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

[Alexander Gordeev]

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 <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(-)

It does speed up. For s390:

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