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Date: Mon, 21 Sep 2009 13:35:46 +0800
From: Wu Fengguang <fengguang.wu@...el.com>
To: Jan Kara <jack@...e.cz>
Cc: Theodore Tso <tytso@....edu>, Jens Axboe <jens.axboe@...cle.com>,
Christoph Hellwig <hch@...radead.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
"chris.mason@...cle.com" <chris.mason@...cle.com>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/7] Per-bdi writeback flusher threads v20
On Mon, Sep 21, 2009 at 11:04:02AM +0800, Wu Fengguang wrote:
> On Mon, Sep 21, 2009 at 03:00:06AM +0800, Jan Kara wrote:
> > On Sat 19-09-09 23:03:51, Wu Fengguang wrote:
> > > On Sat, Sep 19, 2009 at 12:26:07PM +0800, Wu Fengguang wrote:
> > > > On Sat, Sep 19, 2009 at 12:00:51PM +0800, Wu Fengguang wrote:
> > > > > On Sat, Sep 19, 2009 at 11:58:35AM +0800, Wu Fengguang wrote:
> > > > > > On Sat, Sep 19, 2009 at 01:52:52AM +0800, Theodore Tso wrote:
> > > > > > > On Fri, Sep 11, 2009 at 10:39:29PM +0800, Wu Fengguang wrote:
> > > > > > > >
> > > > > > > > That would be good. Sorry for the late work. I'll allocate some time
> > > > > > > > in mid next week to help review and benchmark recent writeback works,
> > > > > > > > and hope to get things done in this merge window.
> > > > > > >
> > > > > > > Did you have some chance to get more work done on the your writeback
> > > > > > > patches?
> > > > > >
> > > > > > Sorry for the delay, I'm now testing the patches with commands
> > > > > >
> > > > > > cp /dev/zero /mnt/test/zero0 &
> > > > > > dd if=/dev/zero of=/mnt/test/zero1 &
> > > > > >
> > > > > > and the attached debug patch.
> > > > > >
> > > > > > One problem I found with ext3/4 is, redirty_tail() is called repeatedly
> > > > > > in the traces, which could slow down the inode writeback significantly.
> > > > >
> > > > > FYI, it's this redirty_tail() called in writeback_single_inode():
> > > > >
> > > > > /*
> > > > > * Someone redirtied the inode while were writing back
> > > > > * the pages.
> > > > > */
> > > > > redirty_tail(inode);
> > > >
> > > > Hmm, this looks like an old fashioned problem get blew up by the
> > > > 128MB MAX_WRITEBACK_PAGES.
> > > >
> > > > The inode was redirtied by the busy cp/dd processes. Now it takes much
> > > > more time to sync 128MB, so that a heavy dirtier can easily redirty
> > > > the inode in that time window.
> > > >
> > > > One single invocation of redirty_tail() could hold up the writeback of
> > > > current inode for up to 30 seconds.
> > >
> > > It seems that this patch helps. However I'm afraid it's too late to
> > > risk merging such kind of patches now..
> > Fenguang, could we maybe write down how the logic should look like
> > and then look at the code and modify it as needed to fit the logic?
> > Because I couldn't find a compact description of the logic anywhere
> > in the code.
>
> Good idea. It makes sense to write something down in Documentation/
> or embedded as code comments.
>
> > Here is how I'd imaging the writeout logic should work:
> > We would have just two lists - b_dirty and b_more_io. Both would be
> > ordered by dirtied_when.
>
> Andrew has a very good description for the dirty/io/more_io queues:
>
> http://lkml.org/lkml/2006/2/7/5
>
> | So the protocol would be:
> |
> | s_io: contains expired and non-expired dirty inodes, with expired ones at
> | the head. Unexpired ones (at least) are in time order.
> |
> | s_more_io: contains dirty expired inodes which haven't been fully written.
> | Ordering doesn't matter (unless someone goes and changes
> | dirty_expire_centisecs - but as long as we don't do anything really bad in
> | response to this we'll be OK).
> |
> | s_dirty: contains expired and non-expired dirty inodes. The non-expired
> | ones are in time-of-dirtying order.
>
> Since then s_io was changed to hold only _expired_ dirty inodes at the
> beginning of a full scan. It serves as a bounded set of dirty inodes.
> So that when finished a full scan of it, the writeback can go on to
> the next superblock, and old dirty files' writeback won't be delayed
> infinitely by poring in newly dirty files.
