mm/mm_init: use node's number of cpus in deferred_page_init_max_threads

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 ecd09650 ("mm: make deferred init's max threads
arch-specific").

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%) |

Michael Ellerman reported:

: On a machine here (1TB, 40 cores, 4KB pages) the existing code gives:
: 
:   [    0.500124] node 2 deferred pages initialised in 210ms
:   [    0.515790] node 3 deferred pages initialised in 230ms
:   [    0.516061] node 0 deferred pages initialised in 230ms
:   [    0.516522] node 7 deferred pages initialised in 230ms
:   [    0.516672] node 4 deferred pages initialised in 230ms
:   [    0.516798] node 6 deferred pages initialised in 230ms
:   [    0.517051] node 5 deferred pages initialised in 230ms
:   [    0.523887] node 1 deferred pages initialised in 240ms
: 
: vs with the patch:
: 
:   [    0.379613] node 0 deferred pages initialised in 90ms
:   [    0.380388] node 1 deferred pages initialised in 90ms
:   [    0.380540] node 4 deferred pages initialised in 100ms
:   [    0.390239] node 6 deferred pages initialised in 100ms
:   [    0.390249] node 2 deferred pages initialised in 100ms
:   [    0.390786] node 3 deferred pages initialised in 110ms
:   [    0.396721] node 5 deferred pages initialised in 110ms
:   [    0.397095] node 7 deferred pages initialised in 110ms
: 
: Which is a nice speedup.

[echanude@redhat.com: v3]
  Link: https://lkml.kernel.org/r/20240528185455.643227-4-echanude@redhat.com
Link: https://lkml.kernel.org/r/20240522203758.626932-4-echanude@redhat.comSigned-off-by: Eric Chanudet <echanude@redhat.com>
Tested-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Reviewed-by: Baoquan He <bhe@redhat.com>
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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)

include/linux/memblock.h

@@ -335,8 +335,6 @@ void __next_mem_pfn_range_in_zone(u64 *idx, struct zone *zone,
 	for (; i != U64_MAX;					  \
 	     __next_mem_pfn_range_in_zone(&i, zone, p_start, p_end))
 
-int __init deferred_page_init_max_threads(const struct cpumask *node_cpumask);
-
 #endif /* CONFIG_DEFERRED_STRUCT_PAGE_INIT */
 
 /**

mm/mm_init.c

@@ -2122,11 +2122,10 @@ deferred_init_memmap_chunk(unsigned long start_pfn, unsigned long end_pfn,
 	}
 }
 
-/* An arch may override for more concurrency. */
-__weak int __init
+static unsigned int __init
 deferred_page_init_max_threads(const struct cpumask *node_cpumask)
 {
-	return 1;
+	return max(cpumask_weight(node_cpumask), 1U);
 }
 
 /* Initialise remaining memory on a node */