Re: [PATCH] mm: cma: allocate pages from CMA if NR_FREE_PAGES approaches low water mark

Hello,

On 11/14/2012 11:58 PM, Andrew Morton wrote:

On Mon, 12 Nov 2012 09:59:42 +0100
Marek Szyprowski [off-list ref] wrote:

quoted

It has been observed that system tends to keep a lot of CMA free pages
even in very high memory pressure use cases. The CMA fallback for movable
pages is used very rarely, only when system is completely pruned from
MOVABLE pages, what usually means that the out-of-memory even will be
triggered very soon. To avoid such situation and make better use of CMA
pages, a heuristics is introduced which turns on CMA fallback for movable
pages when the real number of free pages (excluding CMA free pages)
approaches low water mark.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Kyungmin Park <kyungmin.park@samsung.com>
CC: Michal Nazarewicz <redacted>
---
 mm/page_alloc.c |    9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fcb9719..90b51f3 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -1076,6 +1076,15 @@ static struct page *__rmqueue(struct zone *zone, unsigned int order,
 {
 	struct page *page;

+#ifdef CONFIG_CMA
+	unsigned long nr_free = zone_page_state(zone, NR_FREE_PAGES);
+	unsigned long nr_cma_free = zone_page_state(zone, NR_FREE_CMA_PAGES);
+
+	if (migratetype == MIGRATE_MOVABLE && nr_cma_free &&
+	    nr_free - nr_cma_free < 2 * low_wmark_pages(zone))
+		migratetype = MIGRATE_CMA;
+#endif /* CONFIG_CMA */
+
 retry_reserve:
 	page = __rmqueue_smallest(zone, order, migratetype);

erk, this is right on the page allocator hotpath.  Bad.

Yes, I know that it adds an overhead to allocation hot path, but I found 
no other
place for such change. Do You have any suggestion where such change can 
be applied
to avoid additional load on hot path?

At the very least, we could code it so it is not quite so dreadfully
inefficient:

	if (migratetype == MIGRATE_MOVABLE) {
		unsigned long nr_cma_free;

		nr_cma_free = zone_page_state(zone, NR_FREE_CMA_PAGES);
		if (nr_cma_free) {
			unsigned long nr_free;

			nr_free = zone_page_state(zone, NR_FREE_PAGES);

			if (nr_free - nr_cma_free < 2 * low_wmark_pages(zone))
				migratetype = MIGRATE_CMA;
		}
	}

but it still looks pretty bad.

Do You want me to resend such patch?

Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center


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