mm, compaction: simplify contended compaction handling

Async compaction detects contention either due to failing trylock on
zone->lock or lru_lock, or by need_resched().  Since 1f9efdef4f3f ("mm,
compaction: khugepaged should not give up due to need_resched()") the
code got quite complicated to distinguish these two up to the
__alloc_pages_slowpath() level, so different decisions could be taken
for khugepaged allocations.

After the recent changes, khugepaged allocations don't check for
contended compaction anymore, so we again don't need to distinguish lock
and sched contention, and simplify the current convoluted code a lot.

However, I believe it's also possible to simplify even more and
completely remove the check for contended compaction after the initial
async compaction for costly orders, which was originally aimed at THP
page fault allocations.  There are several reasons why this can be done
now:

- with the new defaults, THP page faults no longer do reclaim/compaction at
  all, unless the system admin has overridden the default, or application has
  indicated via madvise that it can benefit from THP's. In both cases, it
  means that the potential extra latency is expected and worth the benefits.
- even if reclaim/compaction proceeds after this patch where it previously
  wouldn't, the second compaction attempt is still async and will detect the
  contention and back off, if the contention persists
- there are still heuristics like deferred compaction and pageblock skip bits
  in place that prevent excessive THP page fault latencies

Link: http://lkml.kernel.org/r/20160721073614.24395-9-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 11 deletions

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index 0980a6ce4436..d4e106b5dc27 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -55,14 +55,6 @@ enum compact_result {
 	COMPACT_PARTIAL,
 };
 
-/* Used to signal whether compaction detected need_sched() or lock contention */
-/* No contention detected */
-#define COMPACT_CONTENDED_NONE	0
-/* Either need_sched() was true or fatal signal pending */
-#define COMPACT_CONTENDED_SCHED	1
-/* Zone lock or lru_lock was contended in async compaction */
-#define COMPACT_CONTENDED_LOCK	2
-
 struct alloc_context; /* in mm/internal.h */
 
 #ifdef CONFIG_COMPACTION
@@ -76,9 +68,8 @@ extern int sysctl_compact_unevictable_allowed;
 
 extern int fragmentation_index(struct zone *zone, unsigned int order);
 extern enum compact_result try_to_compact_pages(gfp_t gfp_mask,
-			unsigned int order,
-		unsigned int alloc_flags, const struct alloc_context *ac,
-		enum compact_priority prio, int *contended);
+		unsigned int order, unsigned int alloc_flags,
+		const struct alloc_context *ac, enum compact_priority prio);
 extern void compact_pgdat(pg_data_t *pgdat, int order);
 extern void reset_isolation_suitable(pg_data_t *pgdat);
 extern enum compact_result compaction_suitable(struct zone *zone, int order,