Re: 2.6.23-rc6-mm1 -- mkfs stuck in 'D'

Hugh Dickins
2007-09-20T11:59:09Z

On Wed, 19 Sep 2007, Peter Zijlstra wrote:
> On Wed, 19 Sep 2007 21:03:19 +0100 (BST) Hugh Dickins
> <hugh@veritas.com> wrote:
>
> > On Wed, 19 Sep 2007, Andy Whitcroft wrote:
> > > Seems I have a case of a largish i386 NUMA (NUMA-Q) which has a mkfs
> > > stuck in a 'D' wait:
> > >
> > > =======================
> > > mkfs.ext2 D c10220f4 0 6233 6222
> > > [<c12194da>] io_schedule_timeout+0x1e/0x28
> > > [<c10454b4>] congestion_wait+0x62/0x7a
> > > [<c10402af>] get_dirty_limits+0x16a/0x172
> > > [<c104040b>] balance_dirty_pages+0x154/0x1be
> > > [<c103bda3>] generic_perform_write+0x168/0x18a
> > > [<c103be38>] generic_file_buffered_write+0x73/0x107
> > > [<c103c346>] __generic_file_aio_write_nolock+0x47a/0x4a5
> > > [<c103c3b9>] generic_file_aio_write_nolock+0x48/0x9b
> > > [<c105d2d6>] do_sync_write+0xbf/0xfc
> > > [<c105d3a0>] vfs_write+0x8d/0x108
> > > [<c105d4c3>] sys_write+0x41/0x67
> > > [<c100260a>] syscall_call+0x7/0xb
> > > =======================
> >
> > [edited out some bogus lines from stale stack]
> >
> > > This machine and others have run numerous test runs on this kernel and
> > > this is the first time I've see a hang like this.
> >
> > I've been seeing something like that on 4-way PPC64: in my case I've
> > shells hanging in D state trying to append to kernel build log on ext3
> > (the builds themselves going on elsewhere, in tmpfs): one of the shells
> > holding i_mutex and stuck doing congestion_waits from balance_dirty_pages.
> >
> > > I wonder if this is the ultimate cause of the couple of mainline hangs
> > > which were seen, but not diagnosed.
> >
> > My *guess* is that this is peculiar to 2.6.23-rc6-mm1, and from Peter's
> > mm-per-device-dirty-threshold.patch. printks showed bdi_nr_reclaimable
> > 0, bdi_nr_writeback 24, bdi_thresh 1 in balance_dirty_pages (though I've
> > not done enough to check if those really correlate with the hangs),
> > and I'm wondering if the bdi_stat_sum business is needed on the
> > !nr_reclaimable path.
>
> FWIW my tired brain seems to think it the !nr_reclaimable path needs it
> just the same. So this change seems to make sense for now :-)

Thanks.

> > So I'm running now with the patch below, good so far, but can't judge
> > until tomorrow whether it has actually addressed the problem seen.

Last night's run went well: that patch does indeed seem to have fixed it.
Looking at the timings (some variance but _very_ much less than the night
before), there does appear to be some other occasional slight slowdown -
but I've no reason to suspect your patch for it, nor to suppose it's
something new: it may just be an artifact of my heavy swap thrashing.

[PATCH mm] mm per-device dirty threshold fix

Fix occasional hang when a task couldn't get out of balance_dirty_pages:
mm-per-device-dirty-threshold.patch needs to reevaluate bdi_nr_writeback
across all cpus when bdi_thresh is low, even in the case when there was
no bdi_nr_reclaimable.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
---
mm/page-writeback.c | 53 +++++++++++++++++++-----------------------
1 file changed, 24 insertions(+), 29 deletions(-)

--- 2.6.23-rc6-mm1/mm/page-writeback.c 2007-09-18 12:28:25.000000000 +0100
+++ linux/mm/page-writeback.c 2007-09-19 20:00:46.000000000 +0100
@@ -379,7 +379,7 @@ static void balance_dirty_pages(struct a
bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
- break;
+ break;

if (!bdi->dirty_exceeded)
bdi->dirty_exceeded = 1;
@@ -392,39 +392,34 @@ static void balance_dirty_pages(struct a
*/
if (bdi_nr_reclaimable) {
writeback_inodes(&wbc);
-
+ pages_written += write_chunk - wbc.nr_to_write;
get_dirty_limits(&background_thresh, &dirty_thresh,
&bdi_thresh, bdi);
+ }

- /*
- * In order to avoid the stacked BDI deadlock we need
- * to ensure we accurately count the 'dirty' pages when
- * the threshold is low.
- *
- * Otherwise it would be possible to get thresh+n pages
- * reported dirty, even though there are thresh-m pages
- * actually dirty; with m+n sitting in the percpu
- * deltas.
- */
- if (bdi_thresh < 2*bdi_stat_error(bdi)) {
- bdi_nr_reclaimable =
- bdi_stat_sum(bdi, BDI_RECLAIMABLE);
- bdi_nr_writeback =
- bdi_stat_sum(bdi, BDI_WRITEBACK);
- } else {
- bdi_nr_reclaimable =
- bdi_stat(bdi, BDI_RECLAIMABLE);
- bdi_nr_writeback =
- bdi_stat(bdi, BDI_WRITEBACK);
- }
+ /*
+ * In order to avoid the stacked BDI deadlock we need
+ * to ensure we accurately count the 'dirty' pages when
+ * the threshold is low.
+ *
+ * Otherwise it would be possible to get thresh+n pages
+ * reported dirty, even though there are thresh-m pages
+ * actually dirty; with m+n sitting in the percpu
+ * deltas.
+ */
+ if (bdi_thresh < 2*bdi_stat_error(bdi)) {
+ bdi_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
+ bdi_nr_writeback = bdi_stat_sum(bdi, BDI_WRITEBACK);
+ } else if (bdi_nr_reclaimable) {
+ bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
+ bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
+ }

- if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
- break;
+ if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
+ break;
+ if (pages_written >= write_chunk)
+ break; /* We've done our duty */

- pages_written += write_chunk - wbc.nr_to_write;
- if (pages_written >= write_chunk)
- break; /* We've done our duty */
- }
congestion_wait(WRITE, HZ/10);
}