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* [PATCH v7] mm: Add CONFIG_PAGE_BLOCK_ORDER to select page block order
@ 2025-05-21 21:57 Juan Yescas
  2025-05-28  8:21 ` Vlastimil Babka
  2025-06-03 13:03 ` David Hildenbrand
  0 siblings, 2 replies; 8+ messages in thread
From: Juan Yescas @ 2025-05-21 21:57 UTC (permalink / raw)
  To: Andrew Morton, David Hildenbrand, Lorenzo Stoakes,
	Liam R. Howlett, Vlastimil Babka, Mike Rapoport,
	Suren Baghdasaryan, Michal Hocko, Juan Yescas, Zi Yan, linux-mm,
	linux-kernel
  Cc: tjmercier, isaacmanjarres, kaleshsingh, masahiroy, Minchan Kim

Problem: On large page size configurations (16KiB, 64KiB), the CMA
alignment requirement (CMA_MIN_ALIGNMENT_BYTES) increases considerably,
and this causes the CMA reservations to be larger than necessary.
This means that system will have less available MIGRATE_UNMOVABLE and
MIGRATE_RECLAIMABLE page blocks since MIGRATE_CMA can't fallback to them.

The CMA_MIN_ALIGNMENT_BYTES increases because it depends on
MAX_PAGE_ORDER which depends on ARCH_FORCE_MAX_ORDER. The value of
ARCH_FORCE_MAX_ORDER increases on 16k and 64k kernels.

For example, in ARM, the CMA alignment requirement when:

- CONFIG_ARCH_FORCE_MAX_ORDER default value is used
- CONFIG_TRANSPARENT_HUGEPAGE is set:

PAGE_SIZE | MAX_PAGE_ORDER | pageblock_order | CMA_MIN_ALIGNMENT_BYTES
-----------------------------------------------------------------------
   4KiB   |      10        |       9         |  4KiB * (2 ^  9) =   2MiB
  16Kib   |      11        |      11         | 16KiB * (2 ^ 11) =  32MiB
  64KiB   |      13        |      13         | 64KiB * (2 ^ 13) = 512MiB

There are some extreme cases for the CMA alignment requirement when:

- CONFIG_ARCH_FORCE_MAX_ORDER maximum value is set
- CONFIG_TRANSPARENT_HUGEPAGE is NOT set:
- CONFIG_HUGETLB_PAGE is NOT set

PAGE_SIZE | MAX_PAGE_ORDER | pageblock_order |  CMA_MIN_ALIGNMENT_BYTES
------------------------------------------------------------------------
   4KiB   |      15        |      15         |  4KiB * (2 ^ 15) = 128MiB
  16Kib   |      13        |      13         | 16KiB * (2 ^ 13) = 128MiB
  64KiB   |      13        |      13         | 64KiB * (2 ^ 13) = 512MiB

This affects the CMA reservations for the drivers. If a driver in a
4KiB kernel needs 4MiB of CMA memory, in a 16KiB kernel, the minimal
reservation has to be 32MiB due to the alignment requirements:

reserved-memory {
    ...
    cma_test_reserve: cma_test_reserve {
        compatible = "shared-dma-pool";
        size = <0x0 0x400000>; /* 4 MiB */
        ...
    };
};

reserved-memory {
    ...
    cma_test_reserve: cma_test_reserve {
        compatible = "shared-dma-pool";
        size = <0x0 0x2000000>; /* 32 MiB */
        ...
    };
};

Solution: Add a new config CONFIG_PAGE_BLOCK_ORDER that
allows to set the page block order in all the architectures.
The maximum page block order will be given by
ARCH_FORCE_MAX_ORDER.

By default, CONFIG_PAGE_BLOCK_ORDER will have the same
value that ARCH_FORCE_MAX_ORDER. This will make sure that
current kernel configurations won't be affected by this
change. It is a opt-in change.

This patch will allow to have the same CMA alignment
requirements for large page sizes (16KiB, 64KiB) as that
in 4kb kernels by setting a lower pageblock_order.

Tests:

- Verified that HugeTLB pages work when pageblock_order is 1, 7, 10
on 4k and 16k kernels.

- Verified that Transparent Huge Pages work when pageblock_order
is 1, 7, 10 on 4k and 16k kernels.

- Verified that dma-buf heaps allocations work when pageblock_order
is 1, 7, 10 on 4k and 16k kernels.

Benchmarks:

The benchmarks compare 16kb kernels with pageblock_order 10 and 7. The
reason for the pageblock_order 7 is because this value makes the min
CMA alignment requirement the same as that in 4kb kernels (2MB).

