Re: [Alsa-user] new source of MIDI playback slow-down identified -5a03b051ed87e72b959f32a86054e1142ac4cf55 thp: use compaction in kswapd forGFP_ATOMIC order > 0

[Andrea Arcangeli]

On Wed, Feb 23, 2011 at 04:17:44AM +1030, Arthur Marsh wrote:
> OK, these patches applied together against upstream didn't cause a crash 
> but I did observe:
> 
> significant slowdowns of MIDI playback (moreso than in previous cases, 
> and with less than 20 Meg of swap file in use);
> 
> kswapd0 sharing equal top place in CPU usage at times (e.g. 20 percent).
> 
> If I should try only one of the patches or something else entirely, 
> please let me know.

Yes, with irq off, schedule won't run and need_resched won't get set.

So let's try this.

In case this doesn't fix I definitely give it up with compaction in
kswapd as GFP_ATOMIC will likely not get an huge benefit out of
compaction anyway because 1) the allocations from GFP_ATOMIC are
likely short lived, 2) the cost of the compaction scan loop +
migration may be higher than the benefit we get from jumbo frames

So if this doesn't fix it, I'll already post a definitive fix that
removes compaction from kswapd (but leaves it enabled for direct
reclaim for all order sizes including kernel stack). I already
verified that this solves not just the midi latency issue but the
other server benchmark that I'm dealing with. Then we can think at
compaction+kswapd later for 2.6.39 but I think we need to close this
kswapd issue in time for 2.6.38. So if the below isn't enough the next
patch should be applied. I'll get those two patches tested in the
server load too, and I'll let you know how the results are when it
finishes.

Please apply also the attached "kswapd-high-wmark" before the below
one.

====
Subject: compaction: fix high latencies created by kswapd

From: Andrea Arcangeli <aarcange@xxxxxxxxxx>

We need to proper spin_unlock_irq/cond_resched in the compaction loop to avoid
hurting latencies. We must also stop being too aggressive insisting with
compaction within kswapd if compaction don't make progress in every zone.

Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
---
 include/linux/compaction.h |    6 +++++
 mm/compaction.c            |   53 +++++++++++++++++++++++++++++++++++++--------
 mm/vmscan.c                |   39 +++++++++++++++++++--------------
 3 files changed, 73 insertions(+), 25 deletions(-)

--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -271,9 +271,27 @@ static unsigned long isolate_migratepage
 	}
 
 	/* Time to isolate some pages for migration */
+	cond_resched();
 	spin_lock_irq(&zone->lru_lock);
 	for (; low_pfn < end_pfn; low_pfn++) {
 		struct page *page;
+		int unlocked = 0;
+
+		/* give a chance to irqs before checking need_resched() */
+		if (!((low_pfn+1) % SWAP_CLUSTER_MAX)) {
+			spin_unlock_irq(&zone->lru_lock);
+			unlocked = 1;
+		}
+		if (need_resched() || spin_is_contended(&zone->lru_lock)) {
+			if (fatal_signal_pending(current))
+				break;
+			if (!unlocked)
+				spin_unlock_irq(&zone->lru_lock);
+			cond_resched();
+			spin_lock_irq(&zone->lru_lock);
+		} else if (unlocked)
+			spin_lock_irq(&zone->lru_lock);
+
 		if (!pfn_valid_within(low_pfn))
 			continue;
 		nr_scanned++;
@@ -436,6 +454,28 @@ static int compact_finished(struct zone 
 	return COMPACT_CONTINUE;
 }
 
+static unsigned long compaction_watermark(struct zone *zone, int order)
+{
+	/*
+	 * Watermarks for order-0 must be met for compaction. Note the 2UL.
+	 * This is because during migration, copies of pages need to be
+	 * allocated and for a short time, the footprint is higher
+	 */
+	return low_wmark_pages(zone) + (2UL << order);
+}
+
+static int __compaction_need_reclaim(struct zone *zone, int order,
+				     unsigned long watermark)
+{
+	return !zone_watermark_ok(zone, 0, watermark, 0, 0);
+}
+
+int compaction_need_reclaim(struct zone *zone, int order)
+{
+	return __compaction_need_reclaim(zone, order,
+					 compaction_watermark(zone, order));
+}
+
 /*
  * compaction_suitable: Is this suitable to run compaction on this zone now?
  * Returns
@@ -449,21 +489,16 @@ unsigned long compaction_suitable(struct
 	unsigned long watermark;
 
 	/*
-	 * Watermarks for order-0 must be met for compaction. Note the 2UL.
-	 * This is because during migration, copies of pages need to be
-	 * allocated and for a short time, the footprint is higher
-	 */
-	watermark = low_wmark_pages(zone) + (2UL << order);
-	if (!zone_watermark_ok(zone, 0, watermark, 0, 0))
-		return COMPACT_SKIPPED;
-
-	/*
 	 * order == -1 is expected when compacting via
 	 * /proc/sys/vm/compact_memory
 	 */
 	if (order == -1)
 		return COMPACT_CONTINUE;
 
