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mm, compaction: create compact_gap wrapper 

Compaction uses a watermark gap of (2UL << order) pages at various
places and it's not immediately obvious why.  Abstract it through a
compact_gap() wrapper to create a single place with a thorough
explanation.

[vbabka@suse.cz: clarify the comment of compact_gap()]
 Link: http://lkml.kernel.org/r/7b6aed1f-fdf8-2063-9ff4-bbe4de712d37@suse.cz
Link: http://lkml.kernel.org/r/20160810091226.6709-9-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Tested-by: Lorenzo Stoakes <lstoakes@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Vlastimil Babka 2016-10-07 16:57:53 -07:00 committed by Linus Torvalds
parent f2b8228c5f
commit 9861a62c33
3 changed files with 29 additions and 7 deletions
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23
include/linux/compaction.h
View File

@ -58,6 +58,29 @@ enum compact_result {
struct alloc_context; /* in mm/internal.h */
/*
* Number of free order-0 pages that should be available above given watermark
* to make sure compaction has reasonable chance of not running out of free
* pages that it needs to isolate as migration target during its work.
*/
static inline unsigned long compact_gap(unsigned int order)
{
/*
* Although all the isolations for migration are temporary, compaction
* free scanner may have up to 1 << order pages on its list and then
* try to split an (order - 1) free page. At that point, a gap of
* 1 << order might not be enough, so it's safer to require twice that
* amount. Note that the number of pages on the list is also
* effectively limited by COMPACT_CLUSTER_MAX, as that's the maximum
* that the migrate scanner can have isolated on migrate list, and free
* scanner is only invoked when the number of isolated free pages is
* lower than that. But it's not worth to complicate the formula here
* as a bigger gap for higher orders than strictly necessary can also
* improve chances of compaction success.
*/
return 2UL << order;
}
#ifdef CONFIG_COMPACTION
extern int sysctl_compact_memory;
extern int sysctl_compaction_handler(struct ctl_table *table, int write,

7
mm/compaction.c
View File

@ -1391,11 +1391,10 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order,
return COMPACT_SUCCESS;
/*
* Watermarks for order-0 must be met for compaction. Note the 2UL.
* This is because during migration, copies of pages need to be
* allocated and for a short time, the footprint is higher
* Watermarks for order-0 must be met for compaction to be able to
* isolate free pages for migration targets.
*/
watermark = low_wmark_pages(zone) + (2UL << order);
watermark = low_wmark_pages(zone) + compact_gap(order);
if (!__zone_watermark_ok(zone, 0, watermark, classzone_idx,
alloc_flags, wmark_target))
return COMPACT_SKIPPED;

6
mm/vmscan.c
View File

@ -2480,7 +2480,7 @@ static inline bool should_continue_reclaim(struct pglist_data *pgdat,
* If we have not reclaimed enough pages for compaction and the
* inactive lists are large enough, continue reclaiming
*/
pages_for_compaction = (2UL << sc->order);
pages_for_compaction = compact_gap(sc->order);
inactive_lru_pages = node_page_state(pgdat, NR_INACTIVE_FILE);
if (get_nr_swap_pages() > 0)
inactive_lru_pages += node_page_state(pgdat, NR_INACTIVE_ANON);
@ -2612,7 +2612,7 @@ static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
* there is a buffer of free pages available to give compaction
* a reasonable chance of completing and allocating the page
*/
watermark = high_wmark_pages(zone) + (2UL << sc->order);
watermark = high_wmark_pages(zone) + compact_gap(sc->order);
watermark_ok = zone_watermark_ok_safe(zone, 0, watermark, sc->reclaim_idx);
/*
@ -3169,7 +3169,7 @@ static bool kswapd_shrink_node(pg_data_t *pgdat,
* excessive reclaim. Assume that a process requested a high-order
* can direct reclaim/compact.
*/
if (sc->order && sc->nr_reclaimed >= 2UL << sc->order)
if (sc->order && sc->nr_reclaimed >= compact_gap(sc->order))
sc->order = 0;
return sc->nr_scanned >= sc->nr_to_reclaim;