[PATCH AUTOSEL 07/14] mm/page_alloc: fetch the correct pcp buddy during bulk free

Greg Kroah-Hartman
2022-04-28T15:43:26Z

From: Mel Gorman <mgorman@techsingularity.net>

commit ca7b59b1de72450b3e696bada3506a519ac5455c upstream.

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/20220217002227.5739-1-mgorman@techsingularity.net
Link: https://lkml.kernel.org/r/20220217002227.5739-2-mgorman@techsingularity.net
Fixes: 44042b449872 ("mm/page_alloc: allow high-order pages to be stored on the per-cpu lists")
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Aaron Lu <aaron.lu@intel.com>
Tested-by: Aaron Lu <aaron.lu@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
mm/page_alloc.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e6f211dcf82e..b2ef0e75fd29 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1432,10 +1432,10 @@ static bool bulkfree_pcp_prepare(struct page *page)
}
#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);
@@ -1512,7 +1512,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
* 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));
--
2.36.0