Re: [PATCH, RFC] writeback: avoid redirtying when ->write_inode failed to clear I_DIRTY

[Jan Kara <jack@suse.cz>]

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.

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?

								Honza

> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > 
> > Index: linux-2.6/fs/fs-writeback.c
> > ===================================================================
> > --- linux-2.6.orig/fs/fs-writeback.c	2011-08-26 14:47:42.137050059 +0200
> > +++ linux-2.6/fs/fs-writeback.c	2011-08-26 15:06:47.003493601 +0200
> > @@ -464,8 +464,18 @@ writeback_single_inode(struct inode *ino
> >  			 * operations, such as delayed allocation during
> >  			 * submission or metadata updates after data IO
> >  			 * completion.
> > +			 *
> > +			 * For the latter case it is very important to give
> > +			 * the inode another turn on b_more_io instead of
> > +			 * redirtying it.  Constantly moving dirtied_when
> > +			 * forward will prevent us from ever writing out
> > +			 * the metadata dirtied in the I/O completion handler.
> > +			 *
> > +			 * For files on XFS that constantly get appended to
> > +			 * calling redirty_tail means they will never get
> > +			 * their updated i_size written out.
> >  			 */
> > -			redirty_tail(inode, wb);
> > +			requeue_io(inode, wb);
> >  		} else {
> >  			/*
> >  			 * The inode is clean.  At this point we either have

> 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.
> 
> 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           |   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);
>  	}

-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR