Re: [PATCH 0/15] Per-bdi writeback flusher threads v10

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

On Tue, 2009-06-16 at 21:53 +0200, Jens Axboe wrote:
> On Tue, Jun 16 2009, Jens Axboe wrote:
> > On Tue, Jun 16 2009, Zhang, Yanmin wrote:
> > > On Fri, 2009-06-12 at 14:54 +0200, Jens Axboe wrote:
> > > > Hi,
> > > > 
> > > > Here's the 10th version of the writeback patches. Changes since v9:
> > > > 
> > > > - Fix bdi task exit race leaving work on the list, flush it after we
> > > >   know we cannot be found anymore.
> > > > - Rename flusher tasks from bdi-foo to flush-foo. Should make it more
> > > >   clear to the casual observer.
> > > > - Fix a problem with the btrfs bdi register patch that would spew
> > > >   warnings for > 1 mounted btrfs file system.
> > > > - Rebase to current -git, there were some conflicts with the latest work
> > > >   from viro/hch.
> > > > - Fix a block layer core problem were stacked devices would overwrite
> > > >   the bdi state, causing problems and warning spew.
> > > > - In bdi_writeback_all(), in the race occurence of a work allocation
> > > >   failure, restart scanning from the beginning. Then we can drop the
> > > >   bdi_lock mutex before diving into bdi specific writeback.
> > > > - Convert bdi_lock to a spinlock.
> > > > - Use spin_trylock() in bdi_writeback_all(), if this isn't a data
> > > >   integrity writeback. Debatable, I kind of like it...
> > > > - Get rid of BDI_CAP_FLUSH_FORKER, just check for match with the
> > > >   default_backing_dev_info.
> > > > - Fix race in list checking in bdi_forker_task().
> > > > 
> > > > 
> > > > For ease of patching, I've put the full diff here:
> > > > 
> > > >   http://kernel.dk/writeback-v10.patch
> > > Jens,
> > > 
> > > I applied the patch to 2.6.30 and got a confliction. The attachment is
> > > the patch I ported to 2.6.30. Did I miss anything?
> > > 
> > > 
> > > With the patch, kernel reports below messages on 2 machines.
> > > 
> > > INFO: task sync:29984 blocked for more than 120 seconds.
> > > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > > sync          D ffff88002805e300  6168 29984  24581
> > >  ffff88022f84b780 0000000000000082 7fffffffffffffff ffff880133dbfe70
> > >  0000000000000000 ffff88022e2b4c50 ffff88022e2b4fd8 00000001000c7bb8
> > >  ffff88022f513fd0 ffff880133dbfde8 ffff880133dbfec8 ffff88022d5d13c8
> > > Call Trace:
> > >  [<ffffffff802b69e4>] ? bdi_sched_wait+0x0/0xd
> > >  [<ffffffff80780fde>] ? schedule+0x9/0x1d
> > >  [<ffffffff802b69ed>] ? bdi_sched_wait+0x9/0xd
> > >  [<ffffffff8078158d>] ? __wait_on_bit+0x40/0x6f
> > >  [<ffffffff802b69e4>] ? bdi_sched_wait+0x0/0xd
> > >  [<ffffffff80781628>] ? out_of_line_wait_on_bit+0x6c/0x78
> > >  [<ffffffff8024a426>] ? wake_bit_function+0x0/0x23
> > >  [<ffffffff802b67ac>] ? bdi_writeback_all+0x12a/0x152
> > >  [<ffffffff802b6805>] ? generic_sync_sb_inodes+0x31/0xde
> > >  [<ffffffff802b6935>] ? sync_inodes_sb+0x83/0x88
> > >  [<ffffffff802b6980>] ? __sync_inodes+0x46/0x8f
> > >  [<ffffffff802b94f2>] ? do_sync+0x36/0x5a
> > >  [<ffffffff802b9538>] ? sys_sync+0xe/0x12
> > >  [<ffffffff8020b9ab>] ? system_call_fastpath+0x16/0x1b
> > 
> > I don't think it is your backport, for some reason the v10 missed a
> > change that I think could solve this race. If not, there's another in
> > there that I need to look at.
> > 
> > So against your current base, could you try with the below added as
> > well? The printk() is just so we can see if this triggers for you or
> > not.
> 
> OK that wont work, since we need to actually wait for the work to be
> flushed, otherwise we wreak things when we free the bdi immediately
> after that.
> 
> Can you try with this patch?
Jens,

I tested below patch on 4 machines (run all fio sub-test cases twice which
need more than 10 hours). The previous 2 machines don't stop this time.
Unfortunately, the 3rd machine stops. I double-check the disassembled codes
of kernel and make sure bdi_start_fn really calls wb_do_writeback.

