[RFC PATCH 1/6] mm, compaction: periodically drop lock and restore IRQs in scanners

[Vlastimil Babka]

Compaction scanners regularly check for lock contention and need_resched()
through the compact_checklock_irqsave() function. However, if there is no
contention, the lock can be held and IRQ disabled for potentially long time.

This has been addressed by commit b2eef8c0d0 ("mm: compaction: minimise the
time IRQs are disabled while isolating pages for migration") for the migration
scanner. However, the refactoring done by commit 748446bb6b ("mm: compaction:
acquire the zone->lru_lock as late as possible") has changed the conditions so
that the lock is dropped only when there's contention on the lock or
need_resched() is true. Also, need_resched() is checked only when the lock is
already held. The comment "give a chance to irqs before checking need_resched"
is therefore misleading, as IRQs remain disabled when the check is done.

This patch restores the behavior intended by commit b2eef8c0d0 and also tries
to better balance and make more deterministic the time spent by checking for
contention vs the time the scanners might run between the checks. It also
avoids situations where checking has not been done often enough before. The
result should be avoiding both too frequent and too infrequent contention
checking, and especially the potentially long-running scans with IRQs disabled
and no checking of need_resched() or for fatal signal pending, which can happen
when many consecutive pages or pageblocks fail the preliminary tests and do not
reach the later call site to compact_checklock_irqsave(), as explained below.

Before the patch:

In the migration scanner, compact_checklock_irqsave() was called each loop, if
reached. If not reached, some lower-frequency checking could still be done if
the lock was already held, but this would not result in aborting contended
async compaction until reaching compact_checklock_irqsave() or end of
pageblock. In the free scanner, it was similar but completely without the
periodical checking, so lock can be potentially held until reaching the end of
pageblock.

After the patch, in both scanners:

The periodical check is done as the first thing in the loop on each
SWAP_CLUSTER_MAX aligned pfn, using the new compact_unlock_should_abort()
function, which always unlocks the lock (if locked) and aborts async compaction
if scheduling is needed or someone else holds the lock. It also aborts any type
of compaction when a fatal signal is pending.

The compact_checklock_irqsave() function is replaced with a slightly different
compact_trylock_irqsave(). The biggest difference is that the function is not
called at all if the lock is already held. The periodical contention checking
is left solely to compact_unlock_should_abort(). If the lock is not held, the
function however does avoid contended run by making async compaction abort when
sched_needed() holds or trylock fails. Sync compaction will conditionally
schedule, and spin on lock.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
---
 mm/compaction.c | 113 ++++++++++++++++++++++++++++++++++++++------------------
 1 file changed, 77 insertions(+), 36 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index ed7102c..f0fd4b5 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -185,47 +185,74 @@ static void update_pageblock_skip(struct compact_control *cc,
 }
 #endif /* CONFIG_COMPACTION */
 
-static inline bool should_release_lock(spinlock_t *lock)
+/*
+ * Compaction requires the taking of some coarse locks that are potentially
+ * very heavily contended. Check if the process needs to be scheduled or
+ * if the lock is contended. For async compaction, back out if the process
+ * needs to be scheduled, or the lock cannot be taken immediately. For sync
+ * compaction, schedule and spin on the lock if needed.
+ *
+ * Returns true if the lock is held
+ * Returns false if the lock is not held and compaction should abort
+ */
+static inline bool compact_trylock_irqsave(spinlock_t *lock,
+			unsigned long *flags, struct compact_control *cc)
 {
-	return need_resched() || spin_is_contended(lock);
+	if (cc->mode == MIGRATE_ASYNC) {
+		if (need_resched() || !spin_trylock_irqsave(lock, *flags)) {
+			cc->contended = true;
+			return false;
+		}
+	} else {
+		cond_resched();
+		spin_lock_irqsave(lock, *flags);
+	}
+
+	return true;
 }
 
