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

[Jan Kara <jack@suse.cz>]

On Mon 04-07-11 09:06:19, Wu Fengguang wrote:
> Hi Jan,
> 
> On Fri, Jul 01, 2011 at 02:32:44AM +0800, 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.
> 
> The end result, I think, is that the dirty pages are kept more tightly
> under control, with the average number a slightly lowered than before. 
> This reduces the risk to throttle light dirtiers and hence more
> responsive. However it does introduce more overheads by enforcing
> balance_dirty_pages() calls on every 8 pages.
  Yes. I think this was actually the original intention when the code was
written.

> > 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 |   29 ++++++++++++++++++++++-------
> >  1 files changed, 22 insertions(+), 7 deletions(-)
> > 
> >  This is the patch fixing dirty_exceeded logic I promised you last week.
> > I based it on current Linus's tree as it is a relatively direct fix so I
> > expect it can be somewhere in the beginning of the patch series and merged
> > relatively quickly. Can you please add it to your tree? Thanks.
> 
> OK. I noticed that bdi_thresh is no longer used. What if we just
> rename it? But I admit that the patch in its current form looks a bit
> more clear in concept.
  You are right bdi_thresh is only used for computing task_bdi_thresh and
min_task_bdi_thresh now. We could possibly eliminate that one variable but
I guess compiler will optimize it away anyway and I find the code more
legible when written this way...

								Honza

> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > index 31f6988..d8b395f 100644
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> > @@ -274,12 +274,13 @@ 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;
> > @@ -290,6 +291,12 @@ 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;
> > +}
> > +
> >  /*
> >   *
> >   */
> > @@ -483,9 +490,12 @@ static void balance_dirty_pages(struct address_space *mapping,
> >  	unsigned long background_thresh;
> >  	unsigned long dirty_thresh;
> >  	unsigned long bdi_thresh;
> > +	unsigned long task_bdi_thresh;
> > +	unsigned long min_task_bdi_thresh;
> >  	unsigned long pages_written = 0;
> >  	unsigned long pause = 1;
> >  	bool dirty_exceeded = false;
> > +	bool clear_dirty_exceeded = true;
> >  	struct backing_dev_info *bdi = mapping->backing_dev_info;
> >  
> >  	for (;;) {
> > @@ -512,7 +522,8 @@ static void balance_dirty_pages(struct address_space *mapping,
> >  			break;
> >  
> >  		bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
> > -		bdi_thresh = task_dirty_limit(current, bdi_thresh);
> > +		min_task_bdi_thresh = task_min_dirty_limit(bdi_thresh);
> > +		task_bdi_thresh = task_dirty_limit(current, bdi_thresh);
> >  
> >  		/*
> >  		 * In order to avoid the stacked BDI deadlock we need
> > @@ -524,7 +535,7 @@ static void balance_dirty_pages(struct address_space *mapping,
> >  		 * actually dirty; with m+n sitting in the percpu
> >  		 * deltas.
> >  		 */
> > -		if (bdi_thresh < 2*bdi_stat_error(bdi)) {
> > +		if (task_bdi_thresh < 2 * bdi_stat_error(bdi)) {
> >  			bdi_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
> >  			bdi_nr_writeback = bdi_stat_sum(bdi, BDI_WRITEBACK);
> >  		} else {
> > @@ -539,8 +550,11 @@ static void balance_dirty_pages(struct address_space *mapping,
> >  		 * the last resort safeguard.
> >  		 */
> >  		dirty_exceeded =
> > -			(bdi_nr_reclaimable + bdi_nr_writeback > bdi_thresh)
> > -			|| (nr_reclaimable + nr_writeback > dirty_thresh);
> > +		  (bdi_nr_reclaimable + bdi_nr_writeback > task_bdi_thresh)
> > +		  || (nr_reclaimable + nr_writeback > dirty_thresh);
> > +		clear_dirty_exceeded =
> > +		  (bdi_nr_reclaimable + bdi_nr_writeback <= min_task_bdi_thresh)
> > +		  && (nr_reclaimable + nr_writeback <= dirty_thresh);
> >  
> >  		if (!dirty_exceeded)
> >  			break;
> > @@ -558,7 +572,7 @@ static void balance_dirty_pages(struct address_space *mapping,
> >  		 * up.
> >  		 */
> >  		trace_wbc_balance_dirty_start(&wbc, bdi);
> > -		if (bdi_nr_reclaimable > bdi_thresh) {
> > +		if (bdi_nr_reclaimable > task_bdi_thresh) {
> >  			writeback_inodes_wb(&bdi->wb, &wbc);
> >  			pages_written += write_chunk - wbc.nr_to_write;
> >  			trace_wbc_balance_dirty_written(&wbc, bdi);
> > @@ -578,7 +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 (clear_dirty_exceeded && bdi->dirty_exceeded)
> >  		bdi->dirty_exceeded = 0;
> >  
> >  	if (writeback_in_progress(bdi))
> > -- 
> > 1.7.1
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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