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Subject[15/63] mm: compaction: prevent kswapd compacting memory to reduce CPU usage
2.6.38-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Andrea Arcangeli <aarcange@redhat.com>

commit d527caf22e48480b102c7c6ee5b9ba12170148f7 upstream.

This patch reverts 5a03b051 ("thp: use compaction in kswapd for GFP_ATOMIC
order > 0") due to reports stating that kswapd CPU usage was higher and
IRQs were being disabled more frequently. This was reported at
http://www.spinics.net/linux/fedora/alsa-user/msg09885.html.
Without this patch applied, CPU usage by kswapd hovers around the 20% mark
according to the tester (Arthur Marsh:
http://www.spinics.net/linux/fedora/alsa-user/msg09899.html). With this
patch applied, it's around 2%.

The problem is not related to THP which specifies __GFP_NO_KSWAPD but is
triggered by high-order allocations hitting the low watermark for their
order and waking kswapd on kernels with CONFIG_COMPACTION set. The most
common trigger for this is network cards configured for jumbo frames but
it's also possible it'll be triggered by fork-heavy workloads (order-1)
and some wireless cards which depend on order-1 allocations.

The symptoms for the user will be high CPU usage by kswapd in low-memory
situations which could be confused with another writeback problem. While
a patch like 5a03b051 may be reintroduced in the future, this patch plays
it safe for now and reverts it.

[mel@csn.ul.ie: Beefed up the changelog]
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Reported-by: Arthur Marsh <arthur.marsh@internode.on.net>
Tested-by: Arthur Marsh <arthur.marsh@internode.on.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
include/linux/compaction.h | 9 ++-------
mm/compaction.c | 24 +++---------------------
mm/vmscan.c | 18 +-----------------
3 files changed, 6 insertions(+), 45 deletions(-)
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -11,9 +11,6 @@
/* The full zone was compacted */
#define COMPACT_COMPLETE 3

-#define COMPACT_MODE_DIRECT_RECLAIM 0
-#define COMPACT_MODE_KSWAPD 1
-
#ifdef CONFIG_COMPACTION
extern int sysctl_compact_memory;
extern int sysctl_compaction_handler(struct ctl_table *table, int write,
@@ -28,8 +25,7 @@ extern unsigned long try_to_compact_page
bool sync);
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,
- int compact_mode);
+ gfp_t gfp_mask, bool sync);

/* Do not skip compaction more than 64 times */
#define COMPACT_MAX_DEFER_SHIFT 6
@@ -74,8 +70,7 @@ static inline unsigned long compaction_s
}

static inline unsigned long compact_zone_order(struct zone *zone, int order,
- gfp_t gfp_mask, bool sync,
- int compact_mode)
+ gfp_t gfp_mask, bool sync)
{
return COMPACT_CONTINUE;
}
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -42,8 +42,6 @@ struct compact_control {
unsigned int order; /* order a direct compactor needs */
int migratetype; /* MOVABLE, RECLAIMABLE etc */
struct zone *zone;
-
- int compact_mode;
};

static unsigned long release_freepages(struct list_head *freelist)
@@ -397,10 +395,7 @@ static int compact_finished(struct zone
return COMPACT_COMPLETE;

/* Compaction run is not finished if the watermark is not met */
- if (cc->compact_mode != COMPACT_MODE_KSWAPD)
- watermark = low_wmark_pages(zone);
- else
- watermark = high_wmark_pages(zone);
+ watermark = low_wmark_pages(zone);
watermark += (1 << cc->order);

if (!zone_watermark_ok(zone, cc->order, watermark, 0, 0))
@@ -413,15 +408,6 @@ static int compact_finished(struct zone
if (cc->order == -1)
return COMPACT_CONTINUE;

- /*
- * Generating only one page of the right order is not enough
- * for kswapd, we must continue until we're above the high
- * watermark as a pool for high order GFP_ATOMIC allocations
- * too.
- */
- if (cc->compact_mode == COMPACT_MODE_KSWAPD)
- return COMPACT_CONTINUE;
-
/* Direct compactor: Is a suitable page free? */
for (order = cc->order; order < MAX_ORDER; order++) {
/* Job done if page is free of the right migratetype */
@@ -543,8 +529,7 @@ static int compact_zone(struct zone *zon

unsigned long compact_zone_order(struct zone *zone,
int order, gfp_t gfp_mask,
- bool sync,
- int compact_mode)
+ bool sync)
{
struct compact_control cc = {
.nr_freepages = 0,
@@ -553,7 +538,6 @@ unsigned long compact_zone_order(struct
.migratetype = allocflags_to_migratetype(gfp_mask),
.zone = zone,
.sync = sync,
- .compact_mode = compact_mode,
};
INIT_LIST_HEAD(&cc.freepages);
INIT_LIST_HEAD(&cc.migratepages);
@@ -599,8 +583,7 @@ unsigned long try_to_compact_pages(struc
nodemask) {
int status;

- status = compact_zone_order(zone, order, gfp_mask, sync,
- COMPACT_MODE_DIRECT_RECLAIM);
+ status = compact_zone_order(zone, order, gfp_mask, sync);
rc = max(status, rc);

/* If a normal allocation would succeed, stop compacting */
@@ -631,7 +614,6 @@ static int compact_node(int nid)
.nr_freepages = 0,
.nr_migratepages = 0,
.order = -1,
- .compact_mode = COMPACT_MODE_DIRECT_RECLAIM,
};

zone = &pgdat->node_zones[zoneid];
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2397,7 +2397,6 @@ 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;

@@ -2428,24 +2427,9 @@ loop_again:
sc.nr_reclaimed += reclaim_state->reclaimed_slab;
total_scanned += sc.nr_scanned;

- compaction = 0;
- if (order &&
- zone_watermark_ok(zone, 0,
- high_wmark_pages(zone),
- end_zone, 0) &&
- !zone_watermark_ok(zone, order,
- high_wmark_pages(zone),
- end_zone, 0)) {
- compact_zone_order(zone,
- order,
- sc.gfp_mask, false,
- COMPACT_MODE_KSWAPD);
- compaction = 1;
- }
-
if (zone->all_unreclaimable)
continue;
- if (!compaction && nr_slab == 0 &&
+ if (nr_slab == 0 &&
!zone_reclaimable(zone))
zone->all_unreclaimable = 1;
/*



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