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SubjectRe: [PATCH 5/7] writeback: make writeback_control.nr_to_write straight
On Sun, Jun 19, 2011 at 11:35:36PM +0800, Christoph Hellwig wrote:
> I'd rather see this earlier in the series, before the write bandwith
> estimation ones, as it's groundwork independ of those changes.

OK, done.

> > + if (wrote) {
> > + if (jiffies - start_time > HZ / 10UL)
> > + break;
> > + if (work->nr_pages <= 0)
> > + break;
> > + }
>
> This code probably wants some comments.

It's duplicated tests as in writeback_sb_inodes(), which have the comments

/*
* bail out to wb_writeback() often enough to check
* background threshold and other termination conditions.
*/
I'll add a simple comment here:

/* refer to the same tests at the end of writeback_sb_inodes */
> > static void bdi_flush_io(struct backing_dev_info *bdi)
> > {
> > - struct writeback_control wbc = {
> > - .sync_mode = WB_SYNC_NONE,
> > - .older_than_this = NULL,
> > - .range_cyclic = 1,
> > - .nr_to_write = 1024,
> > - };
> > -
> > - writeback_inodes_wb(&bdi->wb, &wbc);
> > + writeback_inodes_wb(&bdi->wb, 1024);
> > }
>
> At this point you could probably also kill the bdi_flush_io wrapper, and
> just call writeback_inodes_wb directly.

Fair enough.

> A comment on the 1024 pages to
> write would be nice, if you remember it from poking the code. I can't
> find any good explanation for it offhand.

I comment it with

Hopefully 1024 is large enough for efficient IO.

Thanks,
Fengguang
---
Subject: writeback: make writeback_control.nr_to_write straight
Date: Wed May 04 19:54:37 CST 2011
Pass struct wb_writeback_work all the way down to writeback_sb_inodes(),
and initialize the struct writeback_control there.

struct writeback_control is basically designed to control writeback of a
single file, but we keep abuse it for writing multiple files in
writeback_sb_inodes() and its callers.

It immediately clean things up, e.g. suddenly wbc.nr_to_write vs
work->nr_pages starts to make sense, and instead of saving and restoring
pages_skipped in writeback_sb_inodes it can always start with a clean
zero value.

It also makes a neat IO pattern change: large dirty files are now
written in the full 4MB writeback chunk size, rather than whatever
remained quota in wbc->nr_to_write.

Acked-by: Jan Kara <jack@suse.cz>
Proposed-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
fs/btrfs/extent_io.c | 2
fs/fs-writeback.c | 196 ++++++++++++++++-------------
include/linux/writeback.h | 6
include/trace/events/writeback.h | 39 ++++-
mm/backing-dev.c | 17 --
mm/page-writeback.c | 17 --
6 files changed, 148 insertions(+), 129 deletions(-)
change set:
- move writeback_control from wb_writeback() down to writeback_sb_inodes()
- change return value of writeback_sb_inodes()/__writeback_inodes_wb()
to the number of pages and/or inodes written
- move writeback_control.older_than_this to struct wb_writeback_work
- remove writeback_control.inodes_written
- remove wbc_writeback_* trace events for now

--- linux-next.orig/fs/fs-writeback.c 2011-06-19 23:57:41.000000000 +0800
+++ linux-next/fs/fs-writeback.c 2011-06-20 00:08:00.000000000 +0800
@@ -30,11 +30,21 @@
#include "internal.h"

/*
+ * The maximum number of pages to writeout in a single bdi flush/kupdate
+ * operation. We do this so we don't hold I_SYNC against an inode for
+ * enormous amounts of time, which would block a userspace task which has
+ * been forced to throttle against that inode. Also, the code reevaluates
+ * the dirty each time it has written this many pages.
+ */
+#define MAX_WRITEBACK_PAGES 1024L
+
+/*
* Passed into wb_writeback(), essentially a subset of writeback_control
*/
struct wb_writeback_work {
long nr_pages;
struct super_block *sb;
+ unsigned long *older_than_this;
enum writeback_sync_modes sync_mode;
unsigned int tagged_writepages:1;
unsigned int for_kupdate:1;
@@ -472,7 +482,6 @@ writeback_single_inode(struct inode *ino
* No need to add it back to the LRU.
*/
list_del_init(&inode->i_wb_list);
- wbc->inodes_written++;
}
}
inode_sync_complete(inode);
@@ -506,6 +515,31 @@ static bool pin_sb_for_writeback(struct
return false;
}