- Perform 100K dma-buf heaps (/dev/dma_heap/system) allocations of
SZ_8M, SZ_4M, SZ_2M, SZ_1M, SZ_64, SZ_8, SZ_4. Use simpleperf
(https://developer.android.com/ndk/guides/simpleperf) to measure
the # of instructions and page-faults on 16k kernels.
The benchmark was executed 10 times. The averages are below:

           # instructions         |     #page-faults
    order 10     |  order 7       | order 10 | order 7
--------------------------------------------------------
 13,891,765,770	 | 11,425,777,314 |    220   |   217
 14,456,293,487	 | 12,660,819,302 |    224   |   219
 13,924,261,018	 | 13,243,970,736 |    217   |   221
 13,910,886,504	 | 13,845,519,630 |    217   |   221
 14,388,071,190	 | 13,498,583,098 |    223   |   224
 13,656,442,167	 | 12,915,831,681 |    216   |   218
 13,300,268,343	 | 12,930,484,776 |    222   |   218
 13,625,470,223	 | 14,234,092,777 |    219   |   218
 13,508,964,965	 | 13,432,689,094 |    225   |   219
 13,368,950,667	 | 13,683,587,37  |    219   |   225
-------------------------------------------------------------------
 13,803,137,433  | 13,131,974,268 |    220   |   220    Averages

There were 4.85% #instructions when order was 7, in comparison
with order 10.

     13,803,137,433 - 13,131,974,268 = -671,163,166 (-4.86%)

The number of page faults in order 7 and 10 were the same.

These results didn't show any significant regression when the
pageblock_order is set to 7 on 16kb kernels.

- Run speedometer 3.1 (https://browserbench.org/Speedometer3.1/) 5 times
 on the 16k kernels with pageblock_order 7 and 10.

order 10 | order 7  | order 7 - order 10 | (order 7 - order 10) %
-------------------------------------------------------------------
  15.8	 |  16.4    |         0.6        |     3.80%
  16.4	 |  16.2    |        -0.2        |    -1.22%
  16.6	 |  16.3    |        -0.3        |    -1.81%
  16.8	 |  16.3    |        -0.5        |    -2.98%
  16.6	 |  16.8    |         0.2        |     1.20%
-------------------------------------------------------------------
  16.44     16.4            -0.04	          -0.24%   Averages

The results didn't show any significant regression when the
pageblock_order is set to 7 on 16kb kernels.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: David Hildenbrand <david@redhat.com>
CC: Mike Rapoport <rppt@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Juan Yescas <jyescas@google.com>
Acked-by: Zi Yan <ziy@nvidia.com>
---
Changes in v7:
  - Update alignment calculation to 2MiB as per David's
    observation.
  - Update page block order calculation in mm/mm_init.c for
    powerpc when CONFIG_HUGETLB_PAGE_SIZE_VARIABLE is set.

Changes in v6:
  - Applied the change provided by Zi Yan to fix
    the Kconfig. The change consists in evaluating
    to true or false in the if expression for range:
    range 1 <symbol> if <expression to eval true/false>.

Changes in v5:
  - Remove the ranges for CONFIG_PAGE_BLOCK_ORDER. The
    ranges with config definitions don't work in Kconfig,
    for example (range 1 MY_CONFIG).
  - Add PAGE_BLOCK_ORDER_MANUAL config for the
    page block order number. The default value was not
    defined.
  - Fix typos reported by Andrew.
  - Test default configs in powerpc. 

Changes in v4:
  - Set PAGE_BLOCK_ORDER in incluxe/linux/mmzone.h to
    validate that MAX_PAGE_ORDER >= PAGE_BLOCK_ORDER at
    compile time.
  - This change fixes the warning in:
    https://lore.kernel.org/oe-kbuild-all/202505091548.FuKO4b4v-lkp@intel.com/

Changes in v3:
  - Rename ARCH_FORCE_PAGE_BLOCK_ORDER to PAGE_BLOCK_ORDER
    as per Matthew's suggestion.
  - Update comments in pageblock-flags.h for pageblock_order
    value when THP or HugeTLB are not used.

Changes in v2:
  - Add Zi's Acked-by tag.
  - Move ARCH_FORCE_PAGE_BLOCK_ORDER config to mm/Kconfig as
    per Zi and Matthew suggestion so it is available to
    all the architectures.
  - Set ARCH_FORCE_PAGE_BLOCK_ORDER to 10 by default when
    ARCH_FORCE_MAX_ORDER is not available.

 include/linux/mmzone.h          | 16 ++++++++++++++++
 include/linux/pageblock-flags.h |  8 ++++----
 mm/Kconfig                      | 34 +++++++++++++++++++++++++++++++++
 mm/mm_init.c                    |  2 +-
 4 files changed, 55 insertions(+), 5 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 6ccec1bf2896..05610337bbb6 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -37,6 +37,22 @@
 
 #define NR_PAGE_ORDERS (MAX_PAGE_ORDER + 1)
 
