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SubjectRe: [PATCH 14/17] writeback: make writeback_control.nr_to_write straight
On Thu 12-05-11 21:57:20, Wu Fengguang wrote:
> 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.
>
> Proposed-by: Christoph Hellwig <hch@infradead.org>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Acked-by: Jan Kara <jack@suse.cz>

Honza
> ---
> fs/btrfs/extent_io.c | 2
> fs/fs-writeback.c | 192 +++++++++++++++--------------
> include/linux/writeback.h | 6
> include/trace/events/writeback.h | 39 ++++-
> mm/backing-dev.c | 9 -
> mm/page-writeback.c | 17 --
> 6 files changed, 140 insertions(+), 125 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_cleaned
> - remove wbc_writeback_* trace events for now
>
> --- linux-next.orig/fs/fs-writeback.c 2011-05-11 22:37:15.000000000 +0800
> +++ linux-next/fs/fs-writeback.c 2011-05-12 11:46:27.000000000 +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_sync:1;
> unsigned int for_kupdate:1;
> @@ -463,7 +473,6 @@ writeback_single_inode(struct inode *ino
> * No need to add it back to the LRU.
> */
> list_del_init(&inode->i_wb_list);
> - wbc->inodes_cleaned++;
> }
> }
> inode_sync_complete(inode);
> @@ -496,6 +505,31 @@ 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_sync)
> + 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.
> *
> @@ -503,18 +537,29 @@ 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 cleaned.
> */
> -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_sync = work->tagged_sync,
> + .for_kupdate = work->for_kupdate,
> + .for_background = work->for_background,
> + .range_cyclic = work->range_cyclic,
> + .range_start = 0,
> + .range_end = LLONG_MAX,
> + };
> + 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
> @@ -529,7 +574,7 @@ static int writeback_sb_inodes(struct su
> * Bounce back to the caller to unpin this and
> * pin the next superblock.
> */
> - return 0;
> + break;
> }
>
> /*
> @@ -543,34 +588,44 @@ 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 (wbc.pages_skipped) {
> /*
> * writeback is not making progress due to locked
> * buffers. Skip this inode for now.
> */
> redirty_tail(inode, wb);
> - }
> + } else if (!(inode->i_state & I_DIRTY))
> + wrote++;
> spin_unlock(&inode->i_lock);
> spin_unlock(&wb->list_lock);
> 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 >= MAX_WRITEBACK_PAGES)
> + 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;
> + long wrote = 0;
>
> while (!list_empty(&wb->b_io)) {
> struct inode *inode = wb_inode(wb->b_io.prev);
> @@ -580,33 +635,34 @@ 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)
> + if (wrote >= MAX_WRITEBACK_PAGES)
> + 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)
> {
> @@ -636,42 +692,13 @@ 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_sync = work->tagged_sync,
> - .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_sync)
> - write_chunk = LONG_MAX;
> + long progress;
>
> oldest_jif = jiffies;
> - wbc.older_than_this = &oldest_jif;
> + work->older_than_this = &oldest_jif;
>
> for (;;) {
> /*
> @@ -700,27 +727,20 @@ static long wb_writeback(struct bdi_writ
> if (work->for_kupdate || work->for_background) {
> 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_cleaned = 0;
> -
> retry:
> - trace_wbc_writeback_start(&wbc, wb->bdi);
> + trace_writeback_start(wb->bdi, work);
> spin_lock(&wb->list_lock);
> 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);
> + progress = __writeback_inodes_wb(wb, work);
> spin_unlock(&wb->list_lock);
> - trace_wbc_writeback_written(&wbc, wb->bdi);
> -
> - work->nr_pages -= write_chunk - wbc.nr_to_write;
> - wrote += write_chunk - wbc.nr_to_write;
> + trace_writeback_written(wb->bdi, work);
>
> /*
> * Did we write something? Try for more
> @@ -730,9 +750,7 @@ retry:
> * 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_cleaned)
> + if (progress)
> continue;
> /*
> * background writeback will start with expired inodes, and
> @@ -741,10 +759,10 @@ retry:
> * lists and cause trouble to the page reclaim.
> */
> if (work->for_background &&
> - wbc.older_than_this &&
> + work->older_than_this &&
> list_empty(&wb->b_io) &&
> list_empty(&wb->b_more_io)) {
> - wbc.older_than_this = NULL;
> + work->older_than_this = NULL;
> goto retry;
> }
> /*
> @@ -759,8 +777,8 @@ retry:
> */
> spin_lock(&wb->list_lock);
> 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);
> @@ -768,7 +786,7 @@ retry:
> spin_unlock(&wb->list_lock);
> }
>
> - return wrote;
> + return nr_pages - work->nr_pages;
> }
>
> /*
> --- linux-next.orig/include/linux/writeback.h 2011-05-11 22:37:15.000000000 +0800
> +++ linux-next/include/linux/writeback.h 2011-05-12 11:45:08.000000000 +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_cleaned; /* # of inodes cleaned */
>
> /*
> * For a_ops->writepages(): is start or end are non-zero then this is
> @@ -58,8 +55,7 @@ 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 2011-05-11 22:37:15.000000000 +0800
> +++ linux-next/mm/backing-dev.c 2011-05-12 11:45:08.000000000 +0800
> @@ -262,14 +262,7 @@ int bdi_has_dirty_io(struct backing_dev_
>
> 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);
> }
>
> /*
> --- linux-next.orig/mm/page-writeback.c 2011-05-11 22:37:15.000000000 +0800
> +++ linux-next/mm/page-writeback.c 2011-05-12 11:46:27.000000000 +0800
> @@ -494,13 +494,6 @@ static void balance_dirty_pages(struct a
> return;
>
> 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);
> @@ -562,17 +555,17 @@ 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 2011-05-11 22:37:15.000000000 +0800
> +++ linux-next/include/trace/events/writeback.h 2011-05-12 11:46:27.000000000 +0800
> @@ -49,6 +49,9 @@ 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),
> @@ -88,6 +91,30 @@ 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),
> @@ -101,7 +128,6 @@ 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)
> ),
> @@ -115,14 +141,12 @@ 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,
> @@ -132,7 +156,6 @@ DECLARE_EVENT_CLASS(wbc_class,
> __entry->for_background,
> __entry->for_reclaim,
> __entry->range_cyclic,
> - __entry->older_than_this,
> __entry->range_start,
> __entry->range_end)
> )
> @@ -141,12 +164,6 @@ 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);
>
> DECLARE_EVENT_CLASS(writeback_congest_waited_template,
> --- linux-next.orig/fs/btrfs/extent_io.c 2011-05-11 22:37:15.000000000 +0800
> +++ linux-next/fs/btrfs/extent_io.c 2011-05-12 11:45:08.000000000 +0800
> @@ -2596,7 +2596,6 @@ 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,
> @@ -2629,7 +2628,6 @@ 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


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