+static long writeback_chunk_size(struct wb_writeback_work *work)
+{
+ long pages;
+
+ /*
+ * WB_SYNC_ALL mode does livelock avoidance by syncing dirty
+ * inodes/pages in one big loop. Setting wbc.nr_to_write=LONG_MAX
+ * here avoids calling into writeback_inodes_wb() more than once.
+ *
+ * The intended call sequence for WB_SYNC_ALL writeback is:
+ *
+ * wb_writeback()
+ * writeback_sb_inodes() <== called only once
+ * write_cache_pages() <== called once for each inode
+ * (quickly) tag currently dirty pages
+ * (maybe slowly) sync all tagged pages
+ */
+ if (work->sync_mode == WB_SYNC_ALL || work->tagged_writepages)
+ pages = LONG_MAX;
+ else
+ pages = min(MAX_WRITEBACK_PAGES, work->nr_pages);
+
+ return pages;
+}
+
/*
* Write a portion of b_io inodes which belong to @sb.
*
@@ -513,18 +547,30 @@ static bool pin_sb_for_writeback(struct
* inodes. Otherwise write only ones which go sequentially
* in reverse order.
*
- * Return 1, if the caller writeback routine should be
- * interrupted. Otherwise return 0.
+ * Return the number of pages and/or inodes written.
*/
-static int writeback_sb_inodes(struct super_block *sb, struct bdi_writeback *wb,
- struct writeback_control *wbc, bool only_this_sb)
+static long writeback_sb_inodes(struct super_block *sb,
+ struct bdi_writeback *wb,
+ struct wb_writeback_work *work)
{
+ struct writeback_control wbc = {
+ .sync_mode = work->sync_mode,
+ .tagged_writepages = work->tagged_writepages,
+ .for_kupdate = work->for_kupdate,
+ .for_background = work->for_background,
+ .range_cyclic = work->range_cyclic,
+ .range_start = 0,
+ .range_end = LLONG_MAX,
+ };
+ unsigned long start_time = jiffies;
+ long write_chunk;
+ long wrote = 0; /* count both pages and inodes */
+
while (!list_empty(&wb->b_io)) {
- long pages_skipped;
struct inode *inode = wb_inode(wb->b_io.prev);

if (inode->i_sb != sb) {
- if (only_this_sb) {
+ if (work->sb) {
/*
* We only want to write back data for this
* superblock, move all inodes not belonging
@@ -539,7 +585,7 @@ static int writeback_sb_inodes(struct su
* Bounce back to the caller to unpin this and
* pin the next superblock.
*/
- return 0;
+ break;
}

/*
@@ -553,12 +599,18 @@ static int writeback_sb_inodes(struct su
requeue_io(inode, wb);
continue;
}
-
__iget(inode);
+ write_chunk = writeback_chunk_size(work);
+ wbc.nr_to_write = write_chunk;
+ wbc.pages_skipped = 0;
+
+ writeback_single_inode(inode, wb, &wbc);

- pages_skipped = wbc->pages_skipped;
- writeback_single_inode(inode, wb, wbc);
- if (wbc->pages_skipped != pages_skipped) {
+ work->nr_pages -= write_chunk - wbc.nr_to_write;
+ wrote += write_chunk - wbc.nr_to_write;
+ if (!(inode->i_state & I_DIRTY))
+ wrote++;
+ if (wbc.pages_skipped) {
/*
* writeback is not making progress due to locked
* buffers. Skip this inode for now.
@@ -570,17 +622,25 @@ static int writeback_sb_inodes(struct su
iput(inode);
cond_resched();
spin_lock(&wb->list_lock);
- if (wbc->nr_to_write <= 0)
- return 1;
+ /*
+ * bail out to wb_writeback() often enough to check
+ * background threshold and other termination conditions.
+ */
+ if (wrote) {
+ if (jiffies - start_time > HZ / 10UL)
+ break;
+ if (work->nr_pages <= 0)
+ break;
+ }
}
- /* b_io is empty */
- return 1;
+ return wrote;
}

-static void __writeback_inodes_wb(struct bdi_writeback *wb,
- struct writeback_control *wbc)
+static long __writeback_inodes_wb(struct bdi_writeback *wb,
+ struct wb_writeback_work *work)
{
- int ret = 0;
+ unsigned long start_time = jiffies;
+ long wrote = 0;

while (!list_empty(&wb->b_io)) {
struct inode *inode = wb_inode(wb->b_io.prev);
@@ -590,33 +650,37 @@ static void __writeback_inodes_wb(struct
requeue_io(inode, wb);
continue;
}
- ret = writeback_sb_inodes(sb, wb, wbc, false);
+ wrote += writeback_sb_inodes(sb, wb, work);
drop_super(sb);

