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From: Michael Ellerman <mpe@ellerman.id.au>
To: Eric Chanudet <echanude@redhat.com>, Mike Rapoport <rppt@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>, Will Deacon
 <will@kernel.org>, Baoquan He <bhe@redhat.com>, Andrew Morton
 <akpm@linux-foundation.org>, Zhen Lei <thunder.leizhen@huawei.com>, Yajun
 Deng <yajun.deng@linux.dev>, Zhang Jianhua <chris.zjh@huawei.com>,
 linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Nick
 Piggin <npiggin@gmail.com>
Subject: Re: [PATCH] arm64: init: override deferred_page_init_max_threads
In-Reply-To: <xu2rcjwh35zdxx7c52ygep26dox3glnurfcuxuhj3sjqbopzij@zrazmowdb3x2>
References: <20240520231555.395979-5-echanude@redhat.com>
 <ZkzHX-4yYybEJdEQ@kernel.org>
 <xu2rcjwh35zdxx7c52ygep26dox3glnurfcuxuhj3sjqbopzij@zrazmowdb3x2>
Date: Wed, 22 May 2024 23:41:07 +1000
Message-ID: <87plte2ccs.fsf@mail.lhotse>
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Eric Chanudet <echanude@redhat.com> writes:
> On Tue, May 21, 2024 at 07:10:07PM +0300, Mike Rapoport wrote:
>> (added powerpc folks)

Thanks Mike.

>> On Mon, May 20, 2024 at 07:15:59PM -0400, Eric Chanudet wrote:
>> > This was the behavior prior to making the function arch-specific with
>> > commit ecd096506922 ("mm: make deferred init's max threads
>> > arch-specific")
>> > 
>> > Architectures can override the generic implementation that uses only one
>> > CPU. 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%) |

How did you measure this, just some printks in page_alloc_init_late() or
something more sophisticated? Just so I can do some comparable measurements.

>> > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>> > index 9b5ab6818f7f..71f5188fe63d 100644
>> > --- a/arch/arm64/mm/init.c
>> > +++ b/arch/arm64/mm/init.c
>> > @@ -158,6 +158,13 @@ static void __init zone_sizes_init(void)
>> >  	free_area_init(max_zone_pfns);
>> >  }
>> >  
>> > +#ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
>> > +int __init deferred_page_init_max_threads(const struct cpumask *node_cpumask)
>> > +{
>> > +	return max_t(int, cpumask_weight(node_cpumask), 1);
>> > +}
>> > +#endif
>> > +
>> 
>> Maybe we should make this default and let architectures that want a single
>> thread override deferred_page_init_max_threads() to return 1?
>
> It would affect more archs than I can try this on. Currently, only x86
> (with this change, arm64) return more than one thread.

I can test powerpc and we can find someone to test s390. No other arches
have it enabled in their defconfig.

> I'm happy to send a v2 inverting the logic if you find it preferable.

That seems preferable. It's a scalability feature, it makes no sense for
the default to be a single thread AFAICS.

cheers