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

[Greg Thelen]

Minchan Kim <minchan.kim@xxxxxxxxx> writes:

> 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.

Good point.  I will add this to the commit 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?

Good catch.  At present, wb isn't used.  I do not have plans to use it.
I will remove it.

>> +				       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
>> 
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