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Message-ID: <20110223162432.GL31195@random.random>
Date: Wed, 23 Feb 2011 17:24:32 +0100
From: Andrea Arcangeli <aarcange@...hat.com>
To: Arthur Marsh <arthur.marsh@...ernode.on.net>
Cc: Clemens Ladisch <cladisch@...glemail.com>,
alsa-user@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
Mel Gorman <mel@....ul.ie>
Subject: Re: [Alsa-user] new source of MIDI playback slow-down identified -
5a03b051ed87e72b959f32a86054e1142ac4cf55 thp: use compaction in kswapd for
GFP_ATOMIC order > 0
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@...hat.com>
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@...hat.com>
---
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;
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