[098/227] mm/page_alloc: fetch the correct pcp buddy during bulk free

Commit Message

Andrew Morton March 22, 2022, 9:43 p.m. UTC

From: Mel Gorman <[e-mail]>
Subject: mm/page_alloc: fetch the correct pcp buddy during bulk free

Patch series "Follow-up on high-order PCP caching", v2.

Commit 44042b449872 ("mm/page_alloc: allow high-order pages to be stored
on the per-cpu lists") was primarily aimed at reducing the cost of SLUB
cache refills of high-order pages in two ways.  Firstly, zone lock
acquisitions was reduced and secondly, there were fewer buddy list
modifications.  This is a follow-up series fixing some issues that became
apparant after merging.

Patch 1 is a functional fix. It's harmless but inefficient.

Patches 2-5 reduce the overhead of bulk freeing of PCP pages.  While the
overhead is small, it's cumulative and noticable when truncating large
files.  The changelog for patch 4 includes results of a microbench that
deletes large sparse files with data in page cache.  Sparse files were
used to eliminate filesystem overhead.

Patch 6 addresses issues with high-order PCP pages being stored on PCP
lists for too long.  Pages freed on a CPU potentially may not be quickly
reused and in some cases this can increase cache miss rates.  Details are
included in the changelog.


This patch (of 6):

free_pcppages_bulk() prefetches buddies about to be freed but the order
must also be passed in as PCP lists store multiple orders.

Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 44042b449872 ("mm/page_alloc: allow high-order pages to be stored on the per-cpu lists")
Signed-off-by: Mel Gorman <[e-mail]>
Reviewed-by: Vlastimil Babka <[e-mail]>
Reviewed-by: Aaron Lu <[e-mail]>
Tested-by: Aaron Lu <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Jesper Dangaard Brouer <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/page_alloc.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

Patch

--- a/mm/page_alloc.c~mm-page_alloc-fetch-the-correct-pcp-buddy-during-bulk-free
+++ a/mm/page_alloc.c
@@ -1429,10 +1429,10 @@  static bool bulkfree_pcp_prepare(struct
 }
 #endif /* CONFIG_DEBUG_VM */
 
-static inline void prefetch_buddy(struct page *page)
+static inline void prefetch_buddy(struct page *page, unsigned int order)
 {
 	unsigned long pfn = page_to_pfn(page);
-	unsigned long buddy_pfn = __find_buddy_pfn(pfn, 0);
+	unsigned long buddy_pfn = __find_buddy_pfn(pfn, order);
 	struct page *buddy = page + (buddy_pfn - pfn);
 
 	prefetch(buddy);
@@ -1509,7 +1509,7 @@  static void free_pcppages_bulk(struct zo
 			 * prefetch buddy for the first pcp->batch nr of pages.
 			 */
 			if (prefetch_nr) {
-				prefetch_buddy(page);
+				prefetch_buddy(page, order);
 				prefetch_nr--;
 			}
 		} while (count > 0 && --batch_free && !list_empty(list));