Re: [PATCH 2/6] writeback: the kupdate expire timestamp should be a moving target

Jan Kara
2011-04-19T09:33:03Z

On Tue 19-04-11 15:20:40, Wu Fengguang wrote:
> On Tue, Apr 19, 2011 at 03:02:47PM +0800, Dave Chinner wrote:
> > On Tue, Apr 19, 2011 at 11:00:05AM +0800, Wu Fengguang wrote:
> > > Dynamically compute the dirty expire timestamp at queue_io() time.
> > >
> > > writeback_control.older_than_this used to be determined at entrance to
> > > the kupdate writeback work. This _static_ timestamp may go stale if the
> > > kupdate work runs on and on. The flusher may then stuck with some old
> > > busy inodes, never considering newly expired inodes thereafter.
> > >
> > > This has two possible problems:
> > >
> > > - It is unfair for a large dirty inode to delay (for a long time) the
> > > writeback of small dirty inodes.
> > >
> > > - As time goes by, the large and busy dirty inode may contain only
> > > _freshly_ dirtied pages. Ignoring newly expired dirty inodes risks
> > > delaying the expired dirty pages to the end of LRU lists, triggering
> > > the evil pageout(). Nevertheless this patch merely addresses part
> > > of the problem.
> >
> > When wb_writeback() is called with for_kupdate set, it initialises
> > wbc->older_than_this appropriately outside the writeback loop.
> > queue_io() is called once per writeback_inodes_wb() call, which is
> > once per loop in wb_writeback. All your change does is re-initialise
> > older_than_this once per loop in wb_writeback, jus tin a different
> > and very non-obvious place.
> >
> > So why didn't you just re-initialise it inside the loop in
> > wb_writeback() and leave all the other code alone?
>
> It helps both readability and efficiency to make it a local var.
>
> I have another patch to kill the wbc->older_than_this (and one more
> for wbc->more_io). They are delayed to avoid possible merge conflicts
> with the IO-less patchset.
>
> But yeah, it seems reasonable to move the first chunk of the below
> patch to this one.
I agree - killing of wbc.older_than_this would be a logical part of
this patch as well.

