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

[richard kennedy <richard@rsk.demon.co.uk>]

On Tue, 2007-10-02 at 10:00 +0800, Fengguang Wu wrote:
> On Mon, Oct 01, 2007 at 11:57:34AM -0400, Chuck Ebbert wrote:
> > On 09/29/2007 07:04 AM, Fengguang Wu wrote:
...
> 
> (expecting real world confirmations...)
> 
> Here is a new safer version.  It's more ugly though.
> 
> ---
> writeback: avoid possible balance_dirty_pages() lockup on a light-load bdi
> 
> On a busy-writing system, a writer could be hold up infinitely on a
> light-load device. It will be trying to sync more than available dirty data.
> 
> The problem case:
> 
> 0. sda/nr_dirty >= dirty_limit;
>    sdb/nr_dirty == 0
> 1. dd writes 32 pages on sdb
> 2. balance_dirty_pages() blocks dd, and tries to write 6MB.
> 3. it never gets there: there's only 128KB dirty data.
> 4. dd may be blocked for a loooong time
> 
> Fix it by returning on 'zero dirty inodes' in the current bdi.
> (In fact there are slight differences between 'dirty inodes' and 'dirty pages'.
> But there is no available counters for 'dirty pages'.)
> 
> But the newly introduced 'break' could make the nr_writeback drift away
> above the dirty limit. The workaround is to limit the error under 1MB.
> 
> Cc: Chuck Ebbert <cebbert@redhat.com>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
> ---
>  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.

Richard