[PATCH 0/4] Stop kswapd consuming 100% CPU when highest zone is small

[PATCH 0/4] Stop kswapd consuming 100% CPU when highest zone is small

[Mel Gorman]

During allocator-intensive workloads, kswapd will be woken frequently
causing free memory to oscillate between the high and min watermark.
This is expected behaviour.  Unfortunately, if the highest zone is
small, a problem occurs.

This seems to happen most with recent sandybridge laptops but it's
probably a co-incidence as some of these laptops just happen to have
a small Normal zone. The reproduction case is almost always during
copying large files that kswapd pegs at 100% CPU until the file is
deleted or cache is dropped.

The problem is mostly down to sleeping_prematurely() keeping kswapd
awake when the highest zone is small and unreclaimable and compounded
by the fact we shrink slabs even when not shrinking zones causing a lot
of time to be spent in shrinkers and a lot of memory to be reclaimed.

Patch 1 corrects sleeping_prematurely to check the zones matching
	the classzone_idx instead of all zones.

Patch 2 avoids shrinking slab when we are not shrinking a zone.

Patch 3 notes that sleeping_prematurely is checking lower zones against
	a high classzone which is not what allocators or balance_pgdat()
	is doing leading to an artifical believe that kswapd should be
	still awake.

Patch 4 notes that when balance_pgdat() gives up on a high zone that the
	decision is not communicated to sleeping_prematurely()

This problem affects 3.0-rc4 and 2.6.38.8 for certain and is expected
to affect 2.6.39 as well. If accepted, they need to go to -stable to
be picked up by distros. This series is against 3.0-rc4. I've cc'd
people that reported similar problems recently to see if they still
suffer from the problem and if this fixes it.

 mm/vmscan.c |   57 ++++++++++++++++++++++++++++++++++-----------------------
 1 files changed, 34 insertions(+), 23 deletions(-)

-- 
1.7.3.4

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[PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in sleeping_prematurely

[Mel Gorman]

During allocator-intensive workloads, kswapd will be woken frequently
causing free memory to oscillate between the high and min watermark.
This is expected behaviour.

A problem occurs if the highest zone is small.  balance_pgdat()
only considers unreclaimable zones when priority is DEF_PRIORITY
but sleeping_prematurely considers all zones. It's possible for this
sequence to occur

  1. kswapd wakes up and enters balance_pgdat()
  2. At DEF_PRIORITY, marks highest zone unreclaimable
  3. At DEF_PRIORITY-1, ignores highest zone setting end_zone
  4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from
        highest zone, clearing all_unreclaimable. Highest zone
        is still unbalanced
  5. kswapd returns and calls sleeping_prematurely
  6. sleeping_prematurely looks at *all* zones, not just the ones
     being considered by balance_pgdat. The highest small zone
     has all_unreclaimable cleared but but the zone is not
     balanced. all_zones_ok is false so kswapd stays awake

This patch corrects the behaviour of sleeping_prematurely to check
the zones balance_pgdat() checked.

Reported-and-tested-by: PA!draig Brady <P@draigBrady.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/vmscan.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8ff834e..841e3bf 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2323,7 +2323,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
 		return true;
 
 	/* Check the watermark levels */
-	for (i = 0; i < pgdat->nr_zones; i++) {
+	for (i = 0; i <= classzone_idx; i++) {
 		struct zone *zone = pgdat->node_zones + i;
 
 		if (!populated_zone(zone))
-- 
1.7.3.4

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[PATCH 2/4] mm: vmscan: Do not apply pressure to slab if we are not applying pressure to zone

[Mel Gorman]

During allocator-intensive workloads, kswapd will be woken frequently
causing free memory to oscillate between the high and min watermark.
This is expected behaviour.

When kswapd applies pressure to zones during node balancing, it checks
if the zone is above a high+balance_gap threshold. If it is, it does
not apply pressure but it unconditionally shrinks slab on a global
basis which is excessive. In the event kswapd is being kept awake due to
a high small unreclaimable zone, it skips zone shrinking but still
calls shrink_slab().

