+ mm-compaction-raise-compaction-priority-after-it-withdrawns.patch added to -mm tree

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The patch titled
     Subject: mm, compaction: raise compaction priority after it withdrawns
has been added to the -mm tree.  Its filename is
     mm-compaction-raise-compaction-priority-after-it-withdrawns.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/mm-compaction-raise-compaction-priority-after-it-withdrawns.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/mm-compaction-raise-compaction-priority-after-it-withdrawns.patch

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------------------------------------------------------
From: Vlastimil Babka <vbabka@xxxxxxx>
Subject: mm, compaction: raise compaction priority after it withdrawns

Mike Kravetz reports that "hugetlb allocations could stall for minutes or
hours when should_compact_retry() would return true more often then it
should.  Specifically, this was in the case where compact_result was
COMPACT_DEFERRED and COMPACT_PARTIAL_SKIPPED and no progress was being
made."

The problem is that the compaction_withdrawn() test in
should_compact_retry() includes compaction outcomes that are only possible
on low compaction priority, and results in a retry without increasing the
priority.  This may result in furter reclaim, and more incomplete
compaction attempts.

With this patch, compaction priority is raised when possible, or
should_compact_retry() returns false.

The COMPACT_SKIPPED result doesn't really fit together with the other
outcomes in compaction_withdrawn(), as that's a result caused by
insufficient order-0 pages, not due to low compaction priority.  With this
patch, it is moved to a new compaction_needs_reclaim() function, and for
that outcome we keep the current logic of retrying if it looks like
reclaim will be able to help.

Link: http://lkml.kernel.org/r/20190806014744.15446-4-mike.kravetz@xxxxxxxxxx
Reported-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Tested-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Hillf Danton <hdanton@xxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 include/linux/compaction.h |   22 +++++++++++++++++-----
 mm/page_alloc.c            |   16 ++++++++++++----
 2 files changed, 29 insertions(+), 9 deletions(-)

--- a/include/linux/compaction.h~mm-compaction-raise-compaction-priority-after-it-withdrawns
+++ a/include/linux/compaction.h
@@ -129,11 +129,8 @@ static inline bool compaction_failed(enu
 	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 needs reclaim to be performed first, so it can continue. */
+static inline bool compaction_needs_reclaim(enum compact_result result)
 {
 	/*
 	 * Compaction backed off due to watermark checks for order-0
@@ -142,6 +139,16 @@ static inline bool compaction_withdrawn(
 	if (result == COMPACT_SKIPPED)
 		return true;
 
+	return false;
+}
+
+/*
+ * Compaction has backed off for some reason after doing some work or none
+ * at all. It might be throttling or lock contention. Retrying might be still
+ * worthwhile, but with a higher priority if allowed.
+ */
+static inline bool compaction_withdrawn(enum compact_result result)
+{
 	/*
 	 * If compaction is deferred for high-order allocations, it is
 	 * because sync compaction recently failed. If this is the case
@@ -206,6 +213,11 @@ static inline bool compaction_failed(enu
 {
 	return false;
 }
+
+static inline bool compaction_needs_reclaim(enum compact_result result)
+{
+	return false;
+}
 
 static inline bool compaction_withdrawn(enum compact_result result)
 {
--- a/mm/page_alloc.c~mm-compaction-raise-compaction-priority-after-it-withdrawns
+++ a/mm/page_alloc.c
@@ -3966,14 +3966,22 @@ should_compact_retry(struct alloc_contex
 		goto check_priority;
 
 	/*
+	 * compaction was skipped because there are not enough order-0 pages
+	 * to work with, so we retry only if it looks like reclaim can help.
+	 */
+	if (compaction_needs_reclaim(compact_result)) {
+		ret = compaction_zonelist_suitable(ac, order, alloc_flags);
+		goto out;
+	}
+
+	/*
 	 * make sure the compaction wasn't deferred or didn't bail out early
 	 * due to locks contention before we declare that we should give up.
-	 * But do not retry if the given zonelist is not suitable for
-	 * compaction.
+	 * But the next retry should use a higher priority if allowed, so
+	 * we don't just keep bailing out endlessly.
 	 */
 	if (compaction_withdrawn(compact_result)) {
-		ret = compaction_zonelist_suitable(ac, order, alloc_flags);
-		goto out;
+		goto check_priority;
 	}
 
 	/*
_

Patches currently in -mm which might be from vbabka@xxxxxxx are

mm-compaction-clear-total_migratefree_scanned-before-scanning-a-new-zone-fix-2.patch
mm-reclaim-cleanup-should_continue_reclaim.patch
mm-compaction-raise-compaction-priority-after-it-withdrawns.patch




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