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    SubjectRe: [PATCH 13/19] mm/migrate: Use xchg instead of spinlock
    On Mon, Jun 04, 2018 at 03:30:22PM +0530, Srikar Dronamraju wrote:
    > diff --git a/mm/migrate.c b/mm/migrate.c
    > index 8c0af0f..1c55956 100644
    > --- a/mm/migrate.c
    > +++ b/mm/migrate.c
    > @@ -1874,11 +1874,9 @@ static bool numamigrate_update_ratelimit(pg_data_t *pgdat,
    > * all the time is being spent migrating!
    > */
    > if (time_after(jiffies, pgdat->numabalancing_migrate_next_window)) {
    > - spin_lock(&pgdat->numabalancing_migrate_lock);
    > - pgdat->numabalancing_migrate_nr_pages = 0;
    > - pgdat->numabalancing_migrate_next_window = jiffies +
    > - msecs_to_jiffies(migrate_interval_millisecs);
    > - spin_unlock(&pgdat->numabalancing_migrate_lock);
    > + if (xchg(&pgdat->numabalancing_migrate_nr_pages, 0))
    > + pgdat->numabalancing_migrate_next_window = jiffies +
    > + msecs_to_jiffies(migrate_interval_millisecs);

    Note that both are in fact wrong. That wants to be something like:

    pgdat->numabalancing_migrate_next_window += interval;

    Otherwise you stretch every interval by 'jiffies - numabalancing_migrate_next_window'.

    Also, that all wants READ_ONCE/WRITE_ONCE, irrespective of the
    spinlock/xchg.

    I suppose the problem here is that PPC has a very nasty test-and-set
    spinlock with fwd progress issues while xchg maps to a fairly simple
    ll/sc that (hopefully) has some hardware fairness.

    And pgdata being a rather course data structure (per node?) there could
    be a lot of CPUs stomping on this here thing.

    So simpler not really, but better for PPC.

    > }
    > if (pgdat->numabalancing_migrate_nr_pages > ratelimit_pages) {
    > trace_mm_numa_migrate_ratelimit(current, pgdat->node_id,
    > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    > index 4526643..464a25c 100644
    > --- a/mm/page_alloc.c
    > +++ b/mm/page_alloc.c
    > @@ -6208,7 +6208,6 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat)
    >
    > pgdat_resize_init(pgdat);
    > #ifdef CONFIG_NUMA_BALANCING
    > - spin_lock_init(&pgdat->numabalancing_migrate_lock);
    > pgdat->numabalancing_migrate_nr_pages = 0;
    > pgdat->active_node_migrate = 0;
    > pgdat->numabalancing_migrate_next_window = jiffies;
    > --
    > 1.8.3.1
    >

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