Once pressure has been applied, the check for zone being unreclaimable
is being made before the check is made if all_unreclaimable should be
set. This miss of unreclaimable can cause has_under_min_watermark_zone
to be set due to an unreclaimable zone preventing kswapd backing off
on congestion_wait().

Reported-and-tested-by: PA!draig Brady <P@draigBrady.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/vmscan.c |   21 ++++++++++++---------
 1 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 841e3bf..38665ec 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2509,16 +2509,16 @@ loop_again:
 					high_wmark_pages(zone) + balance_gap,
 					end_zone, 0))
 				shrink_zone(priority, zone, &sc);
-			reclaim_state->reclaimed_slab = 0;
-			nr_slab = shrink_slab(&shrink, sc.nr_scanned, lru_pages);
-			sc.nr_reclaimed += reclaim_state->reclaimed_slab;
-			total_scanned += sc.nr_scanned;
 
-			if (zone->all_unreclaimable)
-				continue;
-			if (nr_slab == 0 &&
-			    !zone_reclaimable(zone))
-				zone->all_unreclaimable = 1;
+				reclaim_state->reclaimed_slab = 0;
+				nr_slab = shrink_slab(&shrink, sc.nr_scanned, lru_pages);
+				sc.nr_reclaimed += reclaim_state->reclaimed_slab;
+				total_scanned += sc.nr_scanned;
+
+				if (nr_slab == 0 && !zone_reclaimable(zone))
+					zone->all_unreclaimable = 1;
+			}
+
 			/*
 			 * If we've done a decent amount of scanning and
 			 * the reclaim ratio is low, start doing writepage
@@ -2528,6 +2528,9 @@ loop_again:
 			    total_scanned > sc.nr_reclaimed + sc.nr_reclaimed / 2)
 				sc.may_writepage = 1;
 
+			if (zone->all_unreclaimable)
+				continue;
+
 			if (!zone_watermark_ok_safe(zone, order,
 					high_wmark_pages(zone), end_zone, 0)) {
 				all_zones_ok = 0;
-- 
1.7.3.4

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Re: [PATCH 2/4] mm: vmscan: Do not apply pressure to slab if we are not applying pressure to zone

[Mel Gorman]

On Fri, Jun 24, 2011 at 02:43:16PM +0100, Mel Gorman wrote:
> During allocator-intensive workloads, kswapd will be woken frequently
> causing free memory to oscillate between the high and min watermark.
> This is expected behaviour.
> 

Bah, I accidentally exported a branch with a build error in this
patch. Will resend shortly.

-- 
Mel Gorman
SUSE Labs

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[PATCH 3/4] mm: vmscan: Evaluate the watermarks against the correct classzone

[Mel Gorman]

When deciding if kswapd is sleeping prematurely, the classzone is
taken into account but this is different to what balance_pgdat() and
the allocator are doing. Specifically, the DMA zone will be checked
based on the classzone used when waking kswapd which could be for a
GFP_KERNEL or GFP_HIGHMEM request. The lowmem reserve limit kicks in,
the watermark is not met and kswapd thinks its sleeping prematurely
keeping kswapd awake in error.

Reported-and-tested-by: PA!draig Brady <P@draigBrady.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/vmscan.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 38665ec..d859111 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2341,7 +2341,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
 		}
 
 		if (!zone_watermark_ok_safe(zone, order, high_wmark_pages(zone),
-							classzone_idx, 0))
+							i, 0))
 			all_zones_ok = false;
 		else
 			balanced += zone->present_pages;
-- 
1.7.3.4

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[PATCH 4/4] mm: vmscan: Only read new_classzone_idx from pgdat when reclaiming successfully

[Mel Gorman]

During allocator-intensive workloads, kswapd will be woken frequently
causing free memory to oscillate between the high and min watermark.
This is expected behaviour.  Unfortunately, if the highest zone is
small, a problem occurs.

