Re: [PATCH v8 11/12] writeback: make background writeback cgroupaware

[Minchan Kim]

On Fri, Jun 03, 2011 at 09:12:17AM -0700, Greg Thelen wrote:
> When the system is under background dirty memory threshold but a cgroup
> is over its background dirty memory threshold, then only writeback
> inodes associated with the over-limit cgroup(s).
> 
> In addition to checking if the system dirty memory usage is over the
> system background threshold, over_bground_thresh() also checks if any
> cgroups are over their respective background dirty memory thresholds.
> The writeback_control.for_cgroup field is set to distinguish between a
> system and memcg overage.
> 
> If performing cgroup writeback, move_expired_inodes() skips inodes that
> do not contribute dirty pages to the cgroup being written back.
> 
> After writing some pages, wb_writeback() will call
> mem_cgroup_writeback_done() to update the set of over-bg-limits memcg.
> 
> Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx>
Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>

There are some nitpicks at below.

> ---
> Changelog since v7:
> - over_bground_thresh() now sets shared_inodes=1.  In -v7 per memcg
>   background writeback did not, so it did not write pages of shared
>   inodes in background writeback.  In the (potentially common) case
>   where the system dirty memory usage is below the system background
>   dirty threshold but at least one cgroup is over its background dirty
>   limit, then per memcg background writeback is queued for any
>   over-background-threshold cgroups.  Background writeback should be
>   allowed to writeback shared inodes.  The hope is that writing such
>   inodes has good chance of cleaning the inodes so they can transition
>   from shared to non-shared.  Such a transition is good because then the
>   inode will remain unshared until it is written by multiple cgroup.
>   Non-shared inodes offer better isolation.

Above comment should be in description.

> 
>  fs/fs-writeback.c |   32 ++++++++++++++++++++++++--------
>  1 files changed, 24 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 0174fcf..c0bfe62 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -256,14 +256,17 @@ static void move_expired_inodes(struct list_head *delaying_queue,
>  	LIST_HEAD(tmp);
>  	struct list_head *pos, *node;
>  	struct super_block *sb = NULL;
> -	struct inode *inode;
> +	struct inode *inode, *tmp_inode;
>  	int do_sb_sort = 0;
>  
> -	while (!list_empty(delaying_queue)) {
> -		inode = wb_inode(delaying_queue->prev);
> +	list_for_each_entry_safe_reverse(inode, tmp_inode, delaying_queue,
> +					 i_wb_list) {
>  		if (wbc->older_than_this &&
>  		    inode_dirtied_after(inode, *wbc->older_than_this))
>  			break;
> +		if (wbc->for_cgroup &&
> +		    !should_writeback_mem_cgroup_inode(inode, wbc))
> +			continue;
>  		if (sb && sb != inode->i_sb)
>  			do_sb_sort = 1;
>  		sb = inode->i_sb;
> @@ -614,14 +617,22 @@ void writeback_inodes_wb(struct bdi_writeback *wb,
>   */
>  #define MAX_WRITEBACK_PAGES     1024
>  
> -static inline bool over_bground_thresh(void)
> +static inline bool over_bground_thresh(struct bdi_writeback *wb,

At present, wb isn't used.
Do you remain it intentionally for using in future?

> +				       struct writeback_control *wbc)
>  {
>  	unsigned long background_thresh, dirty_thresh;
>  
>  	global_dirty_limits(&background_thresh, &dirty_thresh);
>  
> -	return (global_page_state(NR_FILE_DIRTY) +
> -		global_page_state(NR_UNSTABLE_NFS) > background_thresh);
> +	if (global_page_state(NR_FILE_DIRTY) +
> +	    global_page_state(NR_UNSTABLE_NFS) > background_thresh) {
> +		wbc->for_cgroup = 0;
> +		return true;
> +	}
> +
> +	wbc->for_cgroup = 1;
> +	wbc->shared_inodes = 1;
> +	return mem_cgroups_over_bground_dirty_thresh();
>  }
>  
>  /*
> @@ -700,7 +711,7 @@ static long wb_writeback(struct bdi_writeback *wb,
>  		 * For background writeout, stop when we are below the
>  		 * background dirty threshold
>  		 */
> -		if (work->for_background && !over_bground_thresh())
> +		if (work->for_background && !over_bground_thresh(wb, &wbc))
>  			break;
>  
>  		if (work->for_kupdate || work->for_background) {
> @@ -729,6 +740,9 @@ retry:
>  		work->nr_pages -= write_chunk - wbc.nr_to_write;
>  		wrote += write_chunk - wbc.nr_to_write;
>  
> +		if (write_chunk - wbc.nr_to_write > 0)
> +			mem_cgroup_writeback_done();
> +
>  		/*
>  		 * Did we write something? Try for more
>  		 *
> @@ -809,7 +823,9 @@ static unsigned long get_nr_dirty_pages(void)
>  
>  static long wb_check_background_flush(struct bdi_writeback *wb)
>  {
> -	if (over_bground_thresh()) {
> +	struct writeback_control wbc;
> +
> +	if (over_bground_thresh(wb, &wbc)) {
>  
>  		struct wb_writeback_work work = {
>  			.nr_pages	= LONG_MAX,
> -- 
> 1.7.3.1
> 

-- 
Kind regards
Minchan Kim
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