Re: [PATCH 10/10] vmscan: Kick flusher threads to clean pages whenreclaim is encountering dirty pages

[Mel Gorman]

On Thu, Sep 09, 2010 at 12:22:28PM +0900, KAMEZAWA Hiroyuki wrote:
> On Mon,  6 Sep 2010 11:47:33 +0100
> Mel Gorman <mel@xxxxxxxxx> wrote:
> 
> > There are a number of cases where pages get cleaned but two of concern
> > to this patch are;
> >   o When dirtying pages, processes may be throttled to clean pages if
> >     dirty_ratio is not met.
> >   o Pages belonging to inodes dirtied longer than
> >     dirty_writeback_centisecs get cleaned.
> > 
> > The problem for reclaim is that dirty pages can reach the end of the LRU if
> > pages are being dirtied slowly so that neither the throttling or a flusher
> > thread waking periodically cleans them.
> > 
> > Background flush is already cleaning old or expired inodes first but the
> > expire time is too far in the future at the time of page reclaim. To mitigate
> > future problems, this patch wakes flusher threads to clean 4M of data -
> > an amount that should be manageable without causing congestion in many cases.
> > 
> > Ideally, the background flushers would only be cleaning pages belonging
> > to the zone being scanned but it's not clear if this would be of benefit
> > (less IO) or not (potentially less efficient IO if an inode is scattered
> > across multiple zones).
> > 
> > Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> > ---
> >  mm/vmscan.c |   32 ++++++++++++++++++++++++++++++--
> >  1 files changed, 30 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 408c101..33d27a4 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -148,6 +148,18 @@ static DECLARE_RWSEM(shrinker_rwsem);
> >  /* Direct lumpy reclaim waits up to five seconds for background cleaning */
> >  #define MAX_SWAP_CLEAN_WAIT 50
> >  
> > +/*
> > + * When reclaim encounters dirty data, wakeup flusher threads to clean
> > + * a maximum of 4M of data.
> > + */
> > +#define MAX_WRITEBACK (4194304UL >> PAGE_SHIFT)
> > +#define WRITEBACK_FACTOR (MAX_WRITEBACK / SWAP_CLUSTER_MAX)
> > +static inline long nr_writeback_pages(unsigned long nr_dirty)
> > +{
> > +	return laptop_mode ? 0 :
> > +			min(MAX_WRITEBACK, (nr_dirty * WRITEBACK_FACTOR));
> > +}
> > +
> >  static struct zone_reclaim_stat *get_reclaim_stat(struct zone *zone,
> >  						  struct scan_control *sc)
> >  {
> > @@ -686,12 +698,14 @@ static noinline_for_stack void free_page_list(struct list_head *free_pages)
> >   */
> >  static unsigned long shrink_page_list(struct list_head *page_list,
> >  					struct scan_control *sc,
> > +					int file,
> >  					unsigned long *nr_still_dirty)
> >  {
> >  	LIST_HEAD(ret_pages);
> >  	LIST_HEAD(free_pages);
> >  	int pgactivate = 0;
> >  	unsigned long nr_dirty = 0;
> > +	unsigned long nr_dirty_seen = 0;
> >  	unsigned long nr_reclaimed = 0;
> >  
> >  	cond_resched();
> > @@ -790,6 +804,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> >  		}
> >  
> >  		if (PageDirty(page)) {
> > +			nr_dirty_seen++;
> > +
> >  			/*
> >  			 * Only kswapd can writeback filesystem pages to
> >  			 * avoid risk of stack overflow
> > @@ -923,6 +939,18 @@ keep_lumpy:
> >  
> >  	list_splice(&ret_pages, page_list);
> >  
> > +	/*
> > +	 * If reclaim is encountering dirty pages, it may be because
> > +	 * dirty pages are reaching the end of the LRU even though the
> > +	 * dirty_ratio may be satisified. In this case, wake flusher
> > +	 * threads to pro-actively clean up to a maximum of
> > +	 * 4 * SWAP_CLUSTER_MAX amount of data (usually 1/2MB) unless
> > +	 * !may_writepage indicates that this is a direct reclaimer in
> > +	 * laptop mode avoiding disk spin-ups
> > +	 */
> > +	if (file && nr_dirty_seen && sc->may_writepage)
> > +		wakeup_flusher_threads(nr_writeback_pages(nr_dirty));
> > +
> 
> Thank you. Ok, I'll check what happens in memcg.
> 

Thanks

> Can I add
> 	if (sc->memcg) {
> 		memcg_check_flusher_wakeup()
> 	}
> or some here ?
> 

It seems reasonable.

> Hm, maybe memcg should wake up flusher at starting try_to_free_memory_cgroup_pages().
> 

I'm afraid I cannot make a judgement call on which is the best as I am
not very familiar with how cgroups behave in comparison to normal
reclaim. There could easily be a follow-on patch though that was cgroup
specific?

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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