- if (ret)
- break;
+ /* refer to the same tests at the end of writeback_sb_inodes */
+ if (wrote) {
+ if (jiffies - start_time > HZ / 10UL)
+ break;
+ if (work->nr_pages <= 0)
+ break;
+ }
}
/* Leave any unwritten inodes on b_io */
+ return wrote;
}

-void writeback_inodes_wb(struct bdi_writeback *wb,
- struct writeback_control *wbc)
+long writeback_inodes_wb(struct bdi_writeback *wb, long nr_pages)
{
+ struct wb_writeback_work work = {
+ .nr_pages = nr_pages,
+ .sync_mode = WB_SYNC_NONE,
+ .range_cyclic = 1,
+ };
+
spin_lock(&wb->list_lock);
if (list_empty(&wb->b_io))
- queue_io(wb, wbc->older_than_this);
- __writeback_inodes_wb(wb, wbc);
+ queue_io(wb, NULL);
+ __writeback_inodes_wb(wb, &work);
spin_unlock(&wb->list_lock);
-}

-/*
- * The maximum number of pages to writeout in a single bdi flush/kupdate
- * operation. We do this so we don't hold I_SYNC against an inode for
- * enormous amounts of time, which would block a userspace task which has
- * been forced to throttle against that inode. Also, the code reevaluates
- * the dirty each time it has written this many pages.
- */
-#define MAX_WRITEBACK_PAGES 1024
+ return nr_pages - work.nr_pages;
+}

static inline bool over_bground_thresh(void)
{
@@ -646,42 +710,13 @@ static inline bool over_bground_thresh(v
static long wb_writeback(struct bdi_writeback *wb,
struct wb_writeback_work *work)
{
- struct writeback_control wbc = {
- .sync_mode = work->sync_mode,
- .tagged_writepages = work->tagged_writepages,
- .older_than_this = NULL,
- .for_kupdate = work->for_kupdate,
- .for_background = work->for_background,
- .range_cyclic = work->range_cyclic,
- };
+ long nr_pages = work->nr_pages;
unsigned long oldest_jif;
- long wrote = 0;
- long write_chunk = MAX_WRITEBACK_PAGES;
struct inode *inode;
-
- if (!wbc.range_cyclic) {
- wbc.range_start = 0;
- wbc.range_end = LLONG_MAX;
- }
-
- /*
- * WB_SYNC_ALL mode does livelock avoidance by syncing dirty
- * inodes/pages in one big loop. Setting wbc.nr_to_write=LONG_MAX
- * here avoids calling into writeback_inodes_wb() more than once.
- *
- * The intended call sequence for WB_SYNC_ALL writeback is:
- *
- * wb_writeback()
- * writeback_sb_inodes() <== called only once
- * write_cache_pages() <== called once for each inode
- * (quickly) tag currently dirty pages
- * (maybe slowly) sync all tagged pages
- */
- if (wbc.sync_mode == WB_SYNC_ALL || wbc.tagged_writepages)
- write_chunk = LONG_MAX;
+ long progress;

oldest_jif = jiffies;
- wbc.older_than_this = &oldest_jif;
+ work->older_than_this = &oldest_jif;

spin_lock(&wb->list_lock);
for (;;) {
@@ -711,24 +746,17 @@ static long wb_writeback(struct bdi_writ
if (work->for_kupdate) {
oldest_jif = jiffies -
msecs_to_jiffies(dirty_expire_interval * 10);
- wbc.older_than_this = &oldest_jif;
+ work->older_than_this = &oldest_jif;
}

- wbc.nr_to_write = write_chunk;
- wbc.pages_skipped = 0;
- wbc.inodes_written = 0;
-
- trace_wbc_writeback_start(&wbc, wb->bdi);
+ trace_writeback_start(wb->bdi, work);
if (list_empty(&wb->b_io))
- queue_io(wb, wbc.older_than_this);
+ queue_io(wb, work->older_than_this);
if (work->sb)
- writeback_sb_inodes(work->sb, wb, &wbc, true);
+ progress = writeback_sb_inodes(work->sb, wb, work);
else
- __writeback_inodes_wb(wb, &wbc);
- trace_wbc_writeback_written(&wbc, wb->bdi);
-
- work->nr_pages -= write_chunk - wbc.nr_to_write;
- wrote += write_chunk - wbc.nr_to_write;
+ progress = __writeback_inodes_wb(wb, work);
+ trace_writeback_written(wb->bdi, work);

