[patch 1/3]vmscan: clear ZONE_CONGESTED for zone with good watermark

[patch 1/3]vmscan: clear ZONE_CONGESTED for zone with good watermark

[Shaohua Li]

correctly clear ZONE_CONGESTED. If a zone watermark is ok, we
should clear ZONE_CONGESTED regardless if this is a high order
allocation, because pages can be reclaimed in other tasks but
ZONE_CONGESTED is only cleared in kswapd.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
---
 mm/vmscan.c |   28 +++++++++++++++-------------
 1 file changed, 15 insertions(+), 13 deletions(-)

Index: linux/mm/vmscan.c
===================================================================
--- linux.orig/mm/vmscan.c	2011-07-25 09:37:11.000000000 +0800
+++ linux/mm/vmscan.c	2011-07-28 15:17:56.000000000 +0800
@@ -2494,6 +2494,9 @@ loop_again:
 					high_wmark_pages(zone), 0, 0)) {
 				end_zone = i;
 				break;
+			} else {
+				/* If balanced, clear the congested flag */
+				zone_clear_flag(zone, ZONE_CONGESTED);
 			}
 		}
 		if (i < 0)
@@ -2665,26 +2668,25 @@ out:
 	 * be cleared as kswapd is the only mechanism that clears the flag
 	 * and it is potentially going to sleep here.
 	 */
-	if (order) {
-		for (i = 0; i <= end_zone; i++) {
-			struct zone *zone = pgdat->node_zones + i;
+	for (i = 0; i <= end_zone; i++) {
+		struct zone *zone = pgdat->node_zones + i;
 
-			if (!populated_zone(zone))
-				continue;
+		if (!populated_zone(zone))
+			continue;
 
-			if (zone->all_unreclaimable && priority != DEF_PRIORITY)
-				continue;
+		if (zone->all_unreclaimable && priority != DEF_PRIORITY)
+			continue;
 
-			/* Confirm the zone is balanced for order-0 */
-			if (!zone_watermark_ok(zone, 0,
-					high_wmark_pages(zone), 0, 0)) {
+		/* Confirm the zone is balanced for order-0 */
+		if (!zone_watermark_ok(zone, 0, high_wmark_pages(zone), 0, 0)) {
+			if (order) {
 				order = sc.order = 0;
 				goto loop_again;
 			}
-
-			/* If balanced, clear the congested flag */
-			zone_clear_flag(zone, ZONE_CONGESTED);
 		}
+
+		/* If balanced, clear the congested flag */
+		zone_clear_flag(zone, ZONE_CONGESTED);
 	}
 
 	/*


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Re: [patch 1/3]vmscan: clear ZONE_CONGESTED for zone with good watermark

[Mel Gorman]

On Thu, Jul 28, 2011 at 04:13:01PM +0800, Shaohua Li wrote:
> correctly clear ZONE_CONGESTED. If a zone watermark is ok, we
> should clear ZONE_CONGESTED regardless if this is a high order
> allocation, because pages can be reclaimed in other tasks but
> ZONE_CONGESTED is only cleared in kswapd.
> 

What problem does this solve?

As it is, for high order allocations it takes the following steps

If reclaiming at high order {
	for each zone {
		if all_unreclaimable
			skip
		if watermark is not met
			order = 0
			loop again
		
		/* watermark is met */
		clear congested
	}
}

If high orders are failing, kswapd balances for order-0 where there
is already a cleaning of ZONE_CONGESTED if the zone was shrunk and
became balanced. I see the case for hunk 1 of the patch because now
it'll clear ZONE_CONGESTED for zones that are already balanced which
might have a noticable effect on wait_iff_congested. Is this what
you see? Even if it is, it does not explain hunk 2 of the patch.

-- 
Mel Gorman
SUSE Labs

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Re: [patch 1/3]vmscan: clear ZONE_CONGESTED for zone with good watermark

[Shaohua Li]