When balance_pgdat() returns, it may be at a lower classzone_idx than
it started because the highest zone was unreclaimable. Before checking
if it should go to sleep though, it checks pgdat->classzone_idx which
when there is no other activity will be MAX_NR_ZONES-1. It interprets
this as it has been woken up while reclaiming, skips scheduling and
reclaims again. As there is no useful reclaim work to do, it enters
into a loop of shrinking slab consuming loads of CPU until the highest
zone becomes reclaimable for a long period of time.

There are two problems here. 1) If the returned classzone or order is
lower, it'll continue reclaiming without scheduling. 2) if the highest
zone was marked unreclaimable but balance_pgdat() returns immediately
at DEF_PRIORITY, the new lower classzone is not communicated back to
kswapd() for sleeping.

This patch does two things that are related. If the end_zone is
unreclaimable, this information is communicated back. Second, if
the classzone or order was reduced due to failing to reclaim, new
information is not read from pgdat and instead an attempt is made to go
to sleep. Due to this, it is also necessary that pgdat->classzone_idx
be initialised each time to pgdat->nr_zones - 1 to avoid re-reads
being interpreted as wakeups.

Reported-and-tested-by: PA!draig Brady <P@draigBrady.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/vmscan.c |   34 +++++++++++++++++++++-------------
 1 files changed, 21 insertions(+), 13 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d859111..9297195 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2448,7 +2448,6 @@ loop_again:
 			if (!zone_watermark_ok_safe(zone, order,
 					high_wmark_pages(zone), 0, 0)) {
 				end_zone = i;
-				*classzone_idx = i;
 				break;
 			}
 		}
@@ -2528,8 +2527,11 @@ loop_again:
 			    total_scanned > sc.nr_reclaimed + sc.nr_reclaimed / 2)
 				sc.may_writepage = 1;
 
-			if (zone->all_unreclaimable)
+			if (zone->all_unreclaimable) {
+				if (end_zone && end_zone == i)
+					end_zone--;
 				continue;
+			}
 
 			if (!zone_watermark_ok_safe(zone, order,
 					high_wmark_pages(zone), end_zone, 0)) {
@@ -2709,8 +2711,8 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order, int classzone_idx)
  */
 static int kswapd(void *p)
 {
-	unsigned long order;
-	int classzone_idx;
+	unsigned long order, new_order;
+	int classzone_idx, new_classzone_idx;
 	pg_data_t *pgdat = (pg_data_t*)p;
 	struct task_struct *tsk = current;
 
@@ -2740,17 +2742,23 @@ static int kswapd(void *p)
 	tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
 	set_freezable();
 
-	order = 0;
-	classzone_idx = MAX_NR_ZONES - 1;
+	order = new_order = 0;
+	classzone_idx = new_classzone_idx = pgdat->nr_zones - 1;
 	for ( ; ; ) {
-		unsigned long new_order;
-		int new_classzone_idx;
 		int ret;
 
-		new_order = pgdat->kswapd_max_order;
-		new_classzone_idx = pgdat->classzone_idx;
-		pgdat->kswapd_max_order = 0;
-		pgdat->classzone_idx = MAX_NR_ZONES - 1;
+		/*
+		 * If the last balance_pgdat was unsuccessful it's unlikely a
+		 * new request of a similar or harder type will succeed soon
+		 * so consider going to sleep on the basis we reclaimed at
+		 */
+		if (classzone_idx >= new_classzone_idx && order == new_order) {
+			new_order = pgdat->kswapd_max_order;
+			new_classzone_idx = pgdat->classzone_idx;
+			pgdat->kswapd_max_order =  0;
+			pgdat->classzone_idx = pgdat->nr_zones - 1;
+		}
+
 		if (order < new_order || classzone_idx > new_classzone_idx) {
 			/*
 			 * Don't sleep if someone wants a larger 'order'
@@ -2763,7 +2771,7 @@ static int kswapd(void *p)
 			order = pgdat->kswapd_max_order;
 			classzone_idx = pgdat->classzone_idx;
 			pgdat->kswapd_max_order = 0;
-			pgdat->classzone_idx = MAX_NR_ZONES - 1;
+			pgdat->classzone_idx = pgdat->nr_zones - 1;
 		}
 
 		ret = try_to_freeze();
-- 
1.7.3.4

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[PATCH 0/4] Stop kswapd consuming 100% CPU when highest zone is small

[Mel Gorman]

(Built this time and passed a basic sniff-test.)

