Re: [PATCH] writeback: avoid possible balance_dirty_pages() lockup on a light-load bdi

Fengguang Wu
2007-10-04T01:51:15Z

On Wed, Oct 03, 2007 at 01:46:52PM +0100, richard kennedy wrote:
> On Tue, 2007-10-02 at 10:00 +0800, Fengguang Wu wrote:
> > ---
> > mm/page-writeback.c | 5 +++++
> > 1 file changed, 5 insertions(+)
> >
> > --- linux-2.6.22.orig/mm/page-writeback.c
> > +++ linux-2.6.22/mm/page-writeback.c
> > @@ -250,6 +250,11 @@ static void balance_dirty_pages(struct a
> > pages_written += write_chunk - wbc.nr_to_write;
> > if (pages_written >= write_chunk)
> > break; /* We've done our duty */
> > + if (list_empty(&mapping->host->i_sb->s_dirty) &&
> > + list_empty(&mapping->host->i_sb->s_io) &&
> > + nr_reclaimable + global_page_state(NR_WRITEBACK) <=
> > + dirty_thresh + (1 << (20-PAGE_CACHE_SHIFT)))
> > + break;
> > }
> > congestion_wait(WRITE, HZ/10);
> > }
>
> I've been testing 2.6.23-rc9 + this patch all morning but have just seen
> a lockup. As usual it happened just after a large file copy finished
> and while nr_dirty is still large. I'm sorry to say I didn't have a
> serial console running so I don't have an other info. I will try again
> and see if I can capture some more data.
>
> I did notice that at the beginning of my tests the dirty blocks are
> written back more quickly than usual
>
> nr_dirty count after the copy finished and then 60 seconds later :-
> after copy +60 seconds
> 73520 0
> 73533 0
> 68554 1
>
> but after several iterations of my testcase & just before the lockup
> 68560 57165
> 71974 62896
>
> which is about the same as a unpatched kernel.

Hi Richard,

Thank you for the testing.

However, my patch is kind of duplicate efforts. I was taking the
'do it if simple' attitude. I can continue to improve it if you
really want it. Otherwise I'd recommend you to test the coming
2.6.24-rc1 or backport the -mm writeback patches back to 2.6.23 and
test it there. Peter has did a good job on it.

Fengguang