+/* Defines the order for the number of pages that have a migrate type. */
+#ifndef CONFIG_PAGE_BLOCK_ORDER
+#define PAGE_BLOCK_ORDER MAX_PAGE_ORDER
+#else
+#define PAGE_BLOCK_ORDER CONFIG_PAGE_BLOCK_ORDER
+#endif /* CONFIG_PAGE_BLOCK_ORDER */
+
+/*
+ * The MAX_PAGE_ORDER, which defines the max order of pages to be allocated
+ * by the buddy allocator, has to be larger or equal to the PAGE_BLOCK_ORDER,
+ * which defines the order for the number of pages that can have a migrate type
+ */
+#if (PAGE_BLOCK_ORDER > MAX_PAGE_ORDER)
+#error MAX_PAGE_ORDER must be >= PAGE_BLOCK_ORDER
+#endif
+
 /*
  * PAGE_ALLOC_COSTLY_ORDER is the order at which allocations are deemed
  * costly to service.  That is between allocation orders which should
diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h
index fc6b9c87cb0a..e73a4292ef02 100644
--- a/include/linux/pageblock-flags.h
+++ b/include/linux/pageblock-flags.h
@@ -41,18 +41,18 @@ extern unsigned int pageblock_order;
  * Huge pages are a constant size, but don't exceed the maximum allocation
  * granularity.
  */
-#define pageblock_order		MIN_T(unsigned int, HUGETLB_PAGE_ORDER, MAX_PAGE_ORDER)
+#define pageblock_order		MIN_T(unsigned int, HUGETLB_PAGE_ORDER, PAGE_BLOCK_ORDER)
 
 #endif /* CONFIG_HUGETLB_PAGE_SIZE_VARIABLE */
 
 #elif defined(CONFIG_TRANSPARENT_HUGEPAGE)
 
-#define pageblock_order		MIN_T(unsigned int, HPAGE_PMD_ORDER, MAX_PAGE_ORDER)
+#define pageblock_order		MIN_T(unsigned int, HPAGE_PMD_ORDER, PAGE_BLOCK_ORDER)
 
 #else /* CONFIG_TRANSPARENT_HUGEPAGE */
 
-/* If huge pages are not used, group by MAX_ORDER_NR_PAGES */
-#define pageblock_order		MAX_PAGE_ORDER
+/* If huge pages are not used, group by PAGE_BLOCK_ORDER */
+#define pageblock_order		PAGE_BLOCK_ORDER
 
 #endif /* CONFIG_HUGETLB_PAGE */
 
diff --git a/mm/Kconfig b/mm/Kconfig
index e113f713b493..13a5c4f6e6b6 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -989,6 +989,40 @@ config CMA_AREAS
 
 	  If unsure, leave the default value "8" in UMA and "20" in NUMA.
 
+#
+# Select this config option from the architecture Kconfig, if available, to set
+# the max page order for physically contiguous allocations.
+#
+config ARCH_FORCE_MAX_ORDER
+	int
+
+#
+# When ARCH_FORCE_MAX_ORDER is not defined,
+# the default page block order is MAX_PAGE_ORDER (10) as per
+# include/linux/mmzone.h.
+#
+config PAGE_BLOCK_ORDER
+	int "Page Block Order"
+	range 1 10 if ARCH_FORCE_MAX_ORDER = 0
+	default 10 if ARCH_FORCE_MAX_ORDER = 0
+	range 1 ARCH_FORCE_MAX_ORDER if ARCH_FORCE_MAX_ORDER != 0
+	default ARCH_FORCE_MAX_ORDER if ARCH_FORCE_MAX_ORDER != 0
+	help
+	  The page block order refers to the power of two number of pages that
+	  are physically contiguous and can have a migrate type associated to
+	  them. The maximum size of the page block order is limited by
+	  ARCH_FORCE_MAX_ORDER.
+
+	  This config allows overriding the default page block order when the
+	  page block order is required to be smaller than ARCH_FORCE_MAX_ORDER
+	  or MAX_PAGE_ORDER.
+
+	  Reducing pageblock order can negatively impact THP generation
+	  success rate. If your workloads uses THP heavily, please use this
+	  option with caution.
+
+	  Don't change if unsure.
+
 config MEM_SOFT_DIRTY
 	bool "Track memory changes"
 	depends on CHECKPOINT_RESTORE && HAVE_ARCH_SOFT_DIRTY && PROC_FS
diff --git a/mm/mm_init.c b/mm/mm_init.c
index 327764ca0ee4..ada5374764e4 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1511,7 +1511,7 @@ static inline void setup_usemap(struct zone *zone) {}
 /* Initialise the number of pages represented by NR_PAGEBLOCK_BITS */
 void __init set_pageblock_order(void)
 {
-	unsigned int order = MAX_PAGE_ORDER;
+	unsigned int order = PAGE_BLOCK_ORDER;
 
 	/* Check that pageblock_nr_pages has not already been setup */
 	if (pageblock_order)
-- 
2.49.0.1143.g0be31eac6b-goog



^ permalink raw reply	[flat|nested] 8+ messages in thread
end of thread, other threads:[~2025-06-03 15:42 UTC | newest]

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-- links below jump to the message on this page --
2025-05-21 21:57 [PATCH v7] mm: Add CONFIG_PAGE_BLOCK_ORDER to select page block order Juan Yescas
2025-05-28  8:21 ` Vlastimil Babka
2025-05-28 18:24   ` Andrew Morton
2025-06-03 13:03 ` David Hildenbrand
2025-06-03 14:55   ` Zi Yan
2025-06-03 15:14     ` Zi Yan
2025-06-03 15:42       ` David Hildenbrand
2025-06-03 15:20   ` Juan Yescas

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