On Thu, 2011-07-28 at 18:56 +0800, Mel Gorman wrote:
> On Thu, Jul 28, 2011 at 04:13:01PM +0800, Shaohua Li wrote:
> > correctly clear ZONE_CONGESTED. If a zone watermark is ok, we
> > should clear ZONE_CONGESTED regardless if this is a high order
> > allocation, because pages can be reclaimed in other tasks but
> > ZONE_CONGESTED is only cleared in kswapd.
> > 
> 
> What problem does this solve?
> 
> As it is, for high order allocations it takes the following steps
> 
> If reclaiming at high order {
> 	for each zone {
> 		if all_unreclaimable
> 			skip
> 		if watermark is not met
> 			order = 0
> 			loop again
> 		
> 		/* watermark is met */
> 		clear congested
> 	}
> }
> 
> If high orders are failing, kswapd balances for order-0 where there
> is already a cleaning of ZONE_CONGESTED if the zone was shrunk and
> became balanced. I see the case for hunk 1 of the patch because now
> it'll clear ZONE_CONGESTED for zones that are already balanced which
> might have a noticable effect on wait_iff_congested. Is this what
> you see? Even if it is, it does not explain hunk 2 of the patch.
I first looked at the hunk 2 place and thought we don't clear
ZONE_CONGESTED there. I then figured out we need do the same thing for
the hunk 1. But you are correct, with hunk 1, hunk 2 isn't required.
updated patch.



correctly clear ZONE_CONGESTED. If a zone watermark is ok, we
should clear ZONE_CONGESTED because pages can be reclaimed in
other tasks but ZONE_CONGESTED is only cleared in kswapd.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
---
 mm/vmscan.c |    3 +++
 1 file changed, 3 insertions(+)

Index: linux/mm/vmscan.c
===================================================================
--- linux.orig/mm/vmscan.c	2011-07-29 08:24:10.000000000 +0800
+++ linux/mm/vmscan.c	2011-07-29 08:26:29.000000000 +0800
@@ -2494,6 +2494,9 @@ loop_again:
 					high_wmark_pages(zone), 0, 0)) {
 				end_zone = i;
 				break;
+			} else {
+				/* If balanced, clear the congested flag */
+				zone_clear_flag(zone, ZONE_CONGESTED);
 			}
 		}
 		if (i < 0)


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Re: [patch 1/3]vmscan: clear ZONE_CONGESTED for zone with good watermark

[Mel Gorman]

On Fri, Jul 29, 2011 at 08:35:25AM +0800, Shaohua Li wrote:
> On Thu, 2011-07-28 at 18:56 +0800, Mel Gorman wrote:
> > On Thu, Jul 28, 2011 at 04:13:01PM +0800, Shaohua Li wrote:
> > > correctly clear ZONE_CONGESTED. If a zone watermark is ok, we
> > > should clear ZONE_CONGESTED regardless if this is a high order
> > > allocation, because pages can be reclaimed in other tasks but
> > > ZONE_CONGESTED is only cleared in kswapd.
> > > 
> > 
> > What problem does this solve?
> > 
> > As it is, for high order allocations it takes the following steps
> > 
> > If reclaiming at high order {
> > 	for each zone {
> > 		if all_unreclaimable
> > 			skip
> > 		if watermark is not met
> > 			order = 0
> > 			loop again
> > 		
> > 		/* watermark is met */
> > 		clear congested
> > 	}
> > }
> > 
> > If high orders are failing, kswapd balances for order-0 where there
> > is already a cleaning of ZONE_CONGESTED if the zone was shrunk and
> > became balanced. I see the case for hunk 1 of the patch because now
> > it'll clear ZONE_CONGESTED for zones that are already balanced which
> > might have a noticable effect on wait_iff_congested. Is this what
> > you see? Even if it is, it does not explain hunk 2 of the patch.
> I first looked at the hunk 2 place and thought we don't clear
> ZONE_CONGESTED there. I then figured out we need do the same thing for
> the hunk 1. But you are correct, with hunk 1, hunk 2 isn't required.
> updated patch.
> 
> 
> 
> correctly clear ZONE_CONGESTED. If a zone watermark is ok, we
> should clear ZONE_CONGESTED because pages can be reclaimed in
> other tasks but ZONE_CONGESTED is only cleared in kswapd.
> 
> Signed-off-by: Shaohua Li <shaohua.li@intel.com>

It would be nice if the changelog was expanded a bit to explain
why the patch is necessary. You say this is to "correctly clear
ZONE_CONGESTED" but do not explain why the current code is wrong
or what the user-visible impact is. For example, even cutting and
pasting bits of the discussion like the following would have been
an improvement.

==== CUT HERE ===
kswapd is responsible for clearing ZONE_CONGESTED after it balances
a zone. Unfortunately, if ZONE_CONGESTED was set during a high-order
allocation, it is possible that kswapd misses clearing it.

At the end of balance_pgdat(), kswapd uses the following logic;

 If reclaiming at high order {
     for each zone {
             if all_unreclaimable
                     skip
             if watermark is not met
                     order = 0
                     loop again
             
             /* watermark is met */
             clear congested
     }
 }

i.e. it clears ZONE_CONGESTED if it the zone is balanced. if not,
it restarts balancing at order-0. However, if the higher zones are
balanced for order-0, kswapd will miss clearing ZONE_CONGESTED
as that only happens after a zone is shrunk. This can mean that
wait_iff_congested() stalls unnecessarily. This patch makes kswapd
clear ZONE_CONGESTED during its initial highmem->dma scan for zones
that are already balanced.

==== CUT HERE ====

This makes review a lot easier and will be helpful in the future if
someone uses git blame.

Whether you update the changelog or not;

Acked-by: Mel Gorman <mgorman@suse.de>

Thanks.

-- 
Mel Gorman
SUSE Labs

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Re: [patch 1/3]vmscan: clear ZONE_CONGESTED for zone with good watermark

[Shaohua Li]

On Fri, Jul 29, 2011 at 04:50:43PM +0800, Mel Gorman wrote:
> On Fri, Jul 29, 2011 at 08:35:25AM +0800, Shaohua Li wrote:
> > On Thu, 2011-07-28 at 18:56 +0800, Mel Gorman wrote:
> > > On Thu, Jul 28, 2011 at 04:13:01PM +0800, Shaohua Li wrote:
> > > > correctly clear ZONE_CONGESTED. If a zone watermark is ok, we
> > > > should clear ZONE_CONGESTED regardless if this is a high order
> > > > allocation, because pages can be reclaimed in other tasks but
> > > > ZONE_CONGESTED is only cleared in kswapd.
> > > > 
> > > 
> > > What problem does this solve?
> > > 
> > > As it is, for high order allocations it takes the following steps
> > > 
> > > If reclaiming at high order {
> > > 	for each zone {
> > > 		if all_unreclaimable
> > > 			skip
> > > 		if watermark is not met
> > > 			order = 0
> > > 			loop again
> > > 		
> > > 		/* watermark is met */
> > > 		clear congested
> > > 	}
> > > }
> > > 
> > > If high orders are failing, kswapd balances for order-0 where there
> > > is already a cleaning of ZONE_CONGESTED if the zone was shrunk and
> > > became balanced. I see the case for hunk 1 of the patch because now
> > > it'll clear ZONE_CONGESTED for zones that are already balanced which
> > > might have a noticable effect on wait_iff_congested. Is this what
> > > you see? Even if it is, it does not explain hunk 2 of the patch.
> > I first looked at the hunk 2 place and thought we don't clear
> > ZONE_CONGESTED there. I then figured out we need do the same thing for
> > the hunk 1. But you are correct, with hunk 1, hunk 2 isn't required.
> > updated patch.
> > 
> > 
> > 
> > correctly clear ZONE_CONGESTED. If a zone watermark is ok, we
> > should clear ZONE_CONGESTED because pages can be reclaimed in
> > other tasks but ZONE_CONGESTED is only cleared in kswapd.
> > 
> > Signed-off-by: Shaohua Li <shaohua.li@intel.com>
> 
> It would be nice if the changelog was expanded a bit to explain
> why the patch is necessary. You say this is to "correctly clear
> ZONE_CONGESTED" but do not explain why the current code is wrong
> or what the user-visible impact is. For example, even cutting and
> pasting bits of the discussion like the following would have been
> an improvement.
> 
> ==== CUT HERE ===
> kswapd is responsible for clearing ZONE_CONGESTED after it balances
> a zone. Unfortunately, if ZONE_CONGESTED was set during a high-order
> allocation, it is possible that kswapd misses clearing it.
> 
> At the end of balance_pgdat(), kswapd uses the following logic;
> 
>  If reclaiming at high order {
>      for each zone {
>              if all_unreclaimable
>                      skip
>              if watermark is not met
>                      order = 0
>                      loop again
>              
>              /* watermark is met */
>              clear congested
>      }
>  }
> 
> i.e. it clears ZONE_CONGESTED if it the zone is balanced. if not,
> it restarts balancing at order-0. However, if the higher zones are
> balanced for order-0, kswapd will miss clearing ZONE_CONGESTED
> as that only happens after a zone is shrunk. This can mean that
> wait_iff_congested() stalls unnecessarily. This patch makes kswapd
> clear ZONE_CONGESTED during its initial highmem->dma scan for zones
> that are already balanced.
> 
> ==== CUT HERE ====
> 
> This makes review a lot easier and will be helpful in the future if
> someone uses git blame.
> 
> Whether you update the changelog or not;
> 
> Acked-by: Mel Gorman <mgorman@suse.de>
Ok, updated the changelog.


