Re: [PATCH 33/51] writeback: make bdi_has_dirty_io() take multiple bdi_writeback's into account

[Jan Kara]

On Fri 22-05-15 17:13:47, Tejun Heo wrote:
> bdi_has_dirty_io() used to only reflect whether the root wb
> (bdi_writeback) has dirty inodes.  For cgroup writeback support, it
> needs to take all active wb's into account.  If any wb on the bdi has
> dirty inodes, bdi_has_dirty_io() should return true.
> 
> To achieve that, as inode_wb_list_{move|del}_locked() now keep track
> of the dirty state transition of each wb, the number of dirty wbs can
> be counted in the bdi; however, bdi is already aggregating
> wb->avg_write_bandwidth which can easily be guaranteed to be > 0 when
> there are any dirty inodes by ensuring wb->avg_write_bandwidth can't
> dip below 1.  bdi_has_dirty_io() can simply test whether
> bdi->tot_write_bandwidth is zero or not.
> 
> While this bumps the value of wb->avg_write_bandwidth to one when it
> used to be zero, this shouldn't cause any meaningful behavior
> difference.
> 
> bdi_has_dirty_io() is made an inline function which tests whether
> ->tot_write_bandwidth is non-zero.  Also, WARN_ON_ONCE()'s on its
> value are added to inode_wb_list_{move|del}_locked().

It looks OK although I find using total write bandwidth to detect whether
any wb has any dirty IO rather hacky. Frankly I'd prefer to just iterate
all wbs from bdi_has_dirty_io() since that isn't performance critical
and we iterate all wbs in those paths anyway... Hmm?

								Honza

> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> ---
>  fs/fs-writeback.c                |  5 +++--
>  include/linux/backing-dev-defs.h |  8 ++++++--
>  include/linux/backing-dev.h      | 10 +++++++++-
>  mm/backing-dev.c                 |  5 -----
>  mm/page-writeback.c              | 10 +++++++---
>  5 files changed, 25 insertions(+), 13 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index bbccf68..c98d392 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -99,6 +99,7 @@ static bool wb_io_lists_populated(struct bdi_writeback *wb)
>  		return false;
>  	} else {
>  		set_bit(WB_has_dirty_io, &wb->state);
> +		WARN_ON_ONCE(!wb->avg_write_bandwidth);
>  		atomic_long_add(wb->avg_write_bandwidth,
>  				&wb->bdi->tot_write_bandwidth);
>  		return true;
> @@ -110,8 +111,8 @@ static void wb_io_lists_depopulated(struct bdi_writeback *wb)
>  	if (wb_has_dirty_io(wb) && list_empty(&wb->b_dirty) &&
>  	    list_empty(&wb->b_io) && list_empty(&wb->b_more_io)) {
>  		clear_bit(WB_has_dirty_io, &wb->state);
> -		atomic_long_sub(wb->avg_write_bandwidth,
> -				&wb->bdi->tot_write_bandwidth);
> +		WARN_ON_ONCE(atomic_long_sub_return(wb->avg_write_bandwidth,
> +					&wb->bdi->tot_write_bandwidth) < 0);
>  	}
>  }
>  
> diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
> index d631a61..8c857d7 100644
> --- a/include/linux/backing-dev-defs.h
> +++ b/include/linux/backing-dev-defs.h
> @@ -98,7 +98,7 @@ struct bdi_writeback {
>  	unsigned long dirtied_stamp;
>  	unsigned long written_stamp;	/* pages written at bw_time_stamp */
>  	unsigned long write_bandwidth;	/* the estimated write bandwidth */
> -	unsigned long avg_write_bandwidth; /* further smoothed write bw */
> +	unsigned long avg_write_bandwidth; /* further smoothed write bw, > 0 */
>  
>  	/*
>  	 * The base dirty throttle rate, re-calculated on every 200ms.
> @@ -142,7 +142,11 @@ struct backing_dev_info {
>  	unsigned int min_ratio;
>  	unsigned int max_ratio, max_prop_frac;
>  
> -	atomic_long_t tot_write_bandwidth; /* sum of active avg_write_bw */
> +	/*
> +	 * Sum of avg_write_bw of wbs with dirty inodes.  > 0 if there are
> +	 * any dirty wbs, which is depended upon by bdi_has_dirty().
> +	 */
> +	atomic_long_t tot_write_bandwidth;
>  
>  	struct bdi_writeback wb;  /* the root writeback info for this bdi */
>  	struct bdi_writeback_congested wb_congested; /* its congested state */
> diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> index 3c8403c..0839e44 100644
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -29,7 +29,6 @@ void bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,
>  			enum wb_reason reason);
>  void bdi_start_background_writeback(struct backing_dev_info *bdi);
>  void wb_workfn(struct work_struct *work);
> -bool bdi_has_dirty_io(struct backing_dev_info *bdi);
>  void wb_wakeup_delayed(struct bdi_writeback *wb);
>  
>  extern spinlock_t bdi_lock;
> @@ -42,6 +41,15 @@ static inline bool wb_has_dirty_io(struct bdi_writeback *wb)
>  	return test_bit(WB_has_dirty_io, &wb->state);
>  }
>  
> +static inline bool bdi_has_dirty_io(struct backing_dev_info *bdi)
> +{
> +	/*
> +	 * @bdi->tot_write_bandwidth is guaranteed to be > 0 if there are
> +	 * any dirty wbs.  See wb_update_write_bandwidth().
> +	 */
> +	return atomic_long_read(&bdi->tot_write_bandwidth);
> +}
> +
>  static inline void __add_wb_stat(struct bdi_writeback *wb,
>  				 enum wb_stat_item item, s64 amount)
>  {
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 161ddf1..d2f16fc9 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -256,11 +256,6 @@ static int __init default_bdi_init(void)
>  }
>  subsys_initcall(default_bdi_init);
>  
> -bool bdi_has_dirty_io(struct backing_dev_info *bdi)
> -{
> -	return wb_has_dirty_io(&bdi->wb);
> -}
> -
>  /*
>   * This function is used when the first inode for this wb is marked dirty. It
>   * wakes-up the corresponding bdi thread which should then take care of the
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index c95eb24..99b8846 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -881,9 +881,13 @@ static void wb_update_write_bandwidth(struct bdi_writeback *wb,
>  		avg += (old - avg) >> 3;
>  
>  out:
> -	if (wb_has_dirty_io(wb))
> -		atomic_long_add(avg - wb->avg_write_bandwidth,
> -				&wb->bdi->tot_write_bandwidth);
> +	/* keep avg > 0 to guarantee that tot > 0 if there are dirty wbs */
> +	avg = max(avg, 1LU);
> +	if (wb_has_dirty_io(wb)) {
> +		long delta = avg - wb->avg_write_bandwidth;
> +		WARN_ON_ONCE(atomic_long_add_return(delta,
> +					&wb->bdi->tot_write_bandwidth) <= 0);
> +	}
>  	wb->write_bandwidth = bw;
>  	wb->avg_write_bandwidth = avg;
>  }
> -- 
> 2.4.0
> 
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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