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    SubjectRe: [PATCH 1/4] vmstat: remove zone->lock from walk_zones_in_node
    On Tue, Jan 05, 2010 at 11:04:58AM +0900, KOSAKI Motohiro wrote:
    > 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.
    >

    True. Add a comment explaining that. I considered list_for_each_safe()
    but it wouldn't work in all cases.

    >
    > > 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)?
    >

    I think (2) will make zoneinfo harder to use for examining all the counters
    properly as I explained above. I haven't looked at memory-hotplug in a
    while but IIRC, fields like present_pages should be protected by a lock on
    the pgdat and a seq lock on the zone. If this is not true at the moment,
    it is a problem.

    For the free lists, memory hotplug should be taking the zone->lock properly as
    the final stage of onlining memory is to walk the sections being hot-added,
    init the memmap and then __free_page() each page individually - i.e. the
    normal free path.

    So, if memory hotplug is not protected by proper locking, it's not intentional.

    >
    > > 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.
    >

    Then I believe the zonelock should be preserved so that all entries in
    /proc/zoneinfo are consistent.

    >
    > > > 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.
    >
    >
    >

    --
    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab


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