Re: [PATCH] mm: vmscan: handle isolated pages with lru lock released

[Hugh Dickins <hughd@google.com>]

From: Hillf Danton <dhillf@gmail.com>

When shrinking inactive lru list, isolated pages are queued on locally private
list, so the lock-hold time could be reduced if pages are counted without lock
protection.

To achieve that, firstly updating reclaim stat is delayed until the
putback stage, after reacquiring the lru lock.

Secondly, operations related to vm and zone stats are now proteced with
preemption disabled as they are per-cpu operations.

Signed-off-by: Hillf Danton <dhillf@gmail.com>
Acked-by: Hugh Dickins <hughd@google.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
KAMEZAWA-san and I both admired this patch from Hillf; Rik and David
liked its precursor: I think we'd all be glad to see it in linux-next.

 mm/vmscan.c |   21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

--- a/mm/vmscan.c	Sat Jan 14 14:02:20 2012
+++ b/mm/vmscan.c	Sat Jan 14 20:00:46 2012
@@ -1414,7 +1414,6 @@ update_isolated_counts(struct mem_cgroup
 		       unsigned long *nr_anon,
 		       unsigned long *nr_file)
 {
-	struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(mz);
 	struct zone *zone = mz->zone;
 	unsigned int count[NR_LRU_LISTS] = { 0, };
 	unsigned long nr_active = 0;
@@ -1435,6 +1434,7 @@ update_isolated_counts(struct mem_cgroup
 		count[lru] += numpages;
 	}

+	preempt_disable();
 	__count_vm_events(PGDEACTIVATE, nr_active);

 	__mod_zone_page_state(zone, NR_ACTIVE_FILE,
@@ -1449,8 +1449,9 @@ update_isolated_counts(struct mem_cgroup
 	*nr_anon = count[LRU_ACTIVE_ANON] + count[LRU_INACTIVE_ANON];
 	*nr_file = count[LRU_ACTIVE_FILE] + count[LRU_INACTIVE_FILE];

-	reclaim_stat->recent_scanned[0] += *nr_anon;
-	reclaim_stat->recent_scanned[1] += *nr_file;
+	__mod_zone_page_state(zone, NR_ISOLATED_ANON, *nr_anon);
+	__mod_zone_page_state(zone, NR_ISOLATED_FILE, *nr_file);
+	preempt_enable();
 }

 /*
@@ -1512,6 +1513,7 @@ shrink_inactive_list(unsigned long nr_to
 	unsigned long nr_writeback = 0;
 	isolate_mode_t reclaim_mode = ISOLATE_INACTIVE;
 	struct zone *zone = mz->zone;
+	struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(mz);

 	while (unlikely(too_many_isolated(zone, file, sc))) {
 		congestion_wait(BLK_RW_ASYNC, HZ/10);
@@ -1546,19 +1548,13 @@ shrink_inactive_list(unsigned long nr_to
 			__count_zone_vm_events(PGSCAN_DIRECT, zone,
 					       nr_scanned);
 	}
+	spin_unlock_irq(&zone->lru_lock);

-	if (nr_taken == 0) {
-		spin_unlock_irq(&zone->lru_lock);
+	if (nr_taken == 0)
 		return 0;
-	}

 	update_isolated_counts(mz, &page_list, &nr_anon, &nr_file);

-	__mod_zone_page_state(zone, NR_ISOLATED_ANON, nr_anon);
-	__mod_zone_page_state(zone, NR_ISOLATED_FILE, nr_file);
-
-	spin_unlock_irq(&zone->lru_lock);
-
 	nr_reclaimed = shrink_page_list(&page_list, mz, sc, priority,
 						&nr_dirty, &nr_writeback);

@@ -1570,6 +1566,9 @@ shrink_inactive_list(unsigned long nr_to
 	}

 	spin_lock_irq(&zone->lru_lock);
+
+	reclaim_stat->recent_scanned[0] += nr_anon;
+	reclaim_stat->recent_scanned[1] += nr_file;

 	if (current_is_kswapd())
 		__count_vm_events(KSWAPD_STEAL, nr_reclaimed);

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