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

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Date:   Tue, 12 Apr 2022 19:13:56 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Libo Chen <libo.chen@...cle.com>, 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,

On 4/12/22 18:35, Libo Chen wrote:
> 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.

I'm just talking about the Kconfig change below.  Not talking about whatever else
it might require per architecture.

But you say you have tried that and it doesn't work. What part of it doesn't work?
The Kconfig part or some code execution?

I'll test the Kconfig part of it later (in a few hours).

> 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
> 

-- 
~Randy