[PATCH v2] mm/mm_init: use node's number of cpus in deferred_page_init_max_threads

[Eric Chanudet <echanude@redhat.com>]

x86_64 is already using the node's cpu as maximum threads. Make that the
default for all archs setting DEFERRED_STRUCT_PAGE_INIT.

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

Signed-off-by: Eric Chanudet <echanude@redhat.com>

---
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%) |

- v1: https://lore.kernel.org/linux-arm-kernel/20240520231555.395979-5-echanude@redhat.com
- Changes since v1:
 - Make the generic function return the number of cpus of the node as
   max threads limit instead overriding it for arm64.
- Drop Baoquan He's R-b on v1 since the logic changed.
- Add CCs according to patch changes (ppc and s390 set
  DEFERRED_STRUCT_PAGE_INIT by default).

 arch/x86/mm/init_64.c | 12 ------------
 mm/mm_init.c          |  2 +-
 2 files changed, 1 insertion(+), 13 deletions(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 7e177856ee4f..adec42928ec1 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1354,18 +1354,6 @@ void __init mem_init(void)
 	preallocate_vmalloc_pages();
 }
 
-#ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
-int __init deferred_page_init_max_threads(const struct cpumask *node_cpumask)
-{
-	/*
-	 * More CPUs always led to greater speedups on tested systems, up to
-	 * all the nodes' CPUs.  Use all since the system is otherwise idle
-	 * now.
-	 */
-	return max_t(int, cpumask_weight(node_cpumask), 1);
-}
-#endif
-
 int kernel_set_to_readonly;
 
 void mark_rodata_ro(void)
diff --git a/mm/mm_init.c b/mm/mm_init.c
index f72b852bd5b8..e0023aa68555 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -2126,7 +2126,7 @@ deferred_init_memmap_chunk(unsigned long start_pfn, unsigned long end_pfn,
 __weak int __init
 deferred_page_init_max_threads(const struct cpumask *node_cpumask)
 {
-	return 1;
+	return max_t(int, cpumask_weight(node_cpumask), 1);
 }
 
 /* Initialise remaining memory on a node */
-- 
2.44.0