On Tue, May 28, 2024 at 02:54:58PM -0400, Eric Chanudet wrote:

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 <redacted>
Tested-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)

---
- v1: https://lore.kernel.org/linux-arm-kernel/20240520231555.395979-5-echanude@redhat.com (local)
- 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/ (local)
- 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(-)

It does speed up. For s390:

Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>