During allocator-intensive workloads, kswapd will be woken frequently
causing free memory to oscillate between the high and min watermark.
This is expected behaviour.  Unfortunately, if the highest zone is
small, a problem occurs.

This seems to happen most with recent sandybridge laptops but it's
probably a co-incidence as some of these laptops just happen to have
a small Normal zone. The reproduction case is almost always during
copying large files that kswapd pegs at 100% CPU until the file is
deleted or cache is dropped.

The problem is mostly down to sleeping_prematurely() keeping kswapd
awake when the highest zone is small and unreclaimable and compounded
by the fact we shrink slabs even when not shrinking zones causing a lot
of time to be spent in shrinkers and a lot of memory to be reclaimed.

Patch 1 corrects sleeping_prematurely to check the zones matching
	the classzone_idx instead of all zones.

Patch 2 avoids shrinking slab when we are not shrinking a zone.

Patch 3 notes that sleeping_prematurely is checking lower zones against
	a high classzone which is not what allocators or balance_pgdat()
	is doing leading to an artifical believe that kswapd should be
	still awake.

Patch 4 notes that when balance_pgdat() gives up on a high zone that the
	decision is not communicated to sleeping_prematurely()

This problem affects 2.6.38.8 for certain and is expected to affect
2.6.39 and 3.0-rc4 as well. If accepted, they need to go to -stable
to be picked up by distros and this series is against 3.0-rc4. I've
cc'd people that reported similar problems recently to see if they
still suffer from the problem and if this fixes it.

 mm/vmscan.c |   59 +++++++++++++++++++++++++++++++++++------------------------
 1 files changed, 35 insertions(+), 24 deletions(-)

-- 
1.7.3.4

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[PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in sleeping_prematurely

[Mel Gorman]

During allocator-intensive workloads, kswapd will be woken frequently
causing free memory to oscillate between the high and min watermark.
This is expected behaviour.

A problem occurs if the highest zone is small.  balance_pgdat()
only considers unreclaimable zones when priority is DEF_PRIORITY
but sleeping_prematurely considers all zones. It's possible for this
sequence to occur

  1. kswapd wakes up and enters balance_pgdat()
  2. At DEF_PRIORITY, marks highest zone unreclaimable
  3. At DEF_PRIORITY-1, ignores highest zone setting end_zone
  4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from
        highest zone, clearing all_unreclaimable. Highest zone
        is still unbalanced
  5. kswapd returns and calls sleeping_prematurely
  6. sleeping_prematurely looks at *all* zones, not just the ones
     being considered by balance_pgdat. The highest small zone
     has all_unreclaimable cleared but but the zone is not
     balanced. all_zones_ok is false so kswapd stays awake

This patch corrects the behaviour of sleeping_prematurely to check
the zones balance_pgdat() checked.

Reported-and-tested-by: PA!draig Brady <P@draigBrady.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/vmscan.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8ff834e..841e3bf 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2323,7 +2323,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
 		return true;
 
 	/* Check the watermark levels */
-	for (i = 0; i < pgdat->nr_zones; i++) {
+	for (i = 0; i <= classzone_idx; i++) {
 		struct zone *zone = pgdat->node_zones + i;
 
 		if (!populated_zone(zone))
-- 
1.7.3.4

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Re: [PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in sleeping_prematurely

[Rik van Riel]

