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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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