b_io);
+ list_splice_init(&wb->b_more_io_wait, &wb->b_io);
moved = move_expired_inodes(&wb->b_dirty, &wb->b_io, work);
trace_writeback_queue_io(wb, work, moved);
}
--- linux-next.orig/include/linux/backing-dev.h 2011-10-20 23:13:48.000000000 +0800
+++ linux-next/include/linux/backing-dev.h 2011-10-20 23:13:50.000000000 +0800
@@ -59,6 +59,7 @@ struct bdi_writeback {
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 */
+ struct list_head b_more_io_wait;/* opportunistic retry io */
spinlock_t list_lock; /* protects the b_* lists */
};

@@ -133,9 +134,10 @@ extern struct list_head bdi_pending_list

static inline int wb_has_dirty_io(struct bdi_writeback *wb)
{
- return !list_empty(&wb->b_dirty) ||
- !list_empty(&wb->b_io) ||
- !list_empty(&wb->b_more_io);
+ return !list_empty(&wb->b_dirty) ||
+ !list_empty(&wb->b_io) ||
+ !list_empty(&wb->b_more_io) ||
+ !list_empty(&wb->b_more_io_wait);
}

static inline void __add_bdi_stat(struct backing_dev_info *bdi,
--- linux-next.orig/mm/backing-dev.c 2011-10-20 23:13:48.000000000 +0800
+++ linux-next/mm/backing-dev.c 2011-10-20 23:13:50.000000000 +0800
@@ -74,10 +74,10 @@ static int bdi_debug_stats_show(struct s
unsigned long background_thresh;
unsigned long dirty_thresh;
unsigned long bdi_thresh;
- unsigned long nr_dirty, nr_io, nr_more_io;
+ unsigned long nr_dirty, nr_io, nr_more_io, nr_more_io_wait;
struct inode *inode;

- nr_dirty = nr_io = nr_more_io = 0;
+ nr_dirty = nr_io = nr_more_io = nr_more_io_wait = 0;
spin_lock(&wb->list_lock);
list_for_each_entry(inode, &wb->b_dirty, i_wb_list)
nr_dirty++;
@@ -85,6 +85,8 @@ static int bdi_debug_stats_show(struct s
nr_io++;
list_for_each_entry(inode, &wb->b_more_io, i_wb_list)
nr_more_io++;
+ list_for_each_entry(inode, &wb->b_more_io_wait, i_wb_list)
+ nr_more_io_wait++;
spin_unlock(&wb->list_lock);

global_dirty_limits(&background_thresh, &dirty_thresh);
@@ -103,6 +105,7 @@ static int bdi_debug_stats_show(struct s
"b_dirty: %10lu\n"
"b_io: %10lu\n"
"b_more_io: %10lu\n"
+ "b_more_io_wait: %10lu\n"
"bdi_list: %10u\n"
"state: %10lx\n",
(unsigned long) K(bdi_stat(bdi, BDI_WRITEBACK)),
@@ -116,6 +119,7 @@ static int bdi_debug_stats_show(struct s
nr_dirty,
nr_io,
nr_more_io,
+ nr_more_io_wait,
!list_empty(&bdi->bdi_list), bdi->state);
#undef K

@@ -651,6 +655,7 @@ static void bdi_wb_init(struct bdi_write
INIT_LIST_HEAD(&wb->b_dirty);
INIT_LIST_HEAD(&wb->b_io);
INIT_LIST_HEAD(&wb->b_more_io);
+ INIT_LIST_HEAD(&wb->b_more_io_wait);
spin_lock_init(&wb->list_lock);
setup_timer(&wb->wakeup_timer, wakeup_timer_fn, (unsigned long)bdi);
}
@@ -718,6 +723,7 @@ void bdi_destroy(struct backing_dev_info
list_splice(&bdi->wb.b_dirty, &dst->b_dirty);
list_splice(&bdi->wb.b_io, &dst->b_io);
list_splice(&bdi->wb.b_more_io, &dst->b_more_io);
+ list_splice(&bdi->wb.b_more_io_wait, &dst->b_more_io_wait);
spin_unlock(&bdi->wb.list_lock);
spin_unlock(&dst->list_lock);
}



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