mm, compaction: khugepaged should not give up due to need_resched()

Async compaction aborts when it detects zone lock contention or
need_resched() is true.  David Rientjes has reported that in practice,
most direct async compactions for THP allocation abort due to
need_resched().  This means that a second direct compaction is never
attempted, which might be OK for a page fault, but khugepaged is intended
to attempt a sync compaction in such case and in these cases it won't.

This patch replaces "bool contended" in compact_control with an int that
distinguishes between aborting due to need_resched() and aborting due to
lock contention.  This allows propagating the abort through all compaction
functions as before, but passing the abort reason up to
__alloc_pages_slowpath() which decides when to continue with direct
reclaim and another compaction attempt.

Another problem is that try_to_compact_pages() did not act upon the
reported contention (both need_resched() or lock contention) immediately
and would proceed with another zone from the zonelist.  When
need_resched() is true, that means initializing another zone compaction,
only to check again need_resched() in isolate_migratepages() and aborting.
 For zone lock contention, the unintended consequence is that the lock
contended status reported back to the allocator is detrmined from the last
zone where compaction was attempted, which is rather arbitrary.

This patch fixes the problem in the following way:
- async compaction of a zone aborting due to need_resched() or fatal signal
  pending means that further zones should not be tried. We report
  COMPACT_CONTENDED_SCHED to the allocator.
- aborting zone compaction due to lock contention means we can still try
  another zone, since it has different set of locks. We report back
  COMPACT_CONTENDED_LOCK only if *all* zones where compaction was attempted,
  it was aborted due to lock contention.

As a result of these fixes, khugepaged will proceed with second sync
compaction as intended, when the preceding async compaction aborted due to
need_resched().  Page fault compactions aborting due to need_resched()
will spare some cycles previously wasted by initializing another zone
compaction only to abort again.  Lock contention will be reported only
when compaction in all zones aborted due to lock contention, and therefore
it's not a good idea to try again after reclaim.

In stress-highalloc from mmtests configured to use __GFP_NO_KSWAPD, this
has improved number of THP collapse allocations by 10%, which shows
positive effect on khugepaged.  The benchmark's success rates are
unchanged as it is not recognized as khugepaged.  Numbers of compact_stall
and compact_fail events have however decreased by 20%, with
compact_success still a bit improved, which is good.  With benchmark
configured not to use __GFP_NO_KSWAPD, there is 6% improvement in THP
collapse allocations, and only slight improvement in stalls and failures.

[akpm@linux-foundation.org: fix warnings]
Reported-by: David Rientjes <rientjes@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 76 insertions, 11 deletions

diff --git a/mm/compaction.c b/mm/compaction.c
index 1067c07..26bb20e 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -223,9 +223,21 @@ static void update_pageblock_skip(struct compact_control *cc,
 }
 #endif /* CONFIG_COMPACTION */
 
-static inline bool should_release_lock(spinlock_t *lock)
+static int should_release_lock(spinlock_t *lock)
 {
-	return need_resched() || spin_is_contended(lock);
+	/*
+	 * Sched contention has higher priority here as we may potentially
+	 * have to abort whole compaction ASAP. Returning with lock contention
+	 * means we will try another zone, and further decisions are
+	 * influenced only when all zones are lock contended. That means
+	 * potentially missing a lock contention is less critical.
+	 */
+	if (need_resched())
+		return COMPACT_CONTENDED_SCHED;
+	else if (spin_is_contended(lock))
+		return COMPACT_CONTENDED_LOCK;
+
+	return COMPACT_CONTENDED_NONE;
 }
 
 /*
@@ -240,7 +252,9 @@ static inline bool should_release_lock(spinlock_t *lock)
 static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags,
 				      bool locked, struct compact_control *cc)
 {
-	if (should_release_lock(lock)) {
+	int contended = should_release_lock(lock);
+
+	if (contended) {
 		if (locked) {
 			spin_unlock_irqrestore(lock, *flags);
 			locked = false;
@@ -248,7 +262,7 @@ static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags,
 
 		/* async aborts if taking too long or contended */
 		if (cc->mode == MIGRATE_ASYNC) {
-			cc->contended = true;
+			cc->contended = contended;
 			return false;
 		}
 
@@ -274,7 +288,7 @@ static inline bool compact_should_abort(struct compact_control *cc)
 	/* async compaction aborts if contended */
 	if (need_resched()) {
 		if (cc->mode == MIGRATE_ASYNC) {
-			cc->contended = true;
+			cc->contended = COMPACT_CONTENDED_SCHED;
 			return true;
 		}
 