>
> It seems that the boundary could also be provided by some
> older_than_this timestamp. So removal of b_io is possible
> at least on this purpose.
Yeah, this is a scratch patch to remove b_io, I see no obvious
difficulties in doing so.
Thanks,
Fengguang
---
fs/btrfs/extent_io.c | 2 -
fs/fs-writeback.c | 65 +++++++++-------------------------
include/linux/backing-dev.h | 2 -
include/linux/writeback.h | 4 +-
mm/backing-dev.c | 1
mm/page-writeback.c | 1
6 files changed, 21 insertions(+), 54 deletions(-)
--- linux.orig/fs/fs-writeback.c 2009-09-21 13:12:56.000000000 +0800
+++ linux/fs/fs-writeback.c 2009-09-21 13:12:57.000000000 +0800
@@ -284,7 +284,7 @@ static void redirty_tail(struct inode *i
}
/*
- * requeue inode for re-scanning after bdi->b_io list is exhausted.
+ * requeue inode for re-scanning.
*/
static void requeue_io(struct inode *inode)
{
@@ -317,32 +317,6 @@ static bool inode_dirtied_after(struct i
return ret;
}
-/*
- * Move expired dirty inodes from @delaying_queue to @dispatch_queue.
- */
-static void move_expired_inodes(struct list_head *delaying_queue,
- struct list_head *dispatch_queue,
- unsigned long *older_than_this)
-{
- while (!list_empty(delaying_queue)) {
- struct inode *inode = list_entry(delaying_queue->prev,
- struct inode, i_list);
- if (older_than_this &&
- inode_dirtied_after(inode, *older_than_this))
- break;
- list_move(&inode->i_list, dispatch_queue);
- }
-}
-
-/*
- * Queue all expired dirty inodes for io, eldest first.
- */
-static void queue_io(struct bdi_writeback *wb, unsigned long *older_than_this)
-{
- list_splice_init(&wb->b_more_io, wb->b_io.prev);
- move_expired_inodes(&wb->b_dirty, &wb->b_io, older_than_this);
-}
-
static int write_inode(struct inode *inode, int sync)
{
if (inode->i_sb->s_op->write_inode && !is_bad_inode(inode))
@@ -399,7 +373,7 @@ writeback_single_inode(struct inode *ino
* writeback can proceed with the other inodes on s_io.
*
* We'll have another go at writing back this inode when we
- * completed a full scan of b_io.
+ * completed a full scan.
*/
if (!wait) {
requeue_io(inode);
@@ -540,11 +514,11 @@ static void writeback_inodes_wb(struct b
spin_lock(&inode_lock);
- if (!wbc->for_kupdate || list_empty(&wb->b_io))
- queue_io(wb, wbc->older_than_this);
+ if (list_empty(&wb->b_dirty))
+ list_splice_init(&wb->b_more_io, &wb->b_dirty);
- while (!list_empty(&wb->b_io)) {
- struct inode *inode = list_entry(wb->b_io.prev,
+ while (!list_empty(&wb->b_dirty)) {
+ struct inode *inode = list_entry(wb->b_dirty.prev,
struct inode, i_list);
long pages_skipped;
@@ -590,8 +564,12 @@ static void writeback_inodes_wb(struct b
* Was this inode dirtied after sync_sb_inodes was called?
* This keeps sync from extra jobs and livelock.
*/
- if (inode_dirtied_after(inode, start))
- break;
+ if (inode_dirtied_after(inode, wbc->older_than_this)) {
+ if (list_empty(&wb->b_more_io))
+ break;
+ list_splice_init(&wb->b_more_io, wb->b_dirty.prev);
+ continue;
+ }
if (pin_sb_for_writeback(wbc, inode)) {
requeue_io(inode);
@@ -623,7 +601,7 @@ static void writeback_inodes_wb(struct b
}
spin_unlock(&inode_lock);
- /* Leave any unwritten inodes on b_io */
+ /* Leave any unwritten inodes on b_dirty */
}
void writeback_inodes_wbc(struct writeback_control *wbc)
@@ -674,18 +652,18 @@ static long wb_writeback(struct bdi_writ
.bdi = wb->bdi,
.sb = args->sb,
.sync_mode = args->sync_mode,
- .older_than_this = NULL,
.for_kupdate = args->for_kupdate,
.range_cyclic = args->range_cyclic,
};
unsigned long oldest_jif;
long wrote = 0;
- if (wbc.for_kupdate) {
- wbc.older_than_this = &oldest_jif;
- oldest_jif = jiffies -
+ if (wbc.for_kupdate)
+ wbc.older_than_this = jiffies -
msecs_to_jiffies(dirty_expire_interval * 10);
- }
+ else
+ wbc.older_than_this = jiffies;
+
if (!wbc.range_cyclic) {
wbc.range_start = 0;
wbc.range_end = LLONG_MAX;
@@ -1004,7 +982,7 @@ void __mark_inode_dirty(struct inode *in
goto out;
/*
- * If the inode was already on b_dirty/b_io/b_more_io, don't
+ * If the inode was already on b_dirty/b_more_io, don't
* reposition it (that would break b_dirty time-ordering).
