Re: [PATCH 0/7] Per-bdi writeback flusher threads v20

[Wu Fengguang <fengguang.wu@intel.com>]

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);

> Ideal is to call requeue_[partial_]io() instead of redirty_tail().
> 
> [  131.963885] fs/fs-writeback.c +809 wb_writeback(): comm=flush-8:0 pid=2816 n=0
> [  131.966171] global dirty=4105 writeback=18793 nfs=0 flags=__ towrite=32768 skipped=0 file=0 written=0
> [  132.780826] fs/fs-writeback.c +809 wb_writeback(): comm=flush-0:15 pid=1150 n=0
> [  132.783097] global dirty=4105 writeback=16623 nfs=0 flags=__ towrite=32768 skipped=0 file=0 written=0
> [  134.307094] redirty_tail() +542: inode=12
> [  134.815776] redirty_tail() +542: inode=13
> [  134.817709] fs/fs-writeback.c +809 wb_writeback(): comm=flush-8:0 pid=2816 n=11493
> [  134.821242] global dirty=4192 writeback=16203 nfs=0 flags=__ towrite=21275 skipped=0 file=13 written=4430
> [  135.599954] redirty_tail() +542: inode=12
> [  136.372523] redirty_tail() +542: inode=13
> [  136.386748] fs/fs-writeback.c +809 wb_writeback(): comm=flush-8:0 pid=2816 n=11570
> [  136.432168] global dirty=4308 writeback=15752 nfs=0 flags=__ towrite=21198 skipped=0 file=13 written=4650
> [  137.789115] fs/fs-writeback.c +809 wb_writeback(): comm=flush-0:15 pid=1150 n=0
> [  138.587178] global dirty=9551 writeback=10755 nfs=0 flags=__ towrite=32768 skipped=0 file=0 written=0
> [  138.962743] redirty_tail() +542: inode=12
> [  139.395024] redirty_tail() +542: inode=13
> [  139.403194] fs/fs-writeback.c +809 wb_writeback(): comm=flush-8:0 pid=2816 n=11893
> [  139.413026] global dirty=4101 writeback=16630 nfs=0 flags=__ towrite=20875 skipped=0 file=0 written=2
> [  139.426074] fs/fs-writeback.c +809 wb_writeback(): comm=flush-8:0 pid=2816 n=0
> [  139.435190] global dirty=4101 writeback=16378 nfs=0 flags=__ towrite=32768 skipped=0 file=0 written=0
> [  140.266713] redirty_tail() +542: inode=12
> [  140.449304] redirty_tail() +542: inode=13
> [  140.496241] fs/fs-writeback.c +809 wb_writeback(): comm=flush-8:0 pid=2816 n=11678
> [  140.508339] global dirty=4203 writeback=19220 nfs=0 flags=__ towrite=21090 skipped=0 file=13 written=4254
> [  141.649192] redirty_tail() +542: inode=12
> [  141.971276] redirty_tail() +542: inode=13
> [  141.988572] fs/fs-writeback.c +809 wb_writeback(): comm=flush-8:0 pid=2816 n=11304
> [  142.000107] global dirty=4112 writeback=18362 nfs=0 flags=__ towrite=21464 skipped=0 file=13 written=4541
> 
> btrfs pattern is almost the same, but with an extra (metadata) inode 1.
> 
> [  464.443873] fs/fs-writeback.c +809 wb_writeback(): comm=flush-btrfs-1 pid=2865 n=0
> [  464.450458] global dirty=163 writeback=4375 nfs=0 flags=__ towrite=32768 skipped=0 file=0 written=0
> [  464.655999] fs/fs-writeback.c +809 wb_writeback(): comm=flush-8:0 pid=2816 n=0
> [  464.664478] global dirty=3873 writeback=1175 nfs=0 flags=__ towrite=32768 skipped=0 file=0 written=0
> [  465.291059] redirty_tail() +542: inode=257
> [  465.331584] redirty_tail() +542: inode=258
> [  465.346433] redirty_tail() +560: inode=1
> [  465.352016] fs/fs-writeback.c +809 wb_writeback(): comm=flush-btrfs-1 pid=2865 n=14480
> [  465.355797] global dirty=337 writeback=3980 nfs=0 flags=__ towrite=18288 skipped=0 file=1 written=0
> [  466.226489] redirty_tail() +542: inode=257
> [  466.280894] redirty_tail() +542: inode=258
> [  466.282270] redirty_tail() +560: inode=1
> [  466.288079] fs/fs-writeback.c +809 wb_writeback(): comm=flush-btrfs-1 pid=2865 n=14300
> [  466.291738] global dirty=666 writeback=3807 nfs=0 flags=__ towrite=18468 skipped=0 file=1 written=0
> [  467.101730] redirty_tail() +542: inode=257
> [  467.134303] redirty_tail() +542: inode=258
> [  467.135675] redirty_tail() +560: inode=1
> [  467.144120] fs/fs-writeback.c +809 wb_writeback(): comm=flush-btrfs-1 pid=2865 n=14032
> [  467.147302] global dirty=331 writeback=3665 nfs=0 flags=__ towrite=18736 skipped=0 file=1 written=0
> [  467.964652] redirty_tail() +542: inode=257
> [  468.002423] redirty_tail() +542: inode=258
> [  468.003795] redirty_tail() +560: inode=1
> 
> I'm looking into it.
> 
> Thanks,
> Fengguang

