Re: [PATCH] mm: batch-free pcp list if possible
From: Andrew Morton <akpm@linux-foundation.org>
Date: 2011-02-09 20:38:35
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On Wed, 9 Feb 2011 22:21:17 +0900 Namhyung Kim [off-list ref] wrote:
quoted hunk ↗ jump to hunk
free_pcppages_bulk() frees pages from pcp lists in a round-robin fashion by keeping batch_free counter. But it doesn't need to spin if there is only one non-empty list. This can be checked by batch_free == MIGRATE_PCPTYPES. Signed-off-by: Namhyung Kim <redacted> --- mm/page_alloc.c | 4 ++++ 1 files changed, 4 insertions(+), 0 deletions(-)diff --git a/mm/page_alloc.c b/mm/page_alloc.c index a873e61e312e..470fb42e303c 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c@@ -614,6 +614,10 @@ static void free_pcppages_bulk(struct zone *zone, int count, list = &pcp->lists[migratetype]; } while (list_empty(list)); + /* This is an only non-empty list. Free them all. */ + if (batch_free == MIGRATE_PCPTYPES) + batch_free = to_free; + do { page = list_entry(list->prev, struct page, lru); /* must delete as __free_one_page list manipulates */
free_pcppages_bulk() hurts my brain. What is it actually trying to do, and why? It counts up the number of contiguous empty lists and then frees that number of pages from the first-encountered non-empty list and then advances onto the next list? What's the point in that? What relationship does the number of contiguous empty lists have with the number of pages to free from one list? The comment "This is so more pages are freed off fuller lists instead of spinning excessively around empty lists" makes no sense - the only way this can be true is if the code knows the number of elements on each list, and it doesn't know that. Also, the covering comments over free_pcppages_bulk() regarding the pages_scanned counter and the "all pages pinned" logic appear to be out of date. Or, alternatively, those comments do reflect the desired design, but we broke it. Methinks that free_pcppages_bulk() is an area ripe for simplification and clarification. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>