Honza
> ---
> Subject: writeback: the kupdate expire timestamp should be a moving target
> Date: Wed Jul 21 20:32:30 CST 2010
>
> Remove writeback_control.older_than_this which is no longer used.
>
> [kitayama@cl.bb4u.ne.jp] fix btrfs and ext4 references
>
> Acked-by: Jan Kara <jack@suse.cz>
> Acked-by: Mel Gorman <mel@csn.ul.ie>
> Signed-off-by: Itaru Kitayama <kitayama@cl.bb4u.ne.jp>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
> fs/btrfs/extent_io.c | 2 --
> fs/fs-writeback.c | 13 -------------
> include/linux/writeback.h | 2 --
> include/trace/events/writeback.h | 6 +-----
> mm/backing-dev.c | 1 -
> 5 files changed, 1 insertion(+), 23 deletions(-)
>
> --- linux-next.orig/fs/fs-writeback.c 2011-04-18 08:37:01.000000000 +0800
> +++ linux-next/fs/fs-writeback.c 2011-04-18 08:38:16.000000000 +0800
> @@ -681,30 +681,20 @@ static unsigned long writeback_chunk_siz
> * Try to run once per dirty_writeback_interval. But if a writeback event
> * takes longer than a dirty_writeback_interval interval, then leave a
> * one-second gap.
> - *
> - * older_than_this takes precedence over nr_to_write. So we'll only write back
> - * all dirty pages if they are all attached to "old" mappings.
> */
> static long wb_writeback(struct bdi_writeback *wb,
> struct wb_writeback_work *work)
> {
> struct writeback_control wbc = {
> .sync_mode = work->sync_mode,
> - .older_than_this = NULL,
> .for_kupdate = work->for_kupdate,
> .for_background = work->for_background,
> .range_cyclic = work->range_cyclic,
> };
> - unsigned long oldest_jif;
> long wrote = 0;
> long write_chunk;
> struct inode *inode;
>
> - if (wbc.for_kupdate) {
> - wbc.older_than_this = &oldest_jif;
> - oldest_jif = jiffies -
> - msecs_to_jiffies(dirty_expire_interval * 10);
> - }
> if (!wbc.range_cyclic) {
> wbc.range_start = 0;
> wbc.range_end = LLONG_MAX;
> @@ -1139,9 +1129,6 @@ EXPORT_SYMBOL(__mark_inode_dirty);
> * Write out a superblock's list of dirty inodes. A wait will be performed
> * upon no inodes, all inodes or the final one, depending upon sync_mode.
> *
> - * If older_than_this is non-NULL, then only write out inodes which
> - * had their first dirtying at a time earlier than *older_than_this.
> - *
> * If `bdi' is non-zero then we're being asked to writeback a specific queue.
> * This function assumes that the blockdev superblock's inodes are backed by
> * a variety of queues, so all inodes are searched. For other superblocks,
> --- linux-next.orig/include/linux/writeback.h 2011-04-18 08:36:59.000000000 +0800
> +++ linux-next/include/linux/writeback.h 2011-04-18 08:38:16.000000000 +0800
> @@ -66,8 +66,6 @@ enum writeback_sync_modes {
> */
> struct writeback_control {
> enum writeback_sync_modes sync_mode;
> - unsigned long *older_than_this; /* If !NULL, only write back inodes
> - older than this */
> unsigned long wb_start; /* Time writeback_inodes_wb was
> called. This is needed to avoid
> extra jobs and livelock */
> --- linux-next.orig/include/trace/events/writeback.h 2011-04-18 08:36:59.000000000 +0800
> +++ linux-next/include/trace/events/writeback.h 2011-04-18 08:38:16.000000000 +0800
> @@ -115,7 +115,6 @@ DECLARE_EVENT_CLASS(wbc_class,
> __field(int, for_reclaim)
> __field(int, range_cyclic)
> __field(int, more_io)
> - __field(unsigned long, older_than_this)
> __field(long, range_start)
> __field(long, range_end)
> ),
> @@ -130,14 +129,12 @@ DECLARE_EVENT_CLASS(wbc_class,
> __entry->for_reclaim = wbc->for_reclaim;
> __entry->range_cyclic = wbc->range_cyclic;
> __entry->more_io = wbc->more_io;
> - __entry->older_than_this = wbc->older_than_this ?
> - *wbc->older_than_this : 0;
> __entry->range_start = (long)wbc->range_start;
> __entry->range_end = (long)wbc->range_end;
> ),
>
> TP_printk("bdi %s: towrt=%ld skip=%ld mode=%d kupd=%d "
> - "bgrd=%d reclm=%d cyclic=%d more=%d older=0x%lx "
> + "bgrd=%d reclm=%d cyclic=%d more=%d "
> "start=0x%lx end=0x%lx",
> __entry->name,
> __entry->nr_to_write,
> @@ -148,7 +145,6 @@ DECLARE_EVENT_CLASS(wbc_class,
> __entry->for_reclaim,
> __entry->range_cyclic,
> __entry->more_io,
> - __entry->older_than_this,
> __entry->range_start,
> __entry->range_end)
> )
> --- linux-next.orig/mm/backing-dev.c 2011-04-18 08:36:59.000000000 +0800
> +++ linux-next/mm/backing-dev.c 2011-04-18 08:38:16.000000000 +0800
> @@ -263,7 +263,6 @@ static void bdi_flush_io(struct backing_
> {
> struct writeback_control wbc = {
> .sync_mode = WB_SYNC_NONE,
> - .older_than_this = NULL,
> .range_cyclic = 1,
> .nr_to_write = 1024,
> };
> --- linux-next.orig/fs/btrfs/extent_io.c 2011-04-18 08:36:59.000000000 +0800
> +++ linux-next/fs/btrfs/extent_io.c 2011-04-18 08:38:16.000000000 +0800
> @@ -2556,7 +2556,6 @@ int extent_write_full_page(struct extent
> };
> struct writeback_control wbc_writepages = {
> .sync_mode = wbc->sync_mode,
> - .older_than_this = NULL,
> .nr_to_write = 64,
> .range_start = page_offset(page) + PAGE_CACHE_SIZE,
> .range_end = (loff_t)-1,
> @@ -2589,7 +2588,6 @@ int extent_write_locked_range(struct ext
> };
> struct writeback_control wbc_writepages = {
> .sync_mode = mode,
> - .older_than_this = NULL,
> .nr_to_write = nr_pages * 2,
> .range_start = start,
> .range_end = end + 1,
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR