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    SubjectRe: [PATCH 1/4] vmstat: remove zone->lock from walk_zones_in_node
    Date
    Hi

    > On Mon, Dec 28, 2009 at 04:47:22PM +0900, KOSAKI Motohiro wrote:
    > > 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);
    > > +
    >
    > It's not clear why you feel this information requires the lock and the
    > others do not.

    I think above list operation require lock to prevent NULL pointer access. but other parts
    doesn't protect anything, because memory-hotplug change them without zone lock.


    > For the most part, I agree that the accuracy of the information is
    > not critical. Assuming partial writes of the data are not a problem,
    > the information is not going to go so badly out of sync that it would be
    > noticable, even if the information is out of date within the zone.
    >
    > However, inconsistent reads in zoneinfo really could be a problem. I am
    > concerned that under heavy allocation load that that "pages free" would
    > not match "nr_pages_free" for example. Other examples that adding all the
    > counters together may or may not equal the total number of pages in the zone.
    >
    > Lets say for example there was a subtle bug related to __inc_zone_page_state()
    > that meant that counters were getting slightly out of sync but it was very
    > marginal and/or difficult to reproduce. With this patch applied, we could
    > not be absolutly sure the counters were correct because it could always have
    > raced with someone holding the zone->lock.
    >
    > Minimally, I think zoneinfo should be taking the zone lock.

    Thanks lots comments.
    hmm.. I'd like to clarily your point. My point is memory-hotplug don't take zone lock,
    then zone lock doesn't protect anything. so we have two option

    1) Add zone lock to memroy-hotplug
    2) Remove zone lock from zoneinfo

    I thought (2) is sufficient. Do you mean you prefer to (1)? Or you prefer to ignore rarely event
    (of cource, memory hotplug is rarely)?


    > Secondly, has increased zone->lock contention due to reading /proc
    > really been shown to be a problem? The only situation that I can think
    > of is a badly-written monitor program that is copying all of /proc
    > instead of the files of interest. If a monitor program is doing
    > something like that, it's likely to be incurring performance problems in
    > a large number of different areas. If that is not the trigger case, what
    > is?

    Ah no. I haven't observe such issue. my point is removing meaningless lock.


    > > 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;
    > > +
    >
    > Unnecessary whitespace change.

    Ug. thanks, it's my fault.





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