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SubjectRe: [PATCH 1/4] vmstat: remove zone->lock from walk_zones_in_node
* KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> [2009-12-28 16:47:22]:

> The zone->lock is one of performance critical locks. Then, it shouldn't
> be hold for long time. Currently, we have four walk_zones_in_node()
> usage and almost use-case don't need to hold zone->lock.
>
> Thus, this patch move locking responsibility from walk_zones_in_node
> to its sub function. Also this patch kill unnecessary zone->lock taking.
>
> Cc: Mel Gorman <mel@csn.ul.ie>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> ---
> mm/vmstat.c | 8 +++++---
> 1 files changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 6051fba..a5d45bc 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -418,15 +418,12 @@ static void walk_zones_in_node(struct seq_file *m, pg_data_t *pgdat,
> {
> struct zone *zone;
> struct zone *node_zones = pgdat->node_zones;
> - unsigned long flags;
>
> for (zone = node_zones; zone - node_zones < MAX_NR_ZONES; ++zone) {
> if (!populated_zone(zone))
> continue;
>
> - spin_lock_irqsave(&zone->lock, flags);
> print(m, pgdat, zone);
> - spin_unlock_irqrestore(&zone->lock, flags);
> }
> }
>

> @@ -455,6 +452,7 @@ static void pagetypeinfo_showfree_print(struct seq_file *m,
> pg_data_t *pgdat, struct zone *zone)
> {
> int order, mtype;
> + unsigned long flags;
>
> for (mtype = 0; mtype < MIGRATE_TYPES; mtype++) {
> seq_printf(m, "Node %4d, zone %8s, type %12s ",
> @@ -468,8 +466,11 @@ static void pagetypeinfo_showfree_print(struct seq_file *m,
>
> area = &(zone->free_area[order]);
>
> + spin_lock_irqsave(&zone->lock, flags);
> list_for_each(curr, &area->free_list[mtype])
> freecount++;
> + spin_unlock_irqrestore(&zone->lock, flags);
> +
> seq_printf(m, "%6lu ", freecount);
> }
> seq_putc(m, '\n');
> @@ -709,6 +710,7 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
> struct zone *zone)
> {
> int i;
> +
> seq_printf(m, "Node %d, zone %8s", pgdat->node_id, zone->name);
> seq_printf(m,
> "\n pages free %lu"

While this is a noble cause, is printing all this information correct
without the lock, I am not worried about old
information, but incorrect data. Should the read side be rcu'ed. Is
just removing the lock and accessing data safe across architectures?

--
Balbir


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