*/
if (!was_dirty) {
@@ -1041,11 +1019,6 @@ EXPORT_SYMBOL(__mark_inode_dirty);
* This function assumes that the blockdev superblock's inodes are backed by
* a variety of queues, so all inodes are searched. For other superblocks,
* assume that all inodes are backed by the same queue.
- *
- * The inodes to be written are parked on bdi->b_io. They are moved back onto
- * bdi->b_dirty as they are selected for writing. This way, none can be missed
- * on the writer throttling path, and we get decent balancing between many
- * throttled threads: we don't want them all piling up on inode_sync_wait.
*/
static void wait_sb_inodes(struct super_block *sb)
{
--- linux.orig/fs/btrfs/extent_io.c 2009-09-21 13:12:24.000000000 +0800
+++ linux/fs/btrfs/extent_io.c 2009-09-21 13:12:57.000000000 +0800
@@ -2467,7 +2467,6 @@ int extent_write_full_page(struct extent
struct writeback_control wbc_writepages = {
.bdi = wbc->bdi,
.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,
@@ -2501,7 +2500,6 @@ int extent_write_locked_range(struct ext
struct writeback_control wbc_writepages = {
.bdi = inode->i_mapping->backing_dev_info,
.sync_mode = mode,
- .older_than_this = NULL,
.nr_to_write = nr_pages * 2,
.range_start = start,
.range_end = end + 1,
--- linux.orig/include/linux/writeback.h 2009-09-21 13:12:24.000000000 +0800
+++ linux/include/linux/writeback.h 2009-09-21 13:12:57.000000000 +0800
@@ -32,8 +32,8 @@ struct writeback_control {
struct super_block *sb; /* if !NULL, only write inodes from
this super_block */
enum writeback_sync_modes sync_mode;
- unsigned long *older_than_this; /* If !NULL, only write back inodes
- older than this */
+ unsigned long older_than_this; /* only write back inodes older than
+ this */
long nr_to_write; /* Write this many pages, and decrement
this for each page written */
long pages_skipped; /* Pages which were not written */
--- linux.orig/mm/backing-dev.c 2009-09-21 13:12:24.000000000 +0800
+++ linux/mm/backing-dev.c 2009-09-21 13:12:57.000000000 +0800
@@ -333,7 +333,6 @@ static void bdi_flush_io(struct backing_
struct writeback_control wbc = {
.bdi = bdi,
.sync_mode = WB_SYNC_NONE,
- .older_than_this = NULL,
.range_cyclic = 1,
.nr_to_write = 1024,
};
--- linux.orig/mm/page-writeback.c 2009-09-21 13:12:56.000000000 +0800
+++ linux/mm/page-writeback.c 2009-09-21 13:12:57.000000000 +0800
@@ -492,7 +492,6 @@ static void balance_dirty_pages(struct a
struct writeback_control wbc = {
.bdi = bdi,
.sync_mode = WB_SYNC_NONE,
- .older_than_this = NULL,
.nr_to_write = write_chunk,
.range_cyclic = 1,
};
--- linux.orig/include/linux/backing-dev.h 2009-09-21 13:12:24.000000000 +0800
+++ linux/include/linux/backing-dev.h 2009-09-21 13:12:57.000000000 +0800
@@ -53,7 +53,6 @@ struct bdi_writeback {
struct task_struct *task; /* writeback task */
struct list_head b_dirty; /* dirty inodes */
- struct list_head b_io; /* parked for writeback */
struct list_head b_more_io; /* parked for more writeback */
};
@@ -111,7 +110,6 @@ extern struct list_head bdi_list;
static inline int wb_has_dirty_io(struct bdi_writeback *wb)
{
return !list_empty(&wb->b_dirty) ||
- !list_empty(&wb->b_io) ||
!list_empty(&wb->b_more_io);
}
--
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