On Sat, Sep 03, 2011 at 09:13:15AM +0800, Jan Kara wrote:
> On Sat 27-08-11 21:58:25, Wu Fengguang wrote:
> > Christoph,
> >
> > On Sat, Aug 27, 2011 at 02:14:09PM +0800, Christoph Hellwig wrote:
> > > Right now ->write_inode has no way to safely return a EAGAIN without explicitly
> > > redirtying the inode, as we would lose the dirty state otherwise. Most
> > > filesystems get this wrong, but XFS makes heavy use of it to avoid blocking
> > > the flusher thread when ->write_inode hits contentended inode locks. A
> > > contended ilock is something XFS can hit very easibly when extending files, as
> > > the data I/O completion handler takes the lock to update the size, and the
> > > ->write_inode call can race with it fairly easily if writing enough data
> > > in one go so that the completion for the first write come in just before
> > > we call ->write_inode.
> > >
> > > Change the handling of this case to use requeue_io for a quick retry instead
> > > of redirty_tail, which keeps moving out the dirtied_when data and thus keeps
> > > delaying the writeout more and more with every failed attempt to get the lock.
> >
> > Yeah redirty_tail() does have the problem of possibly delay inodes for
> > too long time. However, you know requeue_io() always has the danger of
> > triggering busy wb_writeback() loops in corner cases.
> >
> > For example, nfs_write_inode()/nfs_commit_unstable_pages() often
> > redirty the inode without doing anything (hence no any progress, a
> > prerequisite for busy loops) depending on the in flight writes, which
> > unfortunately further depends on _external_ network/server states..
> > That means some stalled network/sever state could lead to busy loops
> > in NFS clients.
> >
> > The alternative solution may be to firstly apply the attached patch,
> > and change this one to:
> >
> > - redirty_tail(inode, wb);
> > + requeue_io_wait(inode, wb);
> But your patch doesn't solve the busyloop when the problematic inodes are
> the only ones under writeback, does it? Then b_more_io and b_more_io_wait
> are effectively the same if I understand it right.
The difference lies in the
/*
* No more inodes for IO, bail
*/
if (list_empty(&wb->b_more_io))
break;
check in wb_writeback(). So when what's left are all b_more_io_wait
inodes, the above check will take effect and break the loop. This is
the tricky point of the patch: it relies on the code not touched by
the patch to work. I've updated the changelog to explain this.
> I think that busylooping in cases like these could be fixed improving the
> busyloop prevention at the end of the loop in wb_writeback(). Maybe we
> could just take a short nap before continuting with writeback instead of /
> in addition to waiting for inode writeback. What do you think?
That's a reasonable robust option, however at the cost of keeping the
writeback code in some ambiguous state ;)
Thanks,
Fengguang
---
Subject: writeback: introduce queue b_more_io_wait
Date: Sun Jul 31 18:44:44 CST 2011
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.
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@xxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
fs/fs-writeback.c | 10 ++++++++++
include/linux/backing-dev.h | 8 +++++---
mm/backing-dev.c | 10 ++++++++--
3 files changed, 23 insertions(+), 5 deletions(-)
--- linux-next.orig/fs/fs-writeback.c 2011-08-27 15:28:27.000000000 +0800
+++ linux-next/fs/fs-writeback.c 2011-08-27 15:45:10.000000000 +0800
@@ -220,6 +220,15 @@ 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.
+ */
+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)
{
/*
@@ -307,6 +316,7 @@ 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, older_than_this);
trace_writeback_queue_io(wb, older_than_this, moved);
}
--- linux-next.orig/include/linux/backing-dev.h 2011-08-26 19:27:20.000000000 +0800
+++ linux-next/include/linux/backing-dev.h 2011-08-27 15:45:10.000000000 +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 */
};
@@ -129,9 +130,10 @@ 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 2011-08-26 19:27:20.000000000 +0800
+++ linux-next/mm/backing-dev.c 2011-08-27 15:45:10.000000000 +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
@@ -635,6 +639,7 @@ 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);
}
@@ -701,6 +706,7 @@ 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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