/*
* Did we write something? Try for more
@@ -738,9 +766,7 @@ static long wb_writeback(struct bdi_writ
* mean the overall work is done. So we keep looping as long
* as made some progress on cleaning pages or inodes.
*/
- if (wbc.nr_to_write < write_chunk)
- continue;
- if (wbc.inodes_written)
+ if (progress)
continue;
/*
* No more inodes for IO, bail
@@ -753,8 +779,8 @@ static long wb_writeback(struct bdi_writ
* we'll just busyloop.
*/
if (!list_empty(&wb->b_more_io)) {
+ trace_writeback_wait(wb->bdi, work);
inode = wb_inode(wb->b_more_io.prev);
- trace_wbc_writeback_wait(&wbc, wb->bdi);
spin_lock(&inode->i_lock);
inode_wait_for_writeback(inode, wb);
spin_unlock(&inode->i_lock);
@@ -762,7 +788,7 @@ static long wb_writeback(struct bdi_writ
}
spin_unlock(&wb->list_lock);

- return wrote;
+ return nr_pages - work->nr_pages;
}

/*
--- linux-next.orig/include/linux/writeback.h 2011-06-19 23:57:41.000000000 +0800
+++ linux-next/include/linux/writeback.h 2011-06-20 00:03:25.000000000 +0800
@@ -24,12 +24,9 @@ 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 */
long nr_to_write; /* Write this many pages, and decrement
this for each page written */
long pages_skipped; /* Pages which were not written */
- long inodes_written; /* # of inodes written (at least) */

/*
* For a_ops->writepages(): is start or end are non-zero then this is
@@ -56,8 +53,7 @@ void writeback_inodes_sb_nr(struct super
int writeback_inodes_sb_if_idle(struct super_block *);
int writeback_inodes_sb_nr_if_idle(struct super_block *, unsigned long nr);
void sync_inodes_sb(struct super_block *);
-void writeback_inodes_wb(struct bdi_writeback *wb,
- struct writeback_control *wbc);
+long writeback_inodes_wb(struct bdi_writeback *wb, long nr_pages);
long wb_do_writeback(struct bdi_writeback *wb, int force_wait);
void wakeup_flusher_threads(long nr_pages);

--- linux-next.orig/mm/backing-dev.c 2011-06-19 23:57:41.000000000 +0800
+++ linux-next/mm/backing-dev.c 2011-06-20 00:13:48.000000000 +0800
@@ -260,18 +260,6 @@ int bdi_has_dirty_io(struct backing_dev_
return wb_has_dirty_io(&bdi->wb);
}

-static void bdi_flush_io(struct backing_dev_info *bdi)
-{
- struct writeback_control wbc = {
- .sync_mode = WB_SYNC_NONE,
- .older_than_this = NULL,
- .range_cyclic = 1,
- .nr_to_write = 1024,
- };
-
- writeback_inodes_wb(&bdi->wb, &wbc);
-}
-
/*
* kupdated() used to do this. We cannot do it from the bdi_forker_thread()
* or we risk deadlocking on ->s_umount. The longer term solution would be
@@ -457,9 +445,10 @@ static int bdi_forker_thread(void *ptr)
if (IS_ERR(task)) {
/*
* If thread creation fails, force writeout of
- * the bdi from the thread.
+ * the bdi from the thread. Hopefully 1024 is
+ * large enough for efficient IO.
*/
- bdi_flush_io(bdi);
+ writeback_inodes_wb(&bdi->wb, 1024);
} else {
/*
* The spinlock makes sure we do not lose
--- linux-next.orig/mm/page-writeback.c 2011-06-19 23:58:14.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-06-20 00:03:25.000000000 +0800
@@ -492,13 +492,6 @@ static void balance_dirty_pages(struct a
struct backing_dev_info *bdi = mapping->backing_dev_info;

for (;;) {
- struct writeback_control wbc = {
- .sync_mode = WB_SYNC_NONE,
- .older_than_this = NULL,
- .nr_to_write = write_chunk,
- .range_cyclic = 1,
- };
-
nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
global_page_state(NR_UNSTABLE_NFS);
nr_writeback = global_page_state(NR_WRITEBACK);
@@ -560,17 +553,17 @@ static void balance_dirty_pages(struct a
* threshold otherwise wait until the disk writes catch
* up.
*/
- trace_wbc_balance_dirty_start(&wbc, bdi);
+ trace_balance_dirty_start(bdi);
if (bdi_nr_reclaimable > bdi_thresh) {
- writeback_inodes_wb(&bdi->wb, &wbc);
- pages_written += write_chunk - wbc.nr_to_write;
- trace_wbc_balance_dirty_written(&wbc, bdi);
+ pages_written += writeback_inodes_wb(&bdi->wb,
+ write_chunk);
+ trace_balance_dirty_written(bdi, pages_written);
if (pages_written >= write_chunk)
break; /* We've done our duty */
}
- trace_wbc_balance_dirty_wait(&wbc, bdi);
__set_current_state(TASK_UNINTERRUPTIBLE);
io_schedule_timeout(pause);
+ trace_balance_dirty_wait(bdi);

