Messages in this thread Patch in this message | | | Date | Wed, 23 Sep 2009 18:30:55 +0800 | From | Wu Fengguang <> | Subject | Re: regression in page writeback |
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On Wed, Sep 23, 2009 at 05:37:53PM +0800, Wu Fengguang wrote: > On Wed, Sep 23, 2009 at 05:23:31PM +0800, Peter Zijlstra wrote: > > On Wed, 2009-09-23 at 10:19 +0100, Richard Kennedy wrote: > > > > > > I am concerned that the background writeout no longer stops when it > > > reaches the background threshold, as balance_dirty_pages requests all > > > dirty pages to be written. No doubt this is good for large linear writes > > > but what about more random write workloads? > > > > I've not had time to look over the current code, but write-out not > > stopping on reaching background threshold is a definite bug and needs to > > get fixed. > > Yes, 2.6.31 code stops writeback when background threshold is reached. > But new behavior in latest git is to writeback all pages. > > The code only checks over_bground_thresh() for kupdate works: > > if (args->for_kupdate && args->nr_pages <= 0 && > !over_bground_thresh()) > break; > > However the background work started by balance_dirty_pages() won't check > over_bground_thresh(). So it will move all dirty pages. > > I think it's very weird to check over_bground_thresh() for kupdate > instead of background work. Jens must intended for the latter case.
Here is the patch to fix it. Tested to work OK. This is an RFC.
Thanks, Fengguang --- writeback: stop background writeback when below background threshold
Treat bdi_start_writeback(0) as a special request to do background write, and stop such work when we are below the background dirty threshold.
Also simplify the (nr_pages <= 0) checks. Since we already pass in nr_pages=LONG_MAX for WB_SYNC_ALL and background writes, we don't need to worry about it being decreased to zero.
Reported-by: Richard Kennedy <richard@rsk.demon.co.uk> CC: Jan Kara <jack@suse.cz> CC: Jens Axboe <jens.axboe@oracle.com> CC: Peter Zijlstra <a.p.zijlstra@chello.nl> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com> --- fs/fs-writeback.c | 28 +++++++++++++++++----------- mm/page-writeback.c | 6 +++--- 2 files changed, 20 insertions(+), 14 deletions(-)
--- linux.orig/fs/fs-writeback.c 2009-09-23 17:47:23.000000000 +0800 +++ linux/fs/fs-writeback.c 2009-09-23 18:13:36.000000000 +0800 @@ -41,8 +41,9 @@ struct wb_writeback_args { long nr_pages; struct super_block *sb; enum writeback_sync_modes sync_mode; - int for_kupdate; - int range_cyclic; + int for_kupdate:1; + int range_cyclic:1; + int for_background:1; }; /* @@ -260,6 +261,15 @@ void bdi_start_writeback(struct backing_ .range_cyclic = 1, }; + /* + * We treat @nr_pages=0 as the special case to do background writeback, + * ie. to sync pages until the background dirty threshold is reached. + */ + if (!nr_pages) { + args.nr_pages = LONG_MAX; + args.for_background = 1; + } + bdi_alloc_queue_work(bdi, &args); } @@ -723,20 +733,16 @@ static long wb_writeback(struct bdi_writ for (;;) { /* - * Don't flush anything for non-integrity writeback where - * no nr_pages was given + * Stop writeback when nr_pages has been consumed */ - if (!args->for_kupdate && args->nr_pages <= 0 && - args->sync_mode == WB_SYNC_NONE) + if (args->nr_pages <= 0) break; /* - * If no specific pages were given and this is just a - * periodic background writeout and we are below the - * background dirty threshold, don't do anything + * For background writeout, stop when we are below the + * background dirty threshold */ - if (args->for_kupdate && args->nr_pages <= 0 && - !over_bground_thresh()) + if (args->for_background && !over_bground_thresh()) break; wbc.more_io = 0; --- linux.orig/mm/page-writeback.c 2009-09-23 17:45:58.000000000 +0800 +++ linux/mm/page-writeback.c 2009-09-23 17:47:17.000000000 +0800 @@ -589,10 +589,10 @@ static void balance_dirty_pages(struct a * background_thresh, to keep the amount of dirty memory low. */ if ((laptop_mode && pages_written) || - (!laptop_mode && ((nr_writeback = global_page_state(NR_FILE_DIRTY) - + global_page_state(NR_UNSTABLE_NFS)) + (!laptop_mode && ((global_page_state(NR_FILE_DIRTY) + + global_page_state(NR_UNSTABLE_NFS)) > background_thresh))) - bdi_start_writeback(bdi, nr_writeback); + bdi_start_writeback(bdi, 0); } void set_page_dirty_balance(struct page *page, int page_mkwrite)
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