On Wed, 19 Sep 2007 21:03:19 +0100 (BST) Hugh Dickins
<hugh@veritas.com> wrote:

quoted text
> 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 :-)

quoted text
> 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.
> 
> Not-yet-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);
>  	}
>  
-
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