[PATCH v2 2/3] vmalloc: Optimize vfree

[Muhammad Usama Anjum]

From: Ryan Roberts <ryan.roberts@xxxxxxx>

Whenever vmalloc allocates high order pages (e.g. for a huge mapping) it
must immediately split_page() to order-0 so that it remains compatible
with users that want to access the underlying struct page.
Commit a06157804399 ("mm/vmalloc: request large order pages from buddy
allocator") recently made it much more likely for vmalloc to allocate
high order pages which are subsequently split to order-0.

Unfortunately this had the side effect of causing performance
regressions for tight vmalloc/vfree loops (e.g. test_vmalloc.ko
benchmarks). See Closes: tag. This happens because the high order pages
must be gotten from the buddy but then because they are split to
order-0, when they are freed they are freed to the order-0 pcp.
Previously allocation was for order-0 pages so they were recycled from
the pcp.

It would be preferable if when vmalloc allocates an (e.g.) order-3 page
that it also frees that order-3 page to the order-3 pcp, then the
regression could be removed.

So let's do exactly that; use the new __free_contig_range() API to
batch-free contiguous ranges of pfns. This not only removes the
regression, but significantly improves performance of vfree beyond the
baseline.

A selection of test_vmalloc benchmarks running on arm64 server class
system. mm-new is the baseline. Commit a06157804399 ("mm/vmalloc: request
large order pages from buddy allocator") was added in v6.19-rc1 where we
see regressions. Then with this change performance is much better. (>0
is faster, <0 is slower, (R)/(I) = statistically significant
Regression/Improvement):

+-----------------+----------------------------------------------------------+-------------------+--------------------+
| Benchmark       | Result Class                                             |   mm-new          |  this series       |
+=================+==========================================================+===================+====================+
| micromm/vmalloc | fix_align_alloc_test: p:1, h:0, l:500000 (usec)          |        1331843.33 |         (I) 67.17% |
|                 | fix_size_alloc_test: p:1, h:0, l:500000 (usec)           |         415907.33 |             -5.14% |
|                 | fix_size_alloc_test: p:4, h:0, l:500000 (usec)           |         755448.00 |         (I) 53.55% |
|                 | fix_size_alloc_test: p:16, h:0, l:500000 (usec)          |        1591331.33 |         (I) 57.26% |
|                 | fix_size_alloc_test: p:16, h:1, l:500000 (usec)          |        1594345.67 |         (I) 68.46% |
|                 | fix_size_alloc_test: p:64, h:0, l:100000 (usec)          |        1071826.00 |         (I) 79.27% |
|                 | fix_size_alloc_test: p:64, h:1, l:100000 (usec)          |        1018385.00 |         (I) 84.17% |
|                 | fix_size_alloc_test: p:256, h:0, l:100000 (usec)         |        3970899.67 |         (I) 77.01% |
|                 | fix_size_alloc_test: p:256, h:1, l:100000 (usec)         |        3821788.67 |         (I) 89.44% |
|                 | fix_size_alloc_test: p:512, h:0, l:100000 (usec)         |        7795968.00 |         (I) 82.67% |
|                 | fix_size_alloc_test: p:512, h:1, l:100000 (usec)         |        6530169.67 |        (I) 118.09% |
|                 | full_fit_alloc_test: p:1, h:0, l:500000 (usec)           |         626808.33 |             -0.98% |
|                 | kvfree_rcu_1_arg_vmalloc_test: p:1, h:0, l:500000 (usec) |         532145.67 |             -1.68% |
|                 | kvfree_rcu_2_arg_vmalloc_test: p:1, h:0, l:500000 (usec) |         537032.67 |             -0.96% |
|                 | long_busy_list_alloc_test: p:1, h:0, l:500000 (usec)     |        8805069.00 |         (I) 74.58% |
|                 | pcpu_alloc_test: p:1, h:0, l:500000 (usec)               |         500824.67 |              4.35% |
|                 | random_size_align_alloc_test: p:1, h:0, l:500000 (usec)  |        1637554.67 |         (I) 76.99% |
|                 | random_size_alloc_test: p:1, h:0, l:500000 (usec)        |        4556288.67 |         (I) 72.23% |
|                 | vm_map_ram_test: p:1, h:0, l:500000 (usec)               |         107371.00 |             -0.70% |
+-----------------+----------------------------------------------------------+-------------------+--------------------+

Fixes: a06157804399 ("mm/vmalloc: request large order pages from buddy allocator")
Closes: https://lore.kernel.org/all/66919a28-bc81-49c9-b68f-dd7c73395a0d@xxxxxxx/
Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
Co-developed-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
---
Changes since v1:
- Rebase on mm-new
- Rerun benchmarks
---
 mm/vmalloc.c | 34 +++++++++++++++++++++++++---------
 1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index c607307c657a6..8b935395fb068 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3459,18 +3459,34 @@ void vfree(const void *addr)
 
 	if (unlikely(vm->flags & VM_FLUSH_RESET_PERMS))
 		vm_reset_perms(vm);
-	for (i = 0; i < vm->nr_pages; i++) {
-		struct page *page = vm->pages[i];
+
+	if (vm->nr_pages) {
+		bool account = !(vm->flags & VM_MAP_PUT_PAGES);
+		unsigned long start_pfn, pfn;
+		struct page *page = vm->pages[0];
+		int nr = 1;
 
 		BUG_ON(!page);
-		/*
-		 * High-order allocs for huge vmallocs are split, so
-		 * can be freed as an array of order-0 allocations
-		 */
-		if (!(vm->flags & VM_MAP_PUT_PAGES))
+		start_pfn = page_to_pfn(page);
+		if (account)
 			mod_lruvec_page_state(page, NR_VMALLOC, -1);
-		__free_page(page);
-		cond_resched();
+
+		for (i = 1; i < vm->nr_pages; i++) {
+			page = vm->pages[i];
+			BUG_ON(!page);
+			if (account)
+				mod_lruvec_page_state(page, NR_VMALLOC, -1);
+			pfn = page_to_pfn(page);
+			if (start_pfn + nr == pfn) {
+				nr++;
+				continue;
+			}
+			__free_contig_range(start_pfn, nr);
+			start_pfn = pfn;
+			nr = 1;
+			cond_resched();
+		}
+		__free_contig_range(start_pfn, nr);
 	}
 	kvfree(vm->pages);
 	kfree(vm);
-- 
2.47.3