[PATCH v2 3/3] mm/page_alloc: Optimize __free_contig_frozen_range()

[Muhammad Usama Anjum <usama.anjum@arm.com>]

Apply the same batch-freeing optimization from free_contig_range() to the
frozen page path. The previous __free_contig_frozen_range() freed each
order-0 page individually via free_frozen_pages(), which is slow for the
same reason the old free_contig_range() was: each page goes to the
order-0 pcp list rather than being coalesced into higher-order blocks.

Rewrite __free_contig_frozen_range() to call free_pages_prepare() for
each order-0 page, then batch the prepared pages into the largest
possible power-of-2 aligned chunks via free_prepared_contig_range().
If free_pages_prepare() fails (e.g. HWPoison, bad page) the page is
deliberately not freed; it should not be returned to the allocator.

I've tested CMA through debugfs. The test allocates 16384 pages per
allocation for several iterations. There is 3.5x improvement.

Before: 1406 usec per iteration
After:   402 usec per iteration

Before:

    70.89%     0.69%  cma              [kernel.kallsyms]      [.] free_contig_frozen_range
            |
            |--70.20%--free_contig_frozen_range
            |          |
            |          |--46.41%--__free_frozen_pages
            |          |          |
            |          |           --36.18%--free_frozen_page_commit
            |          |                     |
            |          |                      --29.63%--_raw_spin_unlock_irqrestore
            |          |
            |          |--8.76%--_raw_spin_trylock
            |          |
            |          |--7.03%--__preempt_count_dec_and_test
            |          |
            |          |--4.57%--_raw_spin_unlock
            |          |
            |          |--1.96%--__get_pfnblock_flags_mask.isra.0
            |          |
            |           --1.15%--free_frozen_page_commit
            |
             --0.69%--el0t_64_sync

After:

    23.57%     0.00%  cma              [kernel.kallsyms]      [.] free_contig_frozen_range
            |
            ---free_contig_frozen_range
               |
               |--20.45%--__free_contig_frozen_range
               |          |
               |          |--17.77%--free_pages_prepare
               |          |
               |           --0.72%--free_prepared_contig_range
               |                     |
               |                      --0.55%--__free_frozen_pages
               |
                --3.12%--free_pages_prepare

Suggested-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Muhammad Usama Anjum <usama.anjum@arm.com>
---
 mm/page_alloc.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6a9430f720579..2e99fa85cdc8e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7020,8 +7020,22 @@ static int __alloc_contig_verify_gfp_mask(gfp_t gfp_mask, gfp_t *gfp_cc_mask)
 
 static void __free_contig_frozen_range(unsigned long pfn, unsigned long nr_pages)
 {
-	for (; nr_pages--; pfn++)
-		free_frozen_pages(pfn_to_page(pfn), 0);
+	struct page *page = pfn_to_page(pfn);
+	struct page *start = NULL;
+	unsigned long i;
+
+	for (i = 0; i < nr_pages; i++, page++) {
+		if (free_pages_prepare(page, 0)) {
+			if (!start)
+				start = page;
+		} else if (start) {
+			free_prepared_contig_range(start, page - start);
+			start = NULL;
+		}
+	}
+
+	if (start)
+		free_prepared_contig_range(start, page - start);
 }
 
 /**
-- 
2.47.3