@@ -1140,7 +1154,7 @@ out:
 }
 
 static unsigned long compact_zone_order(struct zone *zone, int order,
-		gfp_t gfp_mask, enum migrate_mode mode, bool *contended)
+		gfp_t gfp_mask, enum migrate_mode mode, int *contended)
 {
 	unsigned long ret;
 	struct compact_control cc = {
@@ -1172,14 +1186,15 @@ int sysctl_extfrag_threshold = 500;
  * @gfp_mask: The GFP mask of the current allocation
  * @nodemask: The allowed nodes to allocate from
  * @mode: The migration mode for async, sync light, or sync migration
- * @contended: Return value that is true if compaction was aborted due to lock contention
+ * @contended: Return value that determines if compaction was aborted due to
+ *	       need_resched() or lock contention
  * @candidate_zone: Return the zone where we think allocation should succeed
  *
  * This is the main entry point for direct page compaction.
  */
 unsigned long try_to_compact_pages(struct zonelist *zonelist,
 			int order, gfp_t gfp_mask, nodemask_t *nodemask,
-			enum migrate_mode mode, bool *contended,
+			enum migrate_mode mode, int *contended,
 			struct zone **candidate_zone)
 {
 	enum zone_type high_zoneidx = gfp_zone(gfp_mask);
@@ -1189,6 +1204,9 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist,
 	struct zone *zone;
 	int rc = COMPACT_DEFERRED;
 	int alloc_flags = 0;
+	int all_zones_contended = COMPACT_CONTENDED_LOCK; /* init for &= op */
+
+	*contended = COMPACT_CONTENDED_NONE;
 
 	/* Check if the GFP flags allow compaction */
 	if (!order || !may_enter_fs || !may_perform_io)
@@ -1202,13 +1220,19 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist,
 	for_each_zone_zonelist_nodemask(zone, z, zonelist, high_zoneidx,
 								nodemask) {
 		int status;
+		int zone_contended;
 
 		if (compaction_deferred(zone, order))
 			continue;
 
 		status = compact_zone_order(zone, order, gfp_mask, mode,
-						contended);
+							&zone_contended);
 		rc = max(status, rc);
+		/*
+		 * It takes at least one zone that wasn't lock contended
+		 * to clear all_zones_contended.
+		 */
+		all_zones_contended &= zone_contended;
 
 		/* If a normal allocation would succeed, stop compacting */
 		if (zone_watermark_ok(zone, order, low_wmark_pages(zone), 0,
@@ -1221,8 +1245,21 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist,
 			 * succeeds in this zone.
 			 */
 			compaction_defer_reset(zone, order, false);
-			break;
-		} else if (mode != MIGRATE_ASYNC) {
+			/*
+			 * It is possible that async compaction aborted due to
+			 * need_resched() and the watermarks were ok thanks to
+			 * somebody else freeing memory. The allocation can
+			 * however still fail so we better signal the
+			 * need_resched() contention anyway (this will not
+			 * prevent the allocation attempt).
+			 */
+			if (zone_contended == COMPACT_CONTENDED_SCHED)
+				*contended = COMPACT_CONTENDED_SCHED;
+
+			goto break_loop;
+		}
+
+		if (mode != MIGRATE_ASYNC) {
 			/*
 			 * We think that allocation won't succeed in this zone
 			 * so we defer compaction there. If it ends up
@@ -1230,8 +1267,36 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist,
 			 */
 			defer_compaction(zone, order);
 		}
+
+		/*
+		 * We might have stopped compacting due to need_resched() in
+		 * async compaction, or due to a fatal signal detected. In that
+		 * case do not try further zones and signal need_resched()
+		 * contention.
+		 */
+		if ((zone_contended == COMPACT_CONTENDED_SCHED)
+					|| fatal_signal_pending(current)) {
+			*contended = COMPACT_CONTENDED_SCHED;
+			goto break_loop;
+		}
+
+		continue;
+break_loop:
+		/*
+		 * We might not have tried all the zones, so  be conservative
+		 * and assume they are not all lock contended.
+		 */
+		all_zones_contended = 0;
+		break;
 	}
 
+	/*
+	 * If at least one zone wasn't deferred or skipped, we report if all
+	 * zones that were tried were lock contended.
+	 */
+	if (rc > COMPACT_SKIPPED && all_zones_contended)
+		*contended = COMPACT_CONTENDED_LOCK;
+
 	return rc;
 }