Re: [PATCH] fs: Fix busyloop in wb_writeback()

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

On Mon, Sep 21, 2009 at 01:43:56AM +0800, Jan Kara wrote:
> On Sun 20-09-09 10:35:28, Wu Fengguang wrote:
> > On Thu, Sep 17, 2009 at 01:22:48AM +0800, Jan Kara wrote:
> > > If all inodes are under writeback (e.g. in case when there's only one inode
> > > with dirty pages), wb_writeback() with WB_SYNC_NONE work basically degrades
> > > to busylooping until I_SYNC flags of the inode is cleared. Fix the problem by
> > > waiting on I_SYNC flags of an inode on b_more_io list in case we failed to
> > > write anything.
> > 
> > Sorry, I realized that inode_wait_for_writeback() waits for I_SYNC.
> > But inodes in b_more_io are not expected to have I_SYNC set. So your
> > patch looks like a big no-op?
>   Hmm, I don't think so. writeback_single_inode() does:
>         if (inode->i_state & I_SYNC) {
>                 /*
>                  * If this inode is locked for writeback and we are not
>                  * doing
>                  * writeback-for-data-integrity, move it to b_more_io so
>                  * that
>                  * writeback can proceed with the other inodes on s_io.
>                  *
>                  * We'll have another go at writing back this inode when we
>                  * completed a full scan of b_io.
>                  */
>                 if (!wait) {
>                         requeue_io(inode);
>                         return 0;
>                 }
> 
>   So when we see inode under writeback, we put it to b_more_io. So I think
> my patch really fixes the issue when two threads are racing on writing the
> same inode.

Ah OK. So it busy loops when there are more syncing threads than dirty
files. For example, one bdi flush thread plus one process running
balance_dirty_pages().

> > The busy loop does exists, when bdi is congested.
> > In this case, write_cache_pages() will refuse to write anything,
> > we used to be calling congestion_wait() to take a breath, but now
> > wb_writeback() purged that call and thus created a busy loop.
>   I don't think congestion is an issue here. The device needen't be
> congested for the busyloop to happen.

bdi congestion is a different case. When there are only one syncing
thread, b_more_io inodes won't have I_SYNC, so your patch is a no-op.
wb_writeback() or any of its sub-routines must wait/yield for a while
to avoid busy looping on the congestion. Where is the wait with Jens'
new code?

Another question is, why wbc.more_io can be ignored for kupdate syncs?
I guess it would lead to slow writeback of large files.

This patch reflects my concerns on the two problems.

Thanks,
Fengguang
---
 fs/fs-writeback.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- linux.orig/fs/fs-writeback.c	2009-09-20 10:44:25.000000000 +0800
+++ linux/fs/fs-writeback.c	2009-09-21 08:53:09.000000000 +0800
@@ -818,8 +818,10 @@ static long wb_writeback(struct bdi_writ
 		/*
 		 * If we ran out of stuff to write, bail unless more_io got set
 		 */
-		if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) {
-			if (wbc.more_io && !wbc.for_kupdate)
+		if (wbc.nr_to_write > 0) {
+			if (wbc.encountered_congestion)
+				congestion_wait(BLK_RW_ASYNC, HZ);
+			if (wbc.more_io)
 				continue;
 			break;
 		}