[PATCH] arm64: init: override deferred_page_init_max_threads

[Eric Chanudet]

This was the behavior prior to making the function arch-specific with
commit ecd096506922 ("mm: make deferred init's max threads
arch-specific")

Architectures can override the generic implementation that uses only one
CPU. Setting DEFERRED_STRUCT_PAGE_INIT and testing on a few arm64
platforms shows faster deferred_init_memmap completions:

|         | x13s        | SA8775p-ride | Ampere R137-P31 | Ampere HR330 |
|         | Metal, 32GB | VM, 36GB     | VM, 58GB        | Metal, 128GB |
|         | 8cpus       | 8cpus        | 8cpus           | 32cpus       |
|---------|-------------|--------------|-----------------|--------------|
| threads |  ms     (%) | ms       (%) |  ms         (%) |  ms      (%) |
|---------|-------------|--------------|-----------------|--------------|
| 1       | 108    (0%) | 72      (0%) | 224        (0%) | 324     (0%) |
| cpus    |  24  (-77%) | 36    (-50%) |  40      (-82%) |  56   (-82%) |

Signed-off-by: Eric Chanudet <echanude@xxxxxxxxxx>
---
 arch/arm64/mm/init.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 9b5ab6818f7f..71f5188fe63d 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -158,6 +158,13 @@ static void __init zone_sizes_init(void)
 	free_area_init(max_zone_pfns);
 }
 
+#ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
+int __init deferred_page_init_max_threads(const struct cpumask *node_cpumask)
+{
+	return max_t(int, cpumask_weight(node_cpumask), 1);
+}
+#endif
+
 int pfn_is_map_memory(unsigned long pfn)
 {
 	phys_addr_t addr = PFN_PHYS(pfn);
-- 
2.44.0