ZONE_CONGESTED is only cleared in kswapd, but pages can be freed in any task.
It's possible ZONE_CONGESTED isn't cleared in some cases:
1. the zone is already balanced just entering balance_pgdat() for order-0 because
concurrent tasks free memory. In this case, later check will skip the zone as
it's balanced so the flag isn't cleared.
2. high order balance fallbacks to order-0. quote from Mel:
At the end of balance_pgdat(), kswapd uses the following logic;

 If reclaiming at high order {
     for each zone {
             if all_unreclaimable
                     skip
             if watermark is not met
                     order = 0
                     loop again

             /* watermark is met */
             clear congested
     }
 }

i.e. it clears ZONE_CONGESTED if it the zone is balanced. if not,
it restarts balancing at order-0. However, if the higher zones are
balanced for order-0, kswapd will miss clearing ZONE_CONGESTED
as that only happens after a zone is shrunk.
This can mean that wait_iff_congested() stalls unnecessarily. This patch
makes kswapd clear ZONE_CONGESTED during its initial highmem->dma scan
for zones that are already balanced.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
---
 mm/vmscan.c |    3 +++
 1 file changed, 3 insertions(+)

Index: linux/mm/vmscan.c
===================================================================
--- linux.orig/mm/vmscan.c	2011-07-29 08:24:10.000000000 +0800
+++ linux/mm/vmscan.c	2011-07-29 08:26:29.000000000 +0800
@@ -2494,6 +2494,9 @@ loop_again:
 					high_wmark_pages(zone), 0, 0)) {
 				end_zone = i;
 				break;
+			} else {
+				/* If balanced, clear the congested flag */
+				zone_clear_flag(zone, ZONE_CONGESTED);
 			}
 		}
 		if (i < 0)

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Re: [patch 1/3]vmscan: clear ZONE_CONGESTED for zone with good watermark

[Minchan Kim]

On Fri, Jul 29, 2011 at 08:35:25AM +0800, Shaohua Li wrote:
> On Thu, 2011-07-28 at 18:56 +0800, Mel Gorman wrote:
> > On Thu, Jul 28, 2011 at 04:13:01PM +0800, Shaohua Li wrote:
> > > correctly clear ZONE_CONGESTED. If a zone watermark is ok, we
> > > should clear ZONE_CONGESTED regardless if this is a high order
> > > allocation, because pages can be reclaimed in other tasks but
> > > ZONE_CONGESTED is only cleared in kswapd.
> > > 
> > 
> > What problem does this solve?
> > 
> > As it is, for high order allocations it takes the following steps
> > 
> > If reclaiming at high order {
> > 	for each zone {
> > 		if all_unreclaimable
> > 			skip
> > 		if watermark is not met
> > 			order = 0
> > 			loop again
> > 		
> > 		/* watermark is met */
> > 		clear congested
> > 	}
> > }
> > 
> > If high orders are failing, kswapd balances for order-0 where there
> > is already a cleaning of ZONE_CONGESTED if the zone was shrunk and
> > became balanced. I see the case for hunk 1 of the patch because now
> > it'll clear ZONE_CONGESTED for zones that are already balanced which
> > might have a noticable effect on wait_iff_congested. Is this what
> > you see? Even if it is, it does not explain hunk 2 of the patch.
> I first looked at the hunk 2 place and thought we don't clear
> ZONE_CONGESTED there. I then figured out we need do the same thing for
> the hunk 1. But you are correct, with hunk 1, hunk 2 isn't required.
> updated patch.
> 
> 
> 
> correctly clear ZONE_CONGESTED. If a zone watermark is ok, we
> should clear ZONE_CONGESTED because pages can be reclaimed in
> other tasks but ZONE_CONGESTED is only cleared in kswapd.
> 
> Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

Even it will fix that when kswapd wakes up lately by order-0 and look at zones,
all zones would become okay so it jumps out with "if (i < 0) goto out" with missing
clearing ZONE_CONGESTED.

-- 
Kind regards,
Minchan Kim

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