[PATCH v3] mm/mm_init: use node's number of cpus in deferred_page_init_max_threads

[Eric Chanudet]

When DEFERRED_STRUCT_PAGE_INIT=y, use a node's cpu count as maximum
thread count for the deferred initialization of struct pages via padata.
This should result in shorter boot times for these configurations by
going through page_alloc_init_late() faster as systems tend not to be
under heavy load that early in the bootstrap.

Only x86_64 does that now. Make it archs agnostic when
DEFERRED_STRUCT_PAGE_INIT is set. With the default defconfigs, that
includes powerpc and s390.

It used to be so before offering archs to override the function for
tuning with commit ecd096506922 ("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 on a powerpc machine (1TB, 40 cores, 4KB pages) reports
faster deferred_init_memmap from 210-240ms to 90-110ms between nodes.

Signed-off-by: Eric Chanudet <echanude at redhat.com>
Tested-by: Michael Ellerman <mpe at ellerman.id.au> (powerpc)

---
- 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).

- v2: https://lore.kernel.org/linux-arm-kernel/20240522203758.626932-4-echanude@redhat.com/
- Changes since v2:
 - deferred_page_init_max_threads returns unsigned and use max instead
   of max_t.
 - Make deferred_page_init_max_threads static since there are no more
   override.
 - Rephrase description.
 - Add T-b and report from Michael Ellerman.

 arch/x86/mm/init_64.c    | 12 ------------
 include/linux/memblock.h |  2 --
 mm/mm_init.c             |  5 ++---
 3 files changed, 2 insertions(+), 17 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/include/linux/memblock.h b/include/linux/memblock.h
index e2082240586d..40c62aca36ec 100644
--- a/include/linux/memblock.h
+++ b/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 */
 
 /**
diff --git a/mm/mm_init.c b/mm/mm_init.c
index f72b852bd5b8..acfeba508796 100644
--- a/mm/mm_init.c
+++ b/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 */
-- 
2.44.0