Re: [PATCH 1/4] Properly account for the number of page cachepages zone_reclaim() can reclaim

[Wu Fengguang]

On Wed, Jun 10, 2009 at 06:31:53PM +0800, Mel Gorman wrote:
> On Wed, Jun 10, 2009 at 09:19:39AM +0800, Wu Fengguang wrote:
> > On Wed, Jun 10, 2009 at 01:01:41AM +0800, Mel Gorman wrote:
> > > On NUMA machines, the administrator can configure zone_reclaim_mode that
> > > is a more targetted form of direct reclaim. On machines with large NUMA
> > > distances for example, a zone_reclaim_mode defaults to 1 meaning that clean
> > > unmapped pages will be reclaimed if the zone watermarks are not being met.
> > > 
> > > There is a heuristic that determines if the scan is worthwhile but the
> > > problem is that the heuristic is not being properly applied and is basically
> > > assuming zone_reclaim_mode is 1 if it is enabled.
> > > 
> > > Historically, once enabled it was depending on NR_FILE_PAGES which may
> > > include swapcache pages that the reclaim_mode cannot deal with.  Patch
> > > vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch by
> > > Kosaki Motohiro noted that zone_page_state(zone, NR_FILE_PAGES) included
> > > pages that were not file-backed such as swapcache and made a calculation
> > > based on the inactive, active and mapped files. This is far superior
> > > when zone_reclaim==1 but if RECLAIM_SWAP is set, then NR_FILE_PAGES is a
> > > reasonable starting figure.
> > > 
> > > This patch alters how zone_reclaim() works out how many pages it might be
> > > able to reclaim given the current reclaim_mode. If RECLAIM_SWAP is set
> > > in the reclaim_mode it will either consider NR_FILE_PAGES as potential
> > > candidates or else use NR_{IN}ACTIVE}_PAGES-NR_FILE_MAPPED to discount
> > > swapcache and other non-file-backed pages.  If RECLAIM_WRITE is not set,
> > > then NR_FILE_DIRTY number of pages are not candidates. If RECLAIM_SWAP is
> > > not set, then NR_FILE_MAPPED are not.
> > > 
> > > Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> > > Acked-by: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
> > > ---
> > >  mm/vmscan.c |   52 ++++++++++++++++++++++++++++++++++++++--------------
> > >  1 files changed, 38 insertions(+), 14 deletions(-)
> > > 
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index 2ddcfc8..2bfc76e 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -2333,6 +2333,41 @@ int sysctl_min_unmapped_ratio = 1;
> > >   */
> > >  int sysctl_min_slab_ratio = 5;
> > >  
> > > +static inline unsigned long zone_unmapped_file_pages(struct zone *zone)
> > > +{
> > > +	return zone_page_state(zone, NR_INACTIVE_FILE) +
> > > +		zone_page_state(zone, NR_ACTIVE_FILE) -
> > > +		zone_page_state(zone, NR_FILE_MAPPED);
> > 
> > This may underflow if too many tmpfs pages are mapped.
> > 
> 
> You're right. This is also a bug now in mmotm for patch
> vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch which
> is where I took this code out of and didn't think deeply enough about.
> Well spotted.
> 
> Should this be something like?
> 
> static unsigned long zone_unmapped_file_pages(struct zone *zone)
> {
> 	unsigned long file_mapped = zone_page_state(zone, NR_FILE_MAPPED);
> 	unsigned long file_lru = zone_page_state(zone, NR_INACTIVE_FILE)
> 			zone_page_state(zone, NR_ACTIVE_FILE);
> 
> 	return (file_lru > file_mapped) ? (file_lru - file_mapped) : 0;
> }
> 
> ?
> 
> If that returns 0, it does mean that there are very few pages that the
> current reclaim_mode is going to be able to deal with so even if the
> count is not perfect, it should be good enough for what we need it for.