 /*
  * Compaction requires the taking of some coarse locks that are potentially
- * very heavily contended. Check if the process needs to be scheduled or
- * if the lock is contended. For async compaction, back out in the event
- * if contention is severe. For sync compaction, schedule.
+ * very heavily contended. The lock should be periodically unlocked to avoid
+ * having disabled IRQs for a long time, even when there is nobody waiting on
+ * the lock. It might also be that allowing the IRQs will result in
+ * need_resched() becoming true. If scheduling is needed, or somebody else
+ * has taken the lock, async compaction aborts. Sync compaction schedules.
+ * Either compaction type will also abort if a fatal signal is pending.
+ * In either case if the lock was locked, it is dropped and not regained.
  *
- * Returns true if the lock is held.
- * Returns false if the lock is released and compaction should abort
+ * Returns true if compaction should abort due to fatal signal pending, or
+ *		async compaction due to lock contention or need to schedule
+ * Returns false when compaction can continue (sync compaction might have
+ *		scheduled)
  */
-static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags,
-				      bool locked, struct compact_control *cc)
+static inline bool compact_unlock_should_abort(spinlock_t *lock,
+		unsigned long flags, bool *locked, struct compact_control *cc)
 {
-	if (should_release_lock(lock)) {
-		if (locked) {
-			spin_unlock_irqrestore(lock, *flags);
-			locked = false;
-		}
+	if (*locked) {
+		spin_unlock_irqrestore(lock, flags);
+		*locked = false;
+	}
 
-		/* async aborts if taking too long or contended */
-		if (cc->mode == MIGRATE_ASYNC) {
+	if (fatal_signal_pending(current))
+		return true;
+
+	if (cc->mode == MIGRATE_ASYNC) {
+		if (need_resched() || spin_is_locked(lock)) {
 			cc->contended = true;
-			return false;
+			return true;
 		}
-
+	} else {
 		cond_resched();
 	}
 
-	if (!locked)
-		spin_lock_irqsave(lock, *flags);
-	return true;
+	return false;
 }
 
 /*
- * Aside from avoiding lock contention, compaction also periodically checks
+ * Aside from avoiding lock contention, compaction should also periodically checks
  * need_resched() and either schedules in sync compaction, or aborts async
- * compaction. This is similar to compact_checklock_irqsave() does, but used
+ * compaction. This is similar to compact_unlock_should_abort() does, but used
  * where no lock is concerned.
  *
  * Returns false when no scheduling was needed, or sync compaction scheduled.
@@ -285,6 +312,16 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
 		int isolated, i;
 		struct page *page = cursor;
 
+		/*
+		 * Periodically drop the lock (if held) regardless of its
+		 * contention, to give chance to IRQs. Abort async compaction
+		 * if contended.
+		 */
+		if (!(blockpfn % SWAP_CLUSTER_MAX)
+		    && compact_unlock_should_abort(&cc->zone->lock, flags,
+								&locked, cc))
+			break;
+
 		nr_scanned++;
 		if (!pfn_valid_within(blockpfn))
 			goto isolate_fail;
@@ -302,8 +339,9 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
 		 * spin on the lock and we acquire the lock as late as
 		 * possible.
 		 */
-		locked = compact_checklock_irqsave(&cc->zone->lock, &flags,
-								locked, cc);
+		if (!locked)
+			locked = compact_trylock_irqsave(&cc->zone->lock,
+								&flags, cc);
 		if (!locked)
 			break;
 
@@ -523,13 +561,15 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
 
 	/* Time to isolate some pages for migration */
 	for (; low_pfn < end_pfn; low_pfn++) {
-		/* give a chance to irqs before checking need_resched() */
-		if (locked && !(low_pfn % SWAP_CLUSTER_MAX)) {
-			if (should_release_lock(&zone->lru_lock)) {
-				spin_unlock_irqrestore(&zone->lru_lock, flags);
-				locked = false;
-			}
-		}
+		/*
+		 * Periodically drop the lock (if held) regardless of its
+		 * contention, to give chance to IRQs. Abort async compaction
+		 * if contended.
+		 */
+		if (!(low_pfn % SWAP_CLUSTER_MAX)
+		    && compact_unlock_should_abort(&zone->lru_lock, flags,
+								&locked, cc))
+			break;
 
 		/*
 		 * migrate_pfn does not necessarily start aligned to a
@@ -631,10 +671,11 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
 		    page_count(page) > page_mapcount(page))
 			continue;
 
-		/* Check if it is ok to still hold the lock */
-		locked = compact_checklock_irqsave(&zone->lru_lock, &flags,
-								locked, cc);
-		if (!locked || fatal_signal_pending(current))
+		/* If the lock is not held, try to take it */
+		if (!locked)
+			locked = compact_trylock_irqsave(&zone->lru_lock,
+								&flags, cc);
+		if (!locked)
 			break;
 
 		/* Recheck PageLRU and PageTransHuge under lock */
-- 
1.8.4.5

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