linux-kernel - Re: [PATCH RESEND 1/1] lib/Kconfig: remove DEBUG_PER_CPU_MAPS dependency for CPUMASK

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Message-ID: <157cb46a-d134-2e72-4a65-14e378dd2b8e@oracle.com>
Date:   Tue, 12 Apr 2022 18:35:27 -0700
From:   Libo Chen <libo.chen@...cle.com>
To:     Randy Dunlap <rdunlap@...radead.org>, gregkh@...uxfoundation.org,
        masahiroy@...nel.org, tglx@...utronix.de, peterz@...radead.org,
        mingo@...nel.org, vbabka@...e.cz, akpm@...ux-foundation.org
Cc:     linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH RESEND 1/1] lib/Kconfig: remove DEBUG_PER_CPU_MAPS
 dependency for CPUMASK_OFFSTACK

Hi Randy,

On 4/12/22 17:18, Randy Dunlap wrote:
> Hi--
>
> On 4/12/22 16:15, Libo Chen wrote:
>> Forcing CPUMASK_OFFSTACK to be conditoned on DEBUG_PER_CPU_MAPS doesn't
>> make a lot of sense nowaday. Even the original patch dating back to 2008,
>> aab46da0520a ("cpumask: Add CONFIG_CPUMASK_OFFSTACK") didn't give any
>> rationale for such dependency.
>>
>> Nowhere in the code supports the presumption that DEBUG_PER_CPU_MAPS is
>> necessary for CONFIG_CPUMASK_OFFSTACK. Make no mistake, it's good to
>> have DEBUG_PER_CPU_MAPS for debugging purpose or precaution, but it's
>> simply not a hard requirement for CPUMASK_OFFSTACK. Moreover, x86 Kconfig
>> already can set CPUMASK_OFFSTACK=y without DEBUG_PER_CPU_MAPS=y.
>> There is no reason other architectures cannot given the fact that they
>> have even fewer, if any, arch-specific CONFIG_DEBUG_PER_CPU_MAPS code than
>> x86.
>>
>> Signed-off-by: Libo Chen <libo.chen@...cle.com>
>> ---
>>   lib/Kconfig | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/lib/Kconfig b/lib/Kconfig
>> index 087e06b4cdfd..7209039dfb59 100644
>> --- a/lib/Kconfig
>> +++ b/lib/Kconfig
>> @@ -511,7 +511,7 @@ config CHECK_SIGNATURE
>>   	bool
>>   
>>   config CPUMASK_OFFSTACK
>> -	bool "Force CPU masks off stack" if DEBUG_PER_CPU_MAPS
> This "if" dependency only controls whether the Kconfig symbol's prompt is
> displayed (presented) in kconfig tools. Removing it makes the prompt always
> be displayed.
>
> Any architecture could select (should be able to) CPUMASK_OFFSTACK independently
> of DEBUG_PER_CPU_MAPS.
Do you mean changing arch/xxxx/Kconfig to select CPUMASK_OFFSTACK under 
some config xxx? That will work but it requires code changes for each 
architecture.
But if you are talking about setting CONFIG_CPUMASK_OFFSTACK=y without 
CONFIG_DEBUG_PER_CPU_MAPS directly in config file, I have tried, it 
doesn't work.

Libo
> Is there another problem here?
>
>> +	bool "Force CPU masks off stack"
>>   	help
>>   	  Use dynamic allocation for cpumask_var_t, instead of putting
>>   	  them on the stack.  This is a bit more expensive, but avoids