>  fs/fs-writeback.c   |   30 +++++++++++++++++++++++++++++-
>  mm/page-writeback.c |    1 +
>  2 files changed, 30 insertions(+), 1 deletion(-)
> 
> --- linux.orig/mm/page-writeback.c	2009-09-19 10:51:09.000000000 +0800
> +++ linux/mm/page-writeback.c	2009-09-19 10:51:47.000000000 +0800
> @@ -536,6 +536,7 @@ static void balance_dirty_pages(struct a
>  			pages_written += write_chunk - wbc.nr_to_write;
>  			get_dirty_limits(&background_thresh, &dirty_thresh,
>  				       &bdi_thresh, bdi);
> +			writeback_debug_report(pages_written, &wbc);
>  		}
>  
>  		/*
> --- linux.orig/fs/fs-writeback.c	2009-09-19 10:51:46.000000000 +0800
> +++ linux/fs/fs-writeback.c	2009-09-19 10:51:47.000000000 +0800
> @@ -68,6 +68,33 @@ enum {
>  #define WS_USED (1 << WS_USED_B)
>  #define WS_ONSTACK (1 << WS_ONSTACK_B)
>  
> +void print_writeback_control(struct writeback_control *wbc)
> +{
> +	printk(KERN_DEBUG
> +			"global dirty=%lu writeback=%lu nfs=%lu "
> +			"flags=%c%c towrite=%ld skipped=%ld "
> +			"file=%lu written=%lu\n",
> +			global_page_state(NR_FILE_DIRTY),
> +			global_page_state(NR_WRITEBACK),
> +			global_page_state(NR_UNSTABLE_NFS),
> +			wbc->encountered_congestion ? 'C':'_',
> +			wbc->more_io ? 'M':'_',
> +			wbc->nr_to_write,
> +			wbc->pages_skipped,
> +			wbc->last_file,
> +			wbc->last_file_written);
> +}
> +
> +void __writeback_debug_report(long n, struct writeback_control *wbc,
> +		const char *file, int line, const char *func)
> +{
> +	printk(KERN_DEBUG "%s +%d %s(): comm=%s pid=%d n=%ld\n",
> +			file, line, func,
> +			current->comm, current->pid,
> +			n);
> +	print_writeback_control(wbc);
> +}
> +
>  static inline bool bdi_work_on_stack(struct bdi_work *work)
>  {
>  	return test_bit(WS_ONSTACK_B, &work->state);
> @@ -302,7 +329,7 @@ static void requeue_io(struct inode *ino
>   */
>  static void requeue_partial_io(struct writeback_control *wbc, struct inode *inode)
>  {
> -	if (time_before(wbc->last_file_time + HZ, jiffies) ||
> +	if (time_before(wbc->last_file_time + 1000 * HZ, jiffies) ||
>  	    wbc->last_file_written == 0 ||
>  	    wbc->last_file_written >= MAX_WRITEBACK_PAGES) {
>  		requeue_io(inode);
> @@ -749,6 +776,7 @@ static long wb_writeback(struct bdi_writ
>  		args->nr_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
>  		wrote += MAX_WRITEBACK_PAGES - wbc.nr_to_write;
>  
> +		writeback_debug_report(wrote, &wbc);
>  		/*
>  		 * If we ran out of stuff to write, bail unless more_io got set
>  		 */

