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Message-ID: <f84f3e60-e71a-4ea4-8c9c-86fd915129a9@huawei.com>
Date: Tue, 16 Jan 2024 15:10:26 +0800
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Subject: Re: [PATCH] ARM64: Dynamically allocate cpumasks and increase
 supported CPUs to 512
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To: "Russell King (Oracle)" <linux@armlinux.org.uk>, "Christoph Lameter
 (Ampere)" <cl@linux.com>
CC: Anshuman Khandual <anshuman.khandual@arm.com>,
	<linux-arm-kernel@lists.infradead.org>, <linux-kernel@vger.kernel.org>,
	<linux-mm@kvack.org>, <Valentin.Schneider@arm.com>, Vanshidhar Konda
	<vanshikonda@os.amperecomputing.com>, Jonathan Cameron
	<Jonathan.Cameron@huawei.com>, Catalin Marinas <catalin.marinas@arm.com>,
	Robin Murphy <robin.murphy@arm.com>, Dave Kleikamp
	<dave.kleikamp@oracle.com>, Matteo Carlini <Matteo.Carlini@arm.com>,
	<akpm@linux-foundation.org>, <yang@os.amperecomputing.com>
References: <794a1211-630b-3ee5-55a3-c06f10df1490@linux.com>
 <ZaVRlHpAOoNIiRiY@shell.armlinux.org.uk>
From: Kefeng Wang <wangkefeng.wang@huawei.com>
In-Reply-To: <ZaVRlHpAOoNIiRiY@shell.armlinux.org.uk>
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On 2024/1/15 23:39, Russell King (Oracle) wrote:
> On Thu, Dec 14, 2023 at 04:05:56PM -0800, Christoph Lameter (Ampere) wrote:
>> Index: linux/arch/arm64/Kconfig
>> ===================================================================
>> --- linux.orig/arch/arm64/Kconfig
>> +++ linux/arch/arm64/Kconfig
>> @@ -1407,7 +1407,21 @@ config SCHED_SMT
>>    config NR_CPUS
>>    	int "Maximum number of CPUs (2-4096)"
>>    	range 2 4096
> 
> I think your mailer got to your patch and messed up the white space.
> There are two spaces before each of these lines rather than the usual
> one.
> 
>> -	default "256"
>> +	default 512
>> +
>> +#
>> +# Determines the placement of cpumasks.
>> +#
>> +# With CPUMASK_OFFSTACK the cpumasks are dynamically allocated.
>> +# Useful for machines with lots of core because it avoids increasing
>> +# the size of many of the data structures in the kernel.
>> +#
>> +# If this is off then the cpumasks have a static sizes and are
>> +# embedded within data structures.
>> +#
>> +config CPUMASK_OFFSTACK
>> +	def_bool y
>> +	depends on NR_CPUS > 256
> 
> Should that be ">= 256" ?

Maybe just select CPUMASK_OFFSTACK if NR_CPUS >= 256,


But could we just make CPUMASK_OFFSTACK configurable and let user/distro
to enable it?

diff --git a/lib/Kconfig b/lib/Kconfig
index 5ddda7c2ed9b..4254be5aa843 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -535,7 +535,9 @@ config CHECK_SIGNATURE
         bool

  config CPUMASK_OFFSTACK
-       bool "Force CPU masks off stack" if DEBUG_PER_CPU_MAPS
+       bool "Force CPU masks off stack"
+       depends on SMP
+       default n
         help
           Use dynamic allocation for cpumask_var_t, instead of putting
           them on the stack.  This is a bit more expensive, but avoids


> 
>>
>>    config HOTPLUG_CPU
>>    	bool "Support for hot-pluggable CPUs"
> 
> Same here.
>