On 06/24/2011 10:44 AM, Mel Gorman wrote:
> During allocator-intensive workloads, kswapd will be woken frequently
> causing free memory to oscillate between the high and min watermark.
> This is expected behaviour.
>
> A problem occurs if the highest zone is small.  balance_pgdat()
> only considers unreclaimable zones when priority is DEF_PRIORITY
> but sleeping_prematurely considers all zones. It's possible for this
> sequence to occur
>
>    1. kswapd wakes up and enters balance_pgdat()
>    2. At DEF_PRIORITY, marks highest zone unreclaimable
>    3. At DEF_PRIORITY-1, ignores highest zone setting end_zone
>    4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from
>          highest zone, clearing all_unreclaimable. Highest zone
>          is still unbalanced
>    5. kswapd returns and calls sleeping_prematurely
>    6. sleeping_prematurely looks at *all* zones, not just the ones
>       being considered by balance_pgdat. The highest small zone
>       has all_unreclaimable cleared but but the zone is not
>       balanced. all_zones_ok is false so kswapd stays awake
>
> This patch corrects the behaviour of sleeping_prematurely to check
> the zones balance_pgdat() checked.
>
> Reported-and-tested-by: PA!draig Brady<P@draigBrady.com>
> Signed-off-by: Mel Gorman<mgorman@suse.de>

Acked-by: Rik van Riel <riel@redhat.com>

-- 
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Re: [PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in sleeping_prematurely

[Minchan Kim]

On Fri, Jun 24, 2011 at 11:44 PM, Mel Gorman <mgorman@suse.de> wrote:
> During allocator-intensive workloads, kswapd will be woken frequently
> causing free memory to oscillate between the high and min watermark.
> This is expected behaviour.
>
> A problem occurs if the highest zone is small.  balance_pgdat()
> only considers unreclaimable zones when priority is DEF_PRIORITY
> but sleeping_prematurely considers all zones. It's possible for this
> sequence to occur
>
>  1. kswapd wakes up and enters balance_pgdat()
>  2. At DEF_PRIORITY, marks highest zone unreclaimable
>  3. At DEF_PRIORITY-1, ignores highest zone setting end_zone
>  4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from
>        highest zone, clearing all_unreclaimable. Highest zone
>        is still unbalanced
>  5. kswapd returns and calls sleeping_prematurely
>  6. sleeping_prematurely looks at *all* zones, not just the ones
>     being considered by balance_pgdat. The highest small zone
>     has all_unreclaimable cleared but but the zone is not
>     balanced. all_zones_ok is false so kswapd stays awake
>
> This patch corrects the behaviour of sleeping_prematurely to check
> the zones balance_pgdat() checked.
>
> Reported-and-tested-by: Pádraig Brady <P@draigBrady.com>
> Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

-- 
Kind regards,
Minchan Kim

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Re: [PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in sleeping_prematurely

[Andrew Morton]

On Fri, 24 Jun 2011 15:44:54 +0100
Mel Gorman <mgorman@suse.de> wrote:

> During allocator-intensive workloads, kswapd will be woken frequently
> causing free memory to oscillate between the high and min watermark.
> This is expected behaviour.
> 
> A problem occurs if the highest zone is small.  balance_pgdat()
> only considers unreclaimable zones when priority is DEF_PRIORITY
> but sleeping_prematurely considers all zones. It's possible for this
> sequence to occur
> 
>   1. kswapd wakes up and enters balance_pgdat()
>   2. At DEF_PRIORITY, marks highest zone unreclaimable
>   3. At DEF_PRIORITY-1, ignores highest zone setting end_zone
>   4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from
>         highest zone, clearing all_unreclaimable. Highest zone
>         is still unbalanced
>   5. kswapd returns and calls sleeping_prematurely
>   6. sleeping_prematurely looks at *all* zones, not just the ones
>      being considered by balance_pgdat. The highest small zone
>      has all_unreclaimable cleared but but the zone is not
>      balanced. all_zones_ok is false so kswapd stays awake
> 
> This patch corrects the behaviour of sleeping_prematurely to check
> the zones balance_pgdat() checked.