Agreed. We opt to give up direct zone reclaim than to risk busy looping ;)

> > > +}
> > > +
> > > +/* Work out how many page cache pages we can reclaim in this reclaim_mode */
> > > +static inline long zone_pagecache_reclaimable(struct zone *zone)
> > > +{
> > > +	long nr_pagecache_reclaimable;
> > > +	long delta = 0;
> > > +
> > > +	/*
> > > +	 * If RECLAIM_SWAP is set, then all file pages are considered
> > > +	 * potentially reclaimable. Otherwise, we have to worry about
> > > +	 * pages like swapcache and zone_unmapped_file_pages() provides
> > > +	 * a better estimate
> > > +	 */
> > > +	if (zone_reclaim_mode & RECLAIM_SWAP)
> > > +		nr_pagecache_reclaimable = zone_page_state(zone, NR_FILE_PAGES);
> > > +	else
> > > +		nr_pagecache_reclaimable = zone_unmapped_file_pages(zone);
> > > +
> > > +	/* If we can't clean pages, remove dirty pages from consideration */
> > > +	if (!(zone_reclaim_mode & RECLAIM_WRITE))
> > > +		delta += zone_page_state(zone, NR_FILE_DIRTY);
> > > +
> > > +	/* Beware of double accounting */
> > 
> > The double accounting happens for NR_FILE_MAPPED but not
> > NR_FILE_DIRTY(dirty tmpfs pages won't be accounted),
> 
> I should have taken that out. In an interim version, delta was altered
> more than once in a way that could have caused underflow.
> 
> > so this comment
> > is more suitable for zone_unmapped_file_pages(). But the double
> > accounting does affects this abstraction. So a more reasonable
> > sequence could be to first substract NR_FILE_DIRTY and then
> > conditionally substract NR_FILE_MAPPED?
> 
> The end result is the same I believe and I prefer having the
> zone_unmapped_file_pages() doing just that and nothing else because it's
> in line with what zone_lru_pages() does.

OK.

> > Or better to introduce a new counter NR_TMPFS_MAPPED to fix this mess?
> > 
> 
> I considered such a counter and dismissed it but maybe it merits wider discussion.
> 
> My problem with it is that it would affect the pagecache add/remove hot paths
> and a few other sites and increase the amount of accouting we do within a
> zone. It seemed unjustified to help a seldom executed slow path that only
> runs on NUMA.

We are not talking about NR_TMPFS_PAGES, but NR_TMPFS_MAPPED :)

We only need to account it in page_add_file_rmap() and page_remove_rmap(),
I don't think they are too hot paths. And the relative cost is low enough.

It will look like this.

---
 include/linux/mmzone.h |    1 +
 mm/rmap.c              |    4 ++++
 2 files changed, 5 insertions(+)

--- linux.orig/include/linux/mmzone.h
+++ linux/include/linux/mmzone.h
@@ -99,6 +99,7 @@ enum zone_stat_item {
 	NR_VMSCAN_WRITE,
 	/* Second 128 byte cacheline */
 	NR_WRITEBACK_TEMP,	/* Writeback using temporary buffers */
+	NR_TMPFS_MAPPED,
 #ifdef CONFIG_NUMA
 	NUMA_HIT,		/* allocated in intended node */
 	NUMA_MISS,		/* allocated in non intended node */
--- linux.orig/mm/rmap.c
+++ linux/mm/rmap.c
@@ -844,6 +844,8 @@ void page_add_file_rmap(struct page *pag
 {
 	if (atomic_inc_and_test(&page->_mapcount)) {
 		__inc_zone_page_state(page, NR_FILE_MAPPED);
+		if (PageSwapBacked(page))
+			__inc_zone_page_state(page, NR_TMPFS_MAPPED);
 		mem_cgroup_update_mapped_file_stat(page, 1);
 	}
 }
@@ -894,6 +896,8 @@ void page_remove_rmap(struct page *page)
 			mem_cgroup_uncharge_page(page);
 		__dec_zone_page_state(page,
 			PageAnon(page) ? NR_ANON_PAGES : NR_FILE_MAPPED);
+		if (!PageAnon(page) && PageSwapBacked(page))
+			__dec_zone_page_state(page, NR_TMPFS_MAPPED);
 		mem_cgroup_update_mapped_file_stat(page, -1);
 		/*
 		 * It would be tidy to reset the PageAnon mapping here,
--
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