Re: [PATCH 2/5] mm: Properly reflect task dirty limits in dirty_exceeded logic

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Mar 08, 2011 at 11:31:12PM +0100, Jan Kara wrote:
> We set bdi->dirty_exceeded (and thus ratelimiting code starts to
> call balance_dirty_pages() every 8 pages) when a per-bdi limit is
> exceeded or global limit is exceeded. But per-bdi limit also depends
> on the task. Thus different tasks reach the limit on that bdi at
> different levels of dirty pages. The result is that with current code
> bdi->dirty_exceeded ping-ponged between 1 and 0 depending on which task
> just got into balance_dirty_pages().
> 
> We fix the issue by clearing bdi->dirty_exceeded only when per-bdi amount
> of dirty pages drops below the threshold (7/8 * bdi_dirty_limit) where task
> limits already do not have any influence.
> 
> CC: Andrew Morton <akpm@linux-foundation.org>
> CC: Christoph Hellwig <hch@infradead.org>
> CC: Dave Chinner <david@fromorbit.com>
> CC: Wu Fengguang <fengguang.wu@intel.com>
> CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  mm/page-writeback.c |   18 ++++++++++++++++--
>  1 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index c472c1c..f388f70 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -275,12 +275,13 @@ static inline void task_dirties_fraction(struct task_struct *tsk,
>   * effectively curb the growth of dirty pages. Light dirtiers with high enough
>   * dirty threshold may never get throttled.
>   */
> +#define TASK_LIMIT_FRACTION 8
>  static unsigned long task_dirty_limit(struct task_struct *tsk,
>  				       unsigned long bdi_dirty)
>  {
>  	long numerator, denominator;
>  	unsigned long dirty = bdi_dirty;
> -	u64 inv = dirty >> 3;
> +	u64 inv = dirty / TASK_LIMIT_FRACTION;
>  
>  	task_dirties_fraction(tsk, &numerator, &denominator);
>  	inv *= numerator;
> @@ -291,6 +292,12 @@ static unsigned long task_dirty_limit(struct task_struct *tsk,
>  	return max(dirty, bdi_dirty/2);
>  }
>  
> +/* Minimum limit for any task */
> +static unsigned long task_min_dirty_limit(unsigned long bdi_dirty)
> +{
> +	return bdi_dirty - bdi_dirty / TASK_LIMIT_FRACTION;
> +}
> +

Hi Jan,

Should the above be called bdi_min_dirty_limit()? In essense we seem to
be setting bdi->bdi_exceeded when dirty pages on bdi cross bdi_thresh and
clear it when dirty pages on bdi are below 7/8*bdi_thresh. So there does
not seem to be any dependency on task dirty limit here hence string
"task" sounds confusing to me. In fact, would bdi_dirty_exceeded_clear_thresh()
be a better name?
 
>  /*
>   *
>   */
> @@ -484,9 +491,11 @@ static void balance_dirty_pages(struct address_space *mapping,
>  	unsigned long background_thresh;
>  	unsigned long dirty_thresh;
>  	unsigned long bdi_thresh;
> +	unsigned long min_bdi_thresh = ULONG_MAX;
>  	unsigned long pages_written = 0;
>  	unsigned long pause = 1;
>  	bool dirty_exceeded = false;
> +	bool min_dirty_exceeded = false;
>  	struct backing_dev_info *bdi = mapping->backing_dev_info;
>  
>  	for (;;) {
> @@ -513,6 +522,7 @@ static void balance_dirty_pages(struct address_space *mapping,
>  			break;
>  
>  		bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
> +		min_bdi_thresh = task_min_dirty_limit(bdi_thresh);
>  		bdi_thresh = task_dirty_limit(current, bdi_thresh);
                ^^^^^
This patch aside, we use bdi_thresh name both for bdi threshold as well
as per task per bdi threshold. will task_bdi_thresh be a better name
here.

>  
>  		/*
> @@ -542,6 +552,9 @@ static void balance_dirty_pages(struct address_space *mapping,
>  		dirty_exceeded =
>  			(bdi_nr_reclaimable + bdi_nr_writeback > bdi_thresh)
>  			|| (nr_reclaimable + nr_writeback > dirty_thresh);
> +		min_dirty_exceeded =
> +			(bdi_nr_reclaimable + bdi_nr_writeback > min_bdi_thresh)
> +			|| (nr_reclaimable + nr_writeback > dirty_thresh);

Would following be easier to understand.

		clear_dirty_exceeded =
			(bdi_nr_reclaimable + bdi_nr_writeback <
				dirty_exceeded_reset_thresh)
			&& (nr_reclaimable + nr_writeback < dirty_thresh);

>  
>  		if (!dirty_exceeded)
>  			break;
> @@ -579,7 +592,8 @@ static void balance_dirty_pages(struct address_space *mapping,
>  			pause = HZ / 10;
>  	}
>  
> -	if (!dirty_exceeded && bdi->dirty_exceeded)
> +	/* Clear dirty_exceeded flag only when no task can exceed the limit */
> +	if (!min_dirty_exceeded && bdi->dirty_exceeded)
>  		bdi->dirty_exceeded = 0;

similiarly...

	if (bdi->dirty_exceeded && clear_dirty_exceeded)
		bdi->dirty_exceeded = 0;

I was confused with the term min_dirty_limit and task_min_dirty_limit()
for sometime as patch said that we are trying to move away from dependence
on task specific bdi_thres for clearing bdi->bdi_thresh. May be it is
just me...
 
Thanks
Vivek 

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