But kswapd is making progress: it's reclaiming slab.  Eventually that
won't work any more and all_unreclaimable will not be cleared and the
condition will fix itself up?



btw,

	if (!sleeping_prematurely(...))
		sleep();

hurts my brain.  My brain would prefer

	if (kswapd_should_sleep(...))
		sleep();

no?

> Reported-and-tested-by: Pádraig Brady <P@draigBrady.com>

But what were the before-and-after observations?  I don't understand
how this can cause a permanent cpuchew by kswapd.

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2323,7 +2323,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
>  		return true;
>  
>  	/* Check the watermark levels */
> -	for (i = 0; i < pgdat->nr_zones; i++) {
> +	for (i = 0; i <= classzone_idx; i++) {
>  		struct zone *zone = pgdat->node_zones + i;
>  
>  		if (!populated_zone(zone))

The patch looks sensible.

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Re: [PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in sleeping_prematurely

[Pádraig Brady]

On 28/06/11 22:49, Andrew Morton wrote:
> On Fri, 24 Jun 2011 15:44:54 +0100
> Mel Gorman <mgorman@suse.de> wrote:
> 
>> During allocator-intensive workloads, kswapd will be woken frequently
>> causing free memory to oscillate between the high and min watermark.
>> This is expected behaviour.
>>
>> A problem occurs if the highest zone is small.  balance_pgdat()
>> only considers unreclaimable zones when priority is DEF_PRIORITY
>> but sleeping_prematurely considers all zones. It's possible for this
>> sequence to occur
>>
>>   1. kswapd wakes up and enters balance_pgdat()
>>   2. At DEF_PRIORITY, marks highest zone unreclaimable
>>   3. At DEF_PRIORITY-1, ignores highest zone setting end_zone
>>   4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from
>>         highest zone, clearing all_unreclaimable. Highest zone
>>         is still unbalanced
>>   5. kswapd returns and calls sleeping_prematurely
>>   6. sleeping_prematurely looks at *all* zones, not just the ones
>>      being considered by balance_pgdat. The highest small zone
>>      has all_unreclaimable cleared but but the zone is not
>>      balanced. all_zones_ok is false so kswapd stays awake
>>
>> This patch corrects the behaviour of sleeping_prematurely to check
>> the zones balance_pgdat() checked.
> 
> But kswapd is making progress: it's reclaiming slab.  Eventually that
> won't work any more and all_unreclaimable will not be cleared and the
> condition will fix itself up?
> 
> 
> 
> btw,
> 
> 	if (!sleeping_prematurely(...))
> 		sleep();
> 
> hurts my brain.  My brain would prefer
> 
> 	if (kswapd_should_sleep(...))
> 		sleep();
> 
> no?
> 
>> Reported-and-tested-by: Padraig Brady <P@draigBrady.com>
> 
> But what were the before-and-after observations?  I don't understand
> how this can cause a permanent cpuchew by kswapd.

Context:
  http://marc.info/?t=130865025500001&r=1&w=2
  https://bugzilla.redhat.com/show_bug.cgi?id=712019

Summary:

This will spin kswapd0 on my SNB laptop with 3GB RAM (with small normal zone):

    dd bs=1M count=3000 if=/dev/zero of=spin.test

Basically once a certain amount of data is cached,
kswapd0 will start spinning, until the data
is removed from cache (by `rm spin.test` for example).

cheers,
Padraig.

