[PATCH 3/4] xfs: revert to using a kthread for AIL pushing

[Markus Trippelsdorf]

On 2011.10.10 at 12:45 +1100, Dave Chinner wrote:
> On Thu, Oct 06, 2011 at 02:33:00PM -0400, Christoph Hellwig wrote:
> > Currently we have a few issues with the way the workqueue code is used to
> > implement AIL pushing:
> > 
> >  - it accidentally uses the same workqueue as the syncer action, and thus
> >    can be prevented from running if there are enough sync actions active
> >    in the system.
> >  - it doesn't use the HIGHPRI flag to queue at the head of the queue of
> >    work items
> > 
> > At this point I'm not confident enough in getting all the workqueue flags and
> > tweaks right to provide a perfectly reliable execution context for AIL
> > pushing, which is the most important piece in XFS to make forward progress
> > when the log fills.
> > 
> > Revert back to use a kthread per filesystem which fixes all the above issues
> > at the cost of having a task struct and stack around for each mounted
> > filesystem.  In addition this also gives us much better ways to diagnose
> > any issues involving hung AIL pushing and removes a small amount of code.
> > 
> > Signed-off-by: Christoph Hellwig <hch at lst.de>
> > Reported-by: Stefan Priebe <s.priebe at profihost.ag>
> > Tested-by: Stefan Priebe <s.priebe at profihost.ag>
> 
> I'd much prefer to fix the problems with the workqueue usage than
> revert back to using a thread, but seeing as I cannot reproduce the
> hangs I can't really track down whatever problem there is. So,
> a bit reluctantly:

Wouldn't it be possible to verify that the problem also goes away with
this simple one liner?

diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 2366c54..daf30c9 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1654,7 +1654,7 @@ xfs_init_workqueues(void)
 	if (!xfs_syncd_wq)
 		goto out;
 
-	xfs_ail_wq = alloc_workqueue("xfsail", WQ_CPU_INTENSIVE, 8);
+	xfs_ail_wq = alloc_workqueue("xfsail", WQ_HIGHPRI | WQ_CPU_INTENSIVE, 8);
 	if (!xfs_ail_wq)
 		goto out_destroy_syncd;