linux-kernel - Re: [PATCH 5/5] arm64: Fix behavior of maxcpus=N

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Date:	Thu, 07 Apr 2016 10:18:54 +0100
From:	James Morse <james.morse@....com>
To:	Suzuki K Poulose <suzuki.poulose@....com>
CC:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	marc.zyngier@....com, will.deacon@....com, catalin.marinas@....com,
	mark.rutland@....com, Vadim.Lomovtsev@...iumnetworks.com
Subject: Re: [PATCH 5/5] arm64: Fix behavior of maxcpus=N

Hi Suzuki,

On 06/04/16 12:24, Suzuki K Poulose wrote:
> maxcpu=n sets the number of CPUs activated at boot time to a max of n,
> but allowing the remaining CPUs to be brought up later if the user
> decides to do so. However, on arm64 due to various reasons, we disallowed
> hotplugging CPUs beyond n, by marking them not present. Now that
> we have checks in place to make sure the hotplugged CPUs have compatible
> features with system and requires no new errata, relax the restriction.

> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index b2d5f4e..0988ccc 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -659,21 +659,12 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
>  	if (max_cpus > ncores)
>  		max_cpus = ncores;

This "if (max_cpus > ncores)" is the only user of max_cpus left in this
function, and 'ncores' isn't used for anything else. Dead code?


> -	/* Don't bother if we're effectively UP */
> -	if (max_cpus <= 1)
> -		return;
> -
>  	/*
>  	 * Initialise the present map (which describes the set of CPUs
>  	 * actually populated at the present time) and release the
>  	 * secondaries from the bootloader.
> -	 *
> -	 * Make sure we online at most (max_cpus - 1) additional CPUs.
>  	 */
> -	max_cpus--;
>  	for_each_possible_cpu(cpu) {
> -		if (max_cpus == 0)
> -			break;
>  
>  		if (cpu == smp_processor_id())
>  			continue;
> @@ -686,7 +677,6 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
>  			continue;
>  
>  		set_cpu_present(cpu, true);
> -		max_cpus--;
>  	}
>  }
>  
> 


Thanks,

James