INFO: task sync:30618 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
sync          D ffffc20000011300  4736 30618  28522
 ffff8800bd25b090 0000000000000082 ffff8800bd763780 ffff8800bd763b08
 00000000bd582e68 0000000000004000 0000000000011300 000000000000c868
 ffff8800bd9c5df8 0000000000000000 ffff8800bd763780 ffff8800bd763b08
Call Trace:
 [<ffffffff802784ba>] ? find_get_pages_tag+0x46/0xdd
 [<ffffffff802c3297>] ? bdi_sched_wait+0x0/0xd
 [<ffffffff80747fe7>] ? schedule+0x9/0x1e
 [<ffffffff802c32a0>] ? bdi_sched_wait+0x9/0xd
 [<ffffffff807485ae>] ? __wait_on_bit+0x41/0x71
 [<ffffffff802c3297>] ? bdi_sched_wait+0x0/0xd
 [<ffffffff80748649>] ? out_of_line_wait_on_bit+0x6b/0x77
 [<ffffffff8024cc0c>] ? wake_bit_function+0x0/0x23
 [<ffffffff802c2f50>] ? bdi_writeback_all+0x134/0x16b
 [<ffffffff802c30ba>] ? generic_sync_sb_inodes+0x31/0xdc
 [<ffffffff802c31e8>] ? sync_inodes_sb+0x83/0x88
 [<ffffffff802c3233>] ? __sync_inodes+0x46/0x8f
 [<ffffffff802c5dac>] ? do_sync+0x36/0x5a
 [<ffffffff802c5df2>] ? sys_sync+0xe/0x14
 [<ffffffff8020ba2b>] ? system_call_fastpath+0x16/0x1b

> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 5a1837f..4a6859e 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -409,7 +409,7 @@ static struct bdi_work *get_next_work_item(struct backing_dev_info *bdi,
>  /*
>   * Retrieve work items and do the writeback they describe
>   */
> -static long wb_writeback(struct bdi_writeback *wb)
> +static long wb_writeback(struct bdi_writeback *wb, int force_wait)
>  {
>  	struct backing_dev_info *bdi = wb->bdi;
>  	struct bdi_work *work;
> @@ -418,7 +418,12 @@ static long wb_writeback(struct bdi_writeback *wb)
>  	while ((work = get_next_work_item(bdi, wb)) != NULL) {
>  		struct super_block *sb = bdi_work_sb(work);
>  		long nr_pages = work->nr_pages;
> -		enum writeback_sync_modes sync_mode = work->sync_mode;
> +		enum writeback_sync_modes sync_mode;
> +
> +		if (force_wait)
> +			sync_mode = WB_SYNC_ALL;
> +		else
> +			sync_mode = work->sync_mode;
>  
>  		/*
>  		 * If this isn't a data integrity operation, just notify
> @@ -444,7 +449,7 @@ static long wb_writeback(struct bdi_writeback *wb)
>   * This will be inlined in bdi_writeback_task() once we get rid of any
>   * dirty inodes on the default_backing_dev_info
>   */
> -long wb_do_writeback(struct bdi_writeback *wb)
> +long wb_do_writeback(struct bdi_writeback *wb, int force_wait)
>  {
>  	long wrote;
>  
> @@ -461,7 +466,7 @@ long wb_do_writeback(struct bdi_writeback *wb)
>  	if (list_empty(&wb->bdi->work_list))
>  		wrote = wb_kupdated(wb);
>  	else
> -		wrote = wb_writeback(wb);
> +		wrote = wb_writeback(wb, force_wait);
>  
>  	return wrote;
>  }
> @@ -477,7 +482,7 @@ int bdi_writeback_task(struct bdi_writeback *wb)
>  	long pages_written;
>  
>  	while (!kthread_should_stop()) {
> -		pages_written = wb_do_writeback(wb);
> +		pages_written = wb_do_writeback(wb, 0);
>  
>  		if (pages_written)
>  			last_active = jiffies;
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index 0d4e31d..e070b91 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -68,7 +68,7 @@ struct writeback_control {
>  void writeback_inodes(struct writeback_control *wbc);
>  int inode_wait(void *);
>  void sync_inodes_sb(struct super_block *, int wait);
> -long wb_do_writeback(struct bdi_writeback *wb);
> +long wb_do_writeback(struct bdi_writeback *wb, int force_wait);
>  
>  /* writeback.h requires fs.h; it, too, is not included from here. */
>  static inline void wait_on_inode(struct inode *inode)
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 23013d5..0c91add 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -389,7 +389,7 @@ static int bdi_start_fn(void *ptr)
>  	 * will be added, since this bdi isn't discoverable anymore.
>  	 */
>  	if (!list_empty(&bdi->work_list))
> -		wb_do_writeback(wb);
> +		wb_do_writeback(wb, 1);
>  
>  	bdi_put_wb(bdi, wb);
>  	return ret;
> @@ -484,7 +484,7 @@ static int bdi_forker_task(void *ptr)
>  		 * dirty data on the default backing_dev_info
>  		 */
>  		if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list))
> -			wb_do_writeback(me);
> +			wb_do_writeback(me, 0);
>  
>  		spin_lock(&bdi_lock);
>  
>