Messages in this thread | | | Date | Mon, 4 Jun 2018 21:28:21 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 13/19] mm/migrate: Use xchg instead of spinlock |
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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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