> Subject: track redirty_tail() calls
> 
> It helps a lot to know how redirty_tail() are called.
> 
> Cc: Ken Chen <kenchen@google.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
> ---
>  fs/fs-writeback.c |   36 +++++++++++++++++++++++++++++++++---
>  1 file changed, 33 insertions(+), 3 deletions(-)
> 
> --- linux.orig/fs/fs-writeback.c	2009-09-19 10:51:47.000000000 +0800
> +++ linux/fs/fs-writeback.c	2009-09-19 10:52:03.000000000 +0800
> @@ -290,6 +290,21 @@ void bdi_start_writeback(struct backing_
>  	bdi_alloc_queue_work(bdi, &args);
>  }
>  
> +#define redirty_tail(inode)						\
> +	do {								\
> +		__redirty_tail(inode, __LINE__);			\
> +	} while (0)
> +
> +#define requeue_io(inode)						\
> +	do {								\
> +		__requeue_io(inode, __LINE__);				\
> +	} while (0)
> +
> +#define requeue_partial_io(wbc, inode)					\
> +	do {								\
> +		__requeue_partial_io(wbc, inode, __LINE__);		\
> +	} while (0)
> +
>  /*
>   * Redirty an inode: set its when-it-was dirtied timestamp and move it to the
>   * furthest end of its superblock's dirty-inode list.
> @@ -299,7 +314,7 @@ void bdi_start_writeback(struct backing_
>   * the case then the inode must have been redirtied while it was being written
>   * out and we don't reset its dirtied_when.
>   */
> -static void redirty_tail(struct inode *inode)
> +static void __redirty_tail(struct inode *inode, int line)
>  {
>  	struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
>  
> @@ -311,23 +326,33 @@ static void redirty_tail(struct inode *i
>  			inode->dirtied_when = jiffies;
>  	}
>  	list_move(&inode->i_list, &wb->b_dirty);
> +
> +	if (sysctl_dirty_debug) {
> +		printk(KERN_DEBUG "redirty_tail() +%d: inode=%lu\n",
> +				line, inode->i_ino);
> +	}
>  }
>  
>  /*
>   * requeue inode for re-scanning after bdi->b_io list is exhausted.
>   */
> -static void requeue_io(struct inode *inode)
> +static void __requeue_io(struct inode *inode, int line)
>  {
>  	struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
>  
>  	list_move(&inode->i_list, &wb->b_more_io);
> +
> +	if (sysctl_dirty_debug) {
> +		printk(KERN_DEBUG "requeue_io() +%d: inode=%lu\n",
> +				line, inode->i_ino);
> +	}
>  }
>  
>  /*
>   * continue io on this inode on next writeback if
>   * it has not accumulated large enough writeback io chunk
>   */
> -static void requeue_partial_io(struct writeback_control *wbc, struct inode *inode)
> +static void __requeue_partial_io(struct writeback_control *wbc, struct inode *inode, int line)
>  {
>  	if (time_before(wbc->last_file_time + 1000 * HZ, jiffies) ||
>  	    wbc->last_file_written == 0 ||
> @@ -337,6 +362,11 @@ static void requeue_partial_io(struct wr
>  	}
>  
>  	list_move_tail(&inode->i_list, &inode_to_bdi(inode)->wb.b_io);
> +
> +	if (sysctl_dirty_debug) {
> +		printk(KERN_DEBUG "requeue_partial_io() +%d: inode=%lu\n",
> +				line, inode->i_ino);
> +	}
>  }
>  
>  static void inode_sync_complete(struct inode *inode)