Re: endless sync on bdi_sched_wait()? 2.6.33.1

[Dave Chinner <david@fromorbit.com>]

On Wed, Apr 21, 2010 at 03:27:18PM +0200, Jan Kara wrote:
> On Wed 21-04-10 11:54:28, Dave Chinner wrote:
> > On Wed, Apr 21, 2010 at 02:33:09AM +0200, Jan Kara wrote:
> > > On Mon 19-04-10 17:04:58, Dave Chinner wrote:
> > > > The third flush - the sync one - does:
.....
> > > > some 75 seconds later having written only 1024 pages. In the mean
> > > > time, the traces show dd blocked in balance_dirty_pages():
.....
> > > > And it appears to stay blocked there without doing any writeback at
> > > > all - there are no wbc_balance_dirty_pages_written traces at all.
> > > > That is, it is blocking until the number of dirty pages is dropping
> > > > below the dirty threshold, then continuing to write and dirty more
> > > > pages.
> > >   I think this happens because sync writeback is running so I_SYNC is set
> > > and thus we cannot do any writeout for the inode from balance_dirty_pages.
> > 
> > It's not even calling into writeback so the I_SYNC flag is way out of
> > scope ;)
>   Are you sure? The tracepoints are in wb_writeback() but
> writeback_inodes_wbc() calls directly into writeback_inodes_wb() so you
> won't see any of the tracepoints to trigger. So how do you know we didn't
> get to writeback_single_inode?

The balance_dirty_pages() tracing code added this hunk:

@@ -536,11 +537,13 @@ static void balance_dirty_pages(struct address_space *mapping,
                 * threshold otherwise wait until the disk writes catch
                 * up.
                 */
+               trace_wbc_balance_dirty_start(&wbc);
                if (bdi_nr_reclaimable > bdi_thresh) {
                        writeback_inodes_wbc(&wbc);
                        pages_written += write_chunk - wbc.nr_to_write;
                        get_dirty_limits(&background_thresh, &dirty_thresh,
                                       &bdi_thresh, bdi);
+                       trace_wbc_balance_dirty_written(&wbc);
                }

                /*

So if we tried to do writeback from here, the
wbc_balance_dirty_written trace would have been emitted, and that is
not showing up very often in any of the traces. e.g:

$ grep balance t.t |grep start |wc -l
4356
$ grep balance t.t |grep wait |wc -l
2171
$ grep balance t.t |grep written |wc -l
7

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com