+	watermark = compaction_watermark(zone, order);
+	if (__compaction_need_reclaim(zone, order, watermark))
+		return COMPACT_SKIPPED;
+
 	/*
 	 * fragmentation index determines if allocation failures are due to
 	 * low memory or external fragmentation
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2385,9 +2385,9 @@ loop_again:
 		 * cause too much scanning of the lower zones.
 		 */
 		for (i = 0; i <= end_zone; i++) {
-			int compaction;
 			struct zone *zone = pgdat->node_zones + i;
 			int nr_slab;
+			int local_order;
 
 			if (!populated_zone(zone))
 				continue;
@@ -2407,20 +2407,21 @@ loop_again:
 			 * We put equal pressure on every zone, unless one
 			 * zone has way too many pages free already.
 			 */
-			if (!zone_watermark_ok_safe(zone, order,
-					high_wmark_pages(zone), end_zone, 0))
+			if (!zone_watermark_ok_safe(zone, 0,
+					high_wmark_pages(zone), end_zone, 0) ||
+			    compaction_need_reclaim(zone, order)) {
 				shrink_zone(priority, zone, &sc);
-			reclaim_state->reclaimed_slab = 0;
-			nr_slab = shrink_slab(sc.nr_scanned, GFP_KERNEL,
-						lru_pages);
-			sc.nr_reclaimed += reclaim_state->reclaimed_slab;
-			total_scanned += sc.nr_scanned;
+				reclaim_state->reclaimed_slab = 0;
+				nr_slab = shrink_slab(sc.nr_scanned,
+						      GFP_KERNEL,
+						      lru_pages);
+				sc.nr_reclaimed += reclaim_state->reclaimed_slab;
+				total_scanned += sc.nr_scanned;
+			}
 
-			compaction = 0;
+			local_order = order;
 			if (order &&
-			    zone_watermark_ok(zone, 0,
-					       high_wmark_pages(zone),
-					      end_zone, 0) &&
+			    !compaction_need_reclaim(zone, order) &&
 			    !zone_watermark_ok(zone, order,
 					       high_wmark_pages(zone),
 					       end_zone, 0)) {
@@ -2428,12 +2429,18 @@ loop_again:
 						   order,
 						   sc.gfp_mask, false,
 						   COMPACT_MODE_KSWAPD);
-				compaction = 1;
+				/*
+				 * Let kswapd stop immediately even if
+				 * compaction didn't succeed to
+				 * generate "high_wmark_pages" of the
+				 * max order wanted in every zone.
+				 */
+				local_order = 0;
 			}
 
 			if (zone->all_unreclaimable)
 				continue;
-			if (!compaction && nr_slab == 0 &&
+			if (nr_slab == 0 &&
 			    !zone_reclaimable(zone))
 				zone->all_unreclaimable = 1;
 			/*
@@ -2445,7 +2452,7 @@ loop_again:
 			    total_scanned > sc.nr_reclaimed + sc.nr_reclaimed / 2)
 				sc.may_writepage = 1;
 
-			if (!zone_watermark_ok_safe(zone, order,
+			if (!zone_watermark_ok_safe(zone, local_order,
 					high_wmark_pages(zone), end_zone, 0)) {
 				all_zones_ok = 0;
 				/*
@@ -2453,7 +2460,7 @@ loop_again:
 				 * means that we have a GFP_ATOMIC allocation
 				 * failure risk. Hurry up!
 				 */
-				if (!zone_watermark_ok_safe(zone, order,
+				if (!zone_watermark_ok_safe(zone, local_order,
 					    min_wmark_pages(zone), end_zone, 0))
 					has_under_min_watermark_zone = 1;
 			} else {
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -26,6 +26,7 @@ extern int fragmentation_index(struct zo
 extern unsigned long try_to_compact_pages(struct zonelist *zonelist,
 			int order, gfp_t gfp_mask, nodemask_t *mask,
 			bool sync);
+extern int compaction_need_reclaim(struct zone *zone, int order);
 extern unsigned long compaction_suitable(struct zone *zone, int order);
 extern unsigned long compact_zone_order(struct zone *zone, int order,
 					gfp_t gfp_mask, bool sync,
@@ -68,6 +69,11 @@ static inline unsigned long try_to_compa
 	return COMPACT_CONTINUE;
 }
 
+static inline int compaction_need_reclaim(struct zone *zone, int order)
+{
+	return 0;
+}
+
 static inline unsigned long compaction_suitable(struct zone *zone, int order)
 {
 	return COMPACT_SKIPPED;
Subject: vmscan: kswapd must not free more than high_wmark pages

From: Andrea Arcangeli <aarcange@xxxxxxxxxx>

When the min_free_kbytes is set with `hugeadm
--set-recommended-min_free_kbytes" or with THP enabled (which runs the
equivalent of "hugeadm --set-recommended-min_free_kbytes" to activate
anti-frag at full effectiveness automatically at boot) the high wmark
of some zone is as high as ~88M. 88M free on a 4G system isn't
horrible, but 88M*8 = 704M free on a 4G system is definitely
unbearable. This only tends to be visible on 4G systems with tiny
over-4g zone where kswapd insists to reach the high wmark on the
over-4g zone but doing so it shrunk up to 704M from the normal zone by
mistake.

For the trivial case where kswapd isn't waken until all zones hit the low wmark
and there is no concurrency between allocator and kswapd freeing, rotating more
the tiny above4g lru than "high-low" despite we only allocated "high-low" cache
into it doesn't sound obviously right either. Bigger gap to me looks like will
do more harm than good and if we need a real guarantee of balancing we should
rotate the allocations across the zones (bigger lru in a zone will require it
to be hit more frequently because it'll rotate slower than the other zones, the
bias should not even dependent on the zone size but on the lru size).

Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
---
 mm/vmscan.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2408,7 +2408,7 @@ loop_again:
 			 * zone has way too many pages free already.
 			 */
 			if (!zone_watermark_ok_safe(zone, order,
-					8*high_wmark_pages(zone), end_zone, 0))
+					high_wmark_pages(zone), end_zone, 0))
 				shrink_zone(priority, zone, &sc);
 			reclaim_state->reclaimed_slab = 0;
 			nr_slab = shrink_slab(sc.nr_scanned, GFP_KERNEL,