/*
* Increase the delay for each loop, up to our previous
--- linux-next.orig/include/trace/events/writeback.h 2011-06-19 23:58:12.000000000 +0800
+++ linux-next/include/trace/events/writeback.h 2011-06-19 23:58:23.000000000 +0800
@@ -62,6 +62,9 @@ DEFINE_EVENT(writeback_work_class, name,
DEFINE_WRITEBACK_WORK_EVENT(writeback_nothread);
DEFINE_WRITEBACK_WORK_EVENT(writeback_queue);
DEFINE_WRITEBACK_WORK_EVENT(writeback_exec);
+DEFINE_WRITEBACK_WORK_EVENT(writeback_start);
+DEFINE_WRITEBACK_WORK_EVENT(writeback_written);
+DEFINE_WRITEBACK_WORK_EVENT(writeback_wait);

TRACE_EVENT(writeback_pages_written,
TP_PROTO(long pages_written),
@@ -101,6 +104,30 @@ DEFINE_WRITEBACK_EVENT(writeback_bdi_reg
DEFINE_WRITEBACK_EVENT(writeback_bdi_unregister);
DEFINE_WRITEBACK_EVENT(writeback_thread_start);
DEFINE_WRITEBACK_EVENT(writeback_thread_stop);
+DEFINE_WRITEBACK_EVENT(balance_dirty_start);
+DEFINE_WRITEBACK_EVENT(balance_dirty_wait);
+
+TRACE_EVENT(balance_dirty_written,
+
+ TP_PROTO(struct backing_dev_info *bdi, int written),
+
+ TP_ARGS(bdi, written),
+
+ TP_STRUCT__entry(
+ __array(char, name, 32)
+ __field(int, written)
+ ),
+
+ TP_fast_assign(
+ strncpy(__entry->name, dev_name(bdi->dev), 32);
+ __entry->written = written;
+ ),
+
+ TP_printk("bdi %s written %d",
+ __entry->name,
+ __entry->written
+ )
+);

DECLARE_EVENT_CLASS(wbc_class,
TP_PROTO(struct writeback_control *wbc, struct backing_dev_info *bdi),
@@ -114,7 +141,6 @@ DECLARE_EVENT_CLASS(wbc_class,
__field(int, for_background)
__field(int, for_reclaim)
__field(int, range_cyclic)
- __field(unsigned long, older_than_this)
__field(long, range_start)
__field(long, range_end)
),
@@ -128,14 +154,12 @@ DECLARE_EVENT_CLASS(wbc_class,
__entry->for_background = wbc->for_background;
__entry->for_reclaim = wbc->for_reclaim;
__entry->range_cyclic = wbc->range_cyclic;
- __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 older=0x%lx "
+ "bgrd=%d reclm=%d cyclic=%d "
"start=0x%lx end=0x%lx",
__entry->name,
__entry->nr_to_write,
@@ -145,7 +169,6 @@ DECLARE_EVENT_CLASS(wbc_class,
__entry->for_background,
__entry->for_reclaim,
__entry->range_cyclic,
- __entry->older_than_this,
__entry->range_start,
__entry->range_end)
)
@@ -154,12 +177,6 @@ DECLARE_EVENT_CLASS(wbc_class,
DEFINE_EVENT(wbc_class, name, \
TP_PROTO(struct writeback_control *wbc, struct backing_dev_info *bdi), \
TP_ARGS(wbc, bdi))
-DEFINE_WBC_EVENT(wbc_writeback_start);
-DEFINE_WBC_EVENT(wbc_writeback_written);
-DEFINE_WBC_EVENT(wbc_writeback_wait);
-DEFINE_WBC_EVENT(wbc_balance_dirty_start);
-DEFINE_WBC_EVENT(wbc_balance_dirty_written);
-DEFINE_WBC_EVENT(wbc_balance_dirty_wait);
DEFINE_WBC_EVENT(wbc_writepage);

TRACE_EVENT(writeback_queue_io,
--- linux-next.orig/fs/btrfs/extent_io.c 2011-06-19 23:57:40.000000000 +0800
+++ linux-next/fs/btrfs/extent_io.c 2011-06-19 23:58:23.000000000 +0800
@@ -2551,7 +2551,6 @@ 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,
@@ -2584,7 +2583,6 @@ 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,

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