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Subject[PATCH v2] mm: modify pgdat_balanced() so that it also handles order=0
On 22.12.2012 19:54, Zlatko Calusic wrote:
> On 20.12.2012 21:58, Andrew Morton wrote:
>> There seems to be some complexity/duplication here between the new
>> unbalanced_zone() and pgdat_balanced().
>>
>> Can we modify pgdat_balanced() so that it also handles order=0, then do
>>
>> - if (!unbalanced_zone || (order && pgdat_balanced(pgdat, balanced, *classzone_idx)))
>> + if (!pgdat_balanced(...))
>>
>> ?
>>
>
> Makes sense, I like the idea! Took me some time to wrap my mind around
> all the logic in balance_pgdat(), while writing my previous patch. Also had
> to revert one if-case logic to avoid double negation, which would be even
> harder to grok. But unbalanced_zone (var, not a function!) has to stay because
> wait_iff_congested() needs a struct zone* param. Here's my take on the subject:
>

And now also with 3 unused variables removed, no other changes.
prepare_kswapd_sleep() now looks so beautiful. ;)

I've been testing the patch on 3 different machines, and no problem at all. One of
those I pushed hard, and it survived.

---8<---
mm: modify pgdat_balanced() so that it also handles order=0

Teach pgdat_balanced() about order-0 allocations so that we can simplify
code in a few places in vmstat.c.

Signed-off-by: Zlatko Calusic <zlatko.calusic@iskon.hr>
---
mm/vmscan.c | 105 ++++++++++++++++++++++++++----------------------------------
1 file changed, 45 insertions(+), 60 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index adc7e90..23291b9 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2452,12 +2452,16 @@ static bool zone_balanced(struct zone *zone, int order,
}

/*
- * pgdat_balanced is used when checking if a node is balanced for high-order
- * allocations. Only zones that meet watermarks and are in a zone allowed
- * by the callers classzone_idx are added to balanced_pages. The total of
- * balanced pages must be at least 25% of the zones allowed by classzone_idx
- * for the node to be considered balanced. Forcing all zones to be balanced
- * for high orders can cause excessive reclaim when there are imbalanced zones.
+ * pgdat_balanced() is used when checking if a node is balanced.
+ *
+ * For order-0, all zones must be balanced!
+ *
+ * For high-order allocations only zones that meet watermarks and are in a
+ * zone allowed by the callers classzone_idx are added to balanced_pages. The
+ * total of balanced pages must be at least 25% of the zones allowed by
+ * classzone_idx for the node to be considered balanced. Forcing all zones to
+ * be balanced for high orders can cause excessive reclaim when there are
+ * imbalanced zones.
* The choice of 25% is due to
* o a 16M DMA zone that is balanced will not balance a zone on any
* reasonable sized machine
@@ -2467,17 +2471,43 @@ static bool zone_balanced(struct zone *zone, int order,
* Similarly, on x86-64 the Normal zone would need to be at least 1G
* to balance a node on its own. These seemed like reasonable ratios.
*/
-static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced_pages,
- int classzone_idx)
+static bool pgdat_balanced(pg_data_t *pgdat, int order, int classzone_idx)
{
unsigned long present_pages = 0;
+ unsigned long balanced_pages = 0;
int i;

- for (i = 0; i <= classzone_idx; i++)
- present_pages += pgdat->node_zones[i].present_pages;
+ /* Check the watermark levels */
+ for (i = 0; i <= classzone_idx; i++) {
+ struct zone *zone = pgdat->node_zones + i;

- /* A special case here: if zone has no page, we think it's balanced */
- return balanced_pages >= (present_pages >> 2);
+ if (!populated_zone(zone))
+ continue;
+
+ present_pages += zone->present_pages;
+
+ /*
+ * A special case here:
+ *
+ * balance_pgdat() skips over all_unreclaimable after
+ * DEF_PRIORITY. Effectively, it considers them balanced so
+ * they must be considered balanced here as well!
+ */
+ if (zone->all_unreclaimable) {
+ balanced_pages += zone->present_pages;
+ continue;
+ }
+
+ if (zone_balanced(zone, order, 0, i))
+ balanced_pages += zone->present_pages;
+ else if (!order)
+ return false;
+ }
+
+ if (order)
+ return balanced_pages >= (present_pages >> 2);
+ else
+ return true;
}

/*
@@ -2489,10 +2519,6 @@ static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced_pages,
static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
int classzone_idx)
{
- int i;
- unsigned long balanced = 0;
- bool all_zones_ok = true;
-
/* If a direct reclaimer woke kswapd within HZ/10, it's premature */
if (remaining)
return false;
@@ -2511,39 +2537,7 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
return false;
}

- /* Check the watermark levels */
- for (i = 0; i <= classzone_idx; i++) {
- struct zone *zone = pgdat->node_zones + i;
-
- if (!populated_zone(zone))
- continue;
-
- /*
- * balance_pgdat() skips over all_unreclaimable after
- * DEF_PRIORITY. Effectively, it considers them balanced so
- * they must be considered balanced here as well if kswapd
- * is to sleep
- */
- if (zone->all_unreclaimable) {
- balanced += zone->present_pages;
- continue;
- }
-
- if (!zone_balanced(zone, order, 0, i))
- all_zones_ok = false;
- else
- balanced += zone->present_pages;
- }
-
- /*
- * For high-order requests, the balanced zones must contain at least
- * 25% of the nodes pages for kswapd to sleep. For order-0, all zones
- * must be balanced
- */
- if (order)
- return pgdat_balanced(pgdat, balanced, classzone_idx);
- else
- return all_zones_ok;
+ return pgdat_balanced(pgdat, order, classzone_idx);
}

/*
@@ -2571,7 +2565,6 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
int *classzone_idx)
{
struct zone *unbalanced_zone;
- unsigned long balanced;
int i;
int end_zone = 0; /* Inclusive. 0 = ZONE_DMA */
unsigned long total_scanned;
@@ -2605,7 +2598,6 @@ loop_again:
int has_under_min_watermark_zone = 0;

unbalanced_zone = NULL;
- balanced = 0;

/*
* Scan in the highmem->dma direction for the highest
@@ -2761,8 +2753,6 @@ loop_again:
* speculatively avoid congestion waits
*/
zone_clear_flag(zone, ZONE_CONGESTED);
- if (i <= *classzone_idx)
- balanced += zone->present_pages;
}

}
@@ -2776,7 +2766,7 @@ loop_again:
pfmemalloc_watermark_ok(pgdat))
wake_up(&pgdat->pfmemalloc_wait);

- if (!unbalanced_zone || (order && pgdat_balanced(pgdat, balanced, *classzone_idx)))
+ if (pgdat_balanced(pgdat, order, *classzone_idx))
break; /* kswapd: all done */
/*
* OK, kswapd is getting into trouble. Take a nap, then take
@@ -2800,12 +2790,7 @@ loop_again:
} while (--sc.priority >= 0);
out:

- /*
- * order-0: All zones must meet high watermark for a balanced node
- * high-order: Balanced zones must make up at least 25% of the node
- * for the node to be balanced
- */
- if (unbalanced_zone && (!order || !pgdat_balanced(pgdat, balanced, *classzone_idx))) {
+ if (!pgdat_balanced(pgdat, order, *classzone_idx)) {
cond_resched();

try_to_freeze();
--
1.8.1.rc0
--
Zlatko


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