vmscan: properly account for the number of page cache pages zone_reclaim() can reclaim

A bug was brought to my attention against a distro kernel but it affects
mainline and I believe problems like this have been reported in various
guises on the mailing lists although I don't have specific examples at the
moment.

The reported problem was that malloc() stalled for a long time (minutes in
some cases) if a large tmpfs mount was occupying a large percentage of
memory overall.  The pages did not get cleaned or reclaimed by
zone_reclaim() because the zone_reclaim_mode was unsuitable, but the lists
are uselessly scanned frequencly making the CPU spin at near 100%.

This patchset intends to address that bug and bring the behaviour of
zone_reclaim() more in line with expectations which were noticed during
investigation.  It is based on top of mmotm and takes advantage of
Kosaki's work with respect to zone_reclaim().

Patch 1 fixes the heuristics that zone_reclaim() uses to determine if the
	scan should go ahead. The broken heuristic is what was causing the
	malloc() stall as it uselessly scanned the LRU constantly. Currently,
	zone_reclaim is assuming zone_reclaim_mode is 1 and historically it
	could not deal with tmpfs pages at all. This fixes up the heuristic so
	that an unnecessary scan is more likely to be correctly avoided.

Patch 2 notes that zone_reclaim() returning a failure automatically means
	the zone is marked full. This is not always true. It could have
	failed because the GFP mask or zone_reclaim_mode were unsuitable.

Patch 3 introduces a counter zreclaim_failed that will increment each
	time the zone_reclaim scan-avoidance heuristics fail. If that
	counter is rapidly increasing, then zone_reclaim_mode should be
	set to 0 as a temporarily resolution and a bug reported because
	the scan-avoidance heuristic is still broken.

This patch:

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.  The lack of
proper detection can manfiest as high CPU usage as the LRU list is scanned
uselessly.

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.

[kosaki.motohiro@jp.fujitsu.com: Estimate unmapped pages minus tmpfs pages]
[fengguang.wu@intel.com: Fix underflow problem in Kosaki's estimate]
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Rik van Riel <riel@redhat.com>
Acked-by: Christoph Lameter <cl@linux-foundation.org>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Mel Gorman 2009-06-16 15:33:20 -07:00 committed by Linus Torvalds
parent 84a8924560
commit 90afa5de6f
2 changed files with 53 additions and 11 deletions

View File

@ -315,10 +315,14 @@ min_unmapped_ratio:
This is available only on NUMA kernels. This is available only on NUMA kernels.
A percentage of the total pages in each zone. Zone reclaim will only This is a percentage of the total pages in each zone. Zone reclaim will
occur if more than this percentage of pages are file backed and unmapped. only occur if more than this percentage of pages are in a state that
This is to insure that a minimal amount of local pages is still available for zone_reclaim_mode allows to be reclaimed.
file I/O even if the node is overallocated.
If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared
against all file-backed unmapped pages including swapcache pages and tmpfs
files. Otherwise, only unmapped pages backed by normal files but not tmpfs
files and similar are considered.
The default is 1 percent. The default is 1 percent.

View File

@ -2356,6 +2356,48 @@ int sysctl_min_unmapped_ratio = 1;
*/ */
int sysctl_min_slab_ratio = 5; int sysctl_min_slab_ratio = 5;
static inline 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);
/*
* It's possible for there to be more file mapped pages than
* accounted for by the pages on the file LRU lists because
* tmpfs pages accounted for as ANON can also be FILE_MAPPED
*/
return (file_lru > file_mapped) ? (file_lru - file_mapped) : 0;
}
/* Work out how many page cache pages we can reclaim in this reclaim_mode */
static 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);
/* Watch for any possible underflows due to delta */
if (unlikely(delta > nr_pagecache_reclaimable))
delta = nr_pagecache_reclaimable;
return nr_pagecache_reclaimable - delta;
}
/* /*
* Try to free up some pages from this zone through reclaim. * Try to free up some pages from this zone through reclaim.
*/ */
@ -2390,9 +2432,7 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
reclaim_state.reclaimed_slab = 0; reclaim_state.reclaimed_slab = 0;
p->reclaim_state = &reclaim_state; p->reclaim_state = &reclaim_state;
if (zone_page_state(zone, NR_FILE_PAGES) - if (zone_pagecache_reclaimable(zone) > zone->min_unmapped_pages) {
zone_page_state(zone, NR_FILE_MAPPED) >
zone->min_unmapped_pages) {
/* /*
* Free memory by calling shrink zone with increasing * Free memory by calling shrink zone with increasing
* priorities until we have enough memory freed. * priorities until we have enough memory freed.
@ -2450,10 +2490,8 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
* if less than a specified percentage of the zone is used by * if less than a specified percentage of the zone is used by
* unmapped file backed pages. * unmapped file backed pages.
*/ */
if (zone_page_state(zone, NR_FILE_PAGES) - if (zone_pagecache_reclaimable(zone) <= zone->min_unmapped_pages &&
zone_page_state(zone, NR_FILE_MAPPED) <= zone->min_unmapped_pages zone_page_state(zone, NR_SLAB_RECLAIMABLE) <= zone->min_slab_pages)
&& zone_page_state(zone, NR_SLAB_RECLAIMABLE)
<= zone->min_slab_pages)
return 0; return 0;
if (zone_is_all_unreclaimable(zone)) if (zone_is_all_unreclaimable(zone))