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SubjectRe: [PATCH v2 13/19] mm/migrate: Use xchg instead of spinlock
On Wed, Jun 20, 2018 at 10:32:54PM +0530, Srikar Dronamraju wrote:
> Currently resetting the migrate rate limit is under a spinlock.
> The spinlock will only serialize the migrate rate limiting and something
> similar can actually be achieved by a simpler xchg.

You're again not explaining things right. The xchg isn't simpler in any
way. It just happens to be faster, esp. so on PPC.

> diff --git a/mm/migrate.c b/mm/migrate.c
> index 8c0af0f..c774990 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1868,17 +1868,25 @@ static struct page *alloc_misplaced_dst_page(struct page *page,
> static bool numamigrate_update_ratelimit(pg_data_t *pgdat,
> unsigned long nr_pages)
> {
> + unsigned long next_window, interval;
> +
> + next_window = READ_ONCE(pgdat->numabalancing_migrate_next_window);
> + interval = msecs_to_jiffies(migrate_interval_millisecs);
> +
> /*
> * Rate-limit the amount of data that is being migrated to a node.
> * Optimal placement is no good if the memory bus is saturated and
> * all the time is being spent migrating!
> */
> + if (time_after(jiffies, next_window)) {
> + if (xchg(&pgdat->numabalancing_migrate_nr_pages, 0)) {
> + do {
> + next_window += interval;
> + } while (unlikely(time_after(jiffies, next_window)));
> +
> + WRITE_ONCE(pgdat->numabalancing_migrate_next_window,
> + next_window);
> + }
> }
> if (pgdat->numabalancing_migrate_nr_pages > ratelimit_pages) {
> trace_mm_numa_migrate_ratelimit(current, pgdat->node_id,

If you maybe write that like:

if (time_after(jiffies, next_window) &&
xchg(&pgdat->numabalancing_migrate_nr_pages, 0UL)) {

do {
next_window += interval;
} while (unlikely(time_after(jiffies, next_window)));

WRITE_ONCE(pgdat->numabalancing_migrate_next_window, next_window);
}

Then you avoid an indent level and line-wrap, resulting imo easier to
read code.

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