Re: [PATCH 4/6] writeback: sync expired inodes first in background writeback

[Wu Fengguang <fengguang.wu@intel.com>]

> > @@ -232,8 +232,15 @@ static void move_expired_inodes(struct l
> >  	while (!list_empty(delaying_queue)) {
> >  		inode = list_entry(delaying_queue->prev, struct inode, i_list);
> >  		if (expire_interval &&
> > -		    inode_dirtied_after(inode, older_than_this))
> > -			break;
> > +		    inode_dirtied_after(inode, older_than_this)) {
> > +			if (wbc->for_background &&
> > +			    list_empty(dispatch_queue) && list_empty(&tmp)) {
> > +				expire_interval >>= 1;
> > +				older_than_this = jiffies - expire_interval;
> > +				continue;
> > +			} else
> > +				break;
> > +		}
> 
> This needs a comment.
> 
> I think what it is saying is that if background flush is active but no
> inodes are old enough, consider newer inodes. This is on the assumption
> that page reclaim has encountered dirty pages and the dirty inodes are
> still too young.

Yes this should be commented. How about this one?

@@ -232,8 +232,20 @@ static void move_expired_inodes(struct l
        while (!list_empty(delaying_queue)) {
                inode = list_entry(delaying_queue->prev, struct inode, i_list);
                if (expire_interval &&
-                   inode_dirtied_after(inode, older_than_this))
+                   inode_dirtied_after(inode, older_than_this)) {
+                       /*
+                        * background writeback will start with expired inodes,
+                        * and then fresh inodes. This order helps reducing
+                        * the number of dirty pages reaching the end of LRU
+                        * lists and cause trouble to the page reclaim.
+                        */
+                       if (wbc->for_background &&
+                           list_empty(dispatch_queue) && list_empty(&tmp)) {
+                               expire_interval = 0;
+                               continue;
+                       }
                        break;
+               }
                if (sb && sb != inode->i_sb)
                        do_sb_sort = 1;
                sb = inode->i_sb;

Thanks,
Fengguang