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

Wu Fengguang
2011-04-19T07:20:47Z

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.

Thanks,
Fengguang
---
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,