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Date: Wed, 22 May 2024 15:46:07 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Eric Chanudet <echanude@redhat.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>, Andy Lutomirski
 <luto@kernel.org>, Peter Zijlstra <peterz@infradead.org>, Thomas Gleixner
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 <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>, Mike Rapoport
 <rppt@kernel.org>, Baoquan He <bhe@redhat.com>, Michael Ellerman
 <mpe@ellerman.id.au>, Nick Piggin <npiggin@gmail.com>, x86@kernel.org,
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 linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
 linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v2] mm/mm_init: use node's number of cpus in
 deferred_page_init_max_threads
Message-Id: <20240522154607.bd5790c0b0dc642aefd3a05c@linux-foundation.org>
In-Reply-To: <20240522203758.626932-4-echanude@redhat.com>
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On Wed, 22 May 2024 16:38:01 -0400 Eric Chanudet <echanude@redhat.com> wrote:

> 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").
> 

It isn't clear to me what is the runtime effect of this change upon our
users.  Can you please prepare a sentence which spells this out?

> 
> ---
> 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%) |

The above is useful info, I'll hoist it into the main changelog.

> --- 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);
>  }

It's an unrelated cleanup , but that could be

	max(cpumask_weight(node_cpumask), 1U);

and the function could/should return unsigned.