[Xenial PATCH 06/11] mm, compaction: abstract compaction feedback to helpers

[Thadeu Lima de Souza Cascardo]

From: Michal Hocko <mhocko at suse.com>

BugLink: https://bugs.launchpad.net/bugs/1655842

Compaction can provide a wild variation of feedback to the caller.  Many
of them are implementation specific and the caller of the compaction
(especially the page allocator) shouldn't be bound to specifics of the
current implementation.

This patch abstracts the feedback into three basic types:
	- compaction_made_progress - compaction was active and made some
	  progress.
	- compaction_failed - compaction failed and further attempts to
	  invoke it would most probably fail and therefore it is not
	  worth retrying
	- compaction_withdrawn - compaction wasn't invoked for an
          implementation specific reasons. In the current implementation
          it means that the compaction was deferred, contended or the
          page scanners met too early without any progress. Retrying is
          still worthwhile.

[vbabka at suse.cz: do not change thp back off behavior]
[akpm at linux-foundation.org: fix typo in comment, per Hillf]
Signed-off-by: Michal Hocko <mhocko at suse.com>
Acked-by: Hillf Danton <hillf.zj at alibaba-inc.com>
Acked-by: Vlastimil Babka <vbabka at suse.cz>
Cc: David Rientjes <rientjes at google.com>
Cc: Johannes Weiner <hannes at cmpxchg.org>
Cc: Joonsoo Kim <js1304 at gmail.com>
Cc: Mel Gorman <mgorman at techsingularity.net>
Cc: Tetsuo Handa <penguin-kernel at I-love.SAKURA.ne.jp>
Cc: Vladimir Davydov <vdavydov at virtuozzo.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(backported from commit cab1802b5f0dddea30547a7451fda8c7e4c593f0)
[cascardo: fixup as we don't have kcompactd]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 include/linux/compaction.h | 77 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index ca262db..781fd9b 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -72,6 +72,69 @@ extern void compaction_defer_reset(struct zone *zone, int order,
 				bool alloc_success);
 extern bool compaction_restarting(struct zone *zone, int order);
 
+/* Compaction has made some progress and retrying makes sense */
+static inline bool compaction_made_progress(enum compact_result result)
+{
+	/*
+	 * Even though this might sound confusing this in fact tells us
+	 * that the compaction successfully isolated and migrated some
+	 * pageblocks.
+	 */
+	if (result == COMPACT_PARTIAL)
+		return true;
+
+	return false;
+}
+
+/* Compaction has failed and it doesn't make much sense to keep retrying. */
+static inline bool compaction_failed(enum compact_result result)
+{
+	/* All zones were scanned completely and still not result. */
+	if (result == COMPACT_COMPLETE)
+		return true;
+
+	return false;
+}
+
+/*
+ * Compaction  has backed off for some reason. It might be throttling or
+ * lock contention. Retrying is still worthwhile.
+ */
+static inline bool compaction_withdrawn(enum compact_result result)
+{
+	/*
+	 * Compaction backed off due to watermark checks for order-0
+	 * so the regular reclaim has to try harder and reclaim something.
+	 */
+	if (result == COMPACT_SKIPPED)
+		return true;
+
+	/*
+	 * If compaction is deferred for high-order allocations, it is
+	 * because sync compaction recently failed. If this is the case
+	 * and the caller requested a THP allocation, we do not want
+	 * to heavily disrupt the system, so we fail the allocation
+	 * instead of entering direct reclaim.
+	 */
+	if (result == COMPACT_DEFERRED)
+		return true;
+
+	/*
+	 * If compaction in async mode encounters contention or blocks higher
+	 * priority task we back off early rather than cause stalls.
+	 */
+	if (result == COMPACT_CONTENDED)
+		return true;
+
+	/*
+	 * Page scanners have met but we haven't scanned full zones so this
+	 * is a back off in fact.
+	 */
+	if (result == COMPACT_PARTIAL_SKIPPED)
+		return true;
+
+	return false;
+}
 #else
 static inline enum compact_result try_to_compact_pages(gfp_t gfp_mask,
 			unsigned int order, int alloc_flags,
@@ -104,6 +167,20 @@ static inline bool compaction_deferred(struct zone *zone, int order)
 	return true;
 }
 
+static inline bool compaction_made_progress(enum compact_result result)
+{
+	return false;
+}
+
+static inline bool compaction_failed(enum compact_result result)
+{
+	return false;
+}
+
+static inline bool compaction_withdrawn(enum compact_result result)
+{
+	return true;
+}
 #endif /* CONFIG_COMPACTION */
 
 #if defined(CONFIG_COMPACTION) && defined(CONFIG_SYSFS) && defined(CONFIG_NUMA)
-- 
2.9.3