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Re: [PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in sleeping_prematurely

[Mel Gorman]

On Tue, Jun 28, 2011 at 02:49:00PM -0700, Andrew Morton wrote:
> On Fri, 24 Jun 2011 15:44:54 +0100
> Mel Gorman <mgorman@suse.de> wrote:
> 
> > During allocator-intensive workloads, kswapd will be woken frequently
> > causing free memory to oscillate between the high and min watermark.
> > This is expected behaviour.
> > 
> > A problem occurs if the highest zone is small.  balance_pgdat()
> > only considers unreclaimable zones when priority is DEF_PRIORITY
> > but sleeping_prematurely considers all zones. It's possible for this
> > sequence to occur
> > 
> >   1. kswapd wakes up and enters balance_pgdat()
> >   2. At DEF_PRIORITY, marks highest zone unreclaimable
> >   3. At DEF_PRIORITY-1, ignores highest zone setting end_zone
> >   4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from
> >         highest zone, clearing all_unreclaimable. Highest zone
> >         is still unbalanced
> >   5. kswapd returns and calls sleeping_prematurely
> >   6. sleeping_prematurely looks at *all* zones, not just the ones
> >      being considered by balance_pgdat. The highest small zone
> >      has all_unreclaimable cleared but but the zone is not
> >      balanced. all_zones_ok is false so kswapd stays awake
> > 
> > This patch corrects the behaviour of sleeping_prematurely to check
> > the zones balance_pgdat() checked.
> 
> But kswapd is making progress: it's reclaiming slab.  Eventually that
> won't work any more and all_unreclaimable will not be cleared and the
> condition will fix itself up?
> 

It might, but at that point we've dumped as much slab as we can which
is very aggressive and there is no guarantee the condition is fixed
up. For example, if fork is happening often enough due to terminal
usage for example, it may be just enough allocation requests satisified
from the highest zone to clear all_unreclaimable during exit.

> btw,
> 
> 	if (!sleeping_prematurely(...))
> 		sleep();
> 
> hurts my brain.  My brain would prefer
> 
> 	if (kswapd_should_sleep(...))
> 		sleep();
> 
> no?
> 

kswapd_try_to_sleep -> should_sleep feel like it would hurt too. I
prefer the sleeping_prematurely name because it indicates what
condition we are checking but I'm biased and generally suck at naming.

> > Reported-and-tested-by: Padraig Brady <P@draigBrady.com>
> 
> But what were the before-and-after observations?  I don't understand
> how this can cause a permanent cpuchew by kswapd.
> 

Padraig has reported on his before-and-after observations.

On its own, this patch doesn't entirely fix his problem because all
the patches are required but I felt that a rolled-up patch would be
too hard to review.

-- 
Mel Gorman
SUSE Labs

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Re: [PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in sleeping_prematurely

[KOSAKI Motohiro]

(2011/06/24 23:44), Mel Gorman wrote:
> During allocator-intensive workloads, kswapd will be woken frequently
> causing free memory to oscillate between the high and min watermark.
> This is expected behaviour.
> 
> A problem occurs if the highest zone is small.  balance_pgdat()
> only considers unreclaimable zones when priority is DEF_PRIORITY
> but sleeping_prematurely considers all zones. It's possible for this
> sequence to occur
> 
>   1. kswapd wakes up and enters balance_pgdat()
>   2. At DEF_PRIORITY, marks highest zone unreclaimable
>   3. At DEF_PRIORITY-1, ignores highest zone setting end_zone
>   4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from
>         highest zone, clearing all_unreclaimable. Highest zone
>         is still unbalanced
>   5. kswapd returns and calls sleeping_prematurely
>   6. sleeping_prematurely looks at *all* zones, not just the ones
>      being considered by balance_pgdat. The highest small zone
>      has all_unreclaimable cleared but but the zone is not
>      balanced. all_zones_ok is false so kswapd stays awake
> 
> This patch corrects the behaviour of sleeping_prematurely to check
> the zones balance_pgdat() checked.
> 
> Reported-and-tested-by: PA!draig Brady <P@draigBrady.com>
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> ---
>  mm/vmscan.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 8ff834e..841e3bf 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2323,7 +2323,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
>  		return true;
>  
>  	/* Check the watermark levels */
> -	for (i = 0; i < pgdat->nr_zones; i++) {
> +	for (i = 0; i <= classzone_idx; i++) {
>  		struct zone *zone = pgdat->node_zones + i;
>  
>  		if (!populated_zone(zone))

sorry for the delay.
	Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>




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