Re: [PATCH 13/19] mm/migrate: Use xchg instead of spinlock

Peter Zijlstra
2018-06-04T19:28:32Z

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
>