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

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Message-ID: <506db9a9-47ff-658a-a821-27315949e8c3@oracle.com>
Date:   Tue, 12 Apr 2022 23:56:23 -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 22:54, Randy Dunlap wrote:
> Hi Libo,
>
> On 4/12/22 19:34, Libo Chen wrote:
>>
>> On 4/12/22 19:13, Randy Dunlap wrote:
>>> 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?
>> oh the Kconfig part. For example, make olddefconfig on a config file with CPUMASK_OFFSTACK=y only turns off CPUMASK_OFFSTACK unless I explicitly set DEBUG_PER_CPU_MAPS=y
> I can enable CPUMASK_OFFSTACK for arm64 without having DEBUG_PER_CPU_MAPS enabled.
> (with a patch, of course.)
> It builds OK. I don't know if it will run OK.

I am a little confused, did you succeed with your patch (replacing "if" 
with "depends on") or my patch (removing "if")? Because I definitely 
cannot enable CPUMASK_OFFSTACK for arm64 without DEBUG_PER_CPUMAPS 
enabled using your change.
> I think that you are arguing for a patch like this:

I am actually arguing for the opposite, I don't think CPUMASK_OFFSTACK 
should require DEBUG_PER_CPU_MAPS. They should be separate and 
independent to each other. So removing "if ..." should be enough in my 
opinion.
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -511,7 +511,8 @@ 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 DEBUG_PER_CPU_MAPS

This forces every arch to enable DEBUG_PER_CPU_MAPS if they want to 
enable CPUMASK_OFFSTACK, it's even stronger than "if". My whole argument 
is CPUMASK_OFFSTACK should be enable/disabled independent of 
DEBUG_PER_CPU_MASK
>   	help
>   	  Use dynamic allocation for cpumask_var_t, instead of putting
>   	  them on the stack.  This is a bit more expensive, but avoids
>
>
> As I said earlier, the "if" on the "bool" line just controls the prompt message.
> This patch make CPUMASK_OFFSTACK require DEBUG_PER_CPU_MAPS -- which might be overkill.
>

Okay I understand now "if" on the "boot" is not a dependency and it only 
controls the prompt message, then the question is why we cannot enable 
CPUMASK_OFFSTACK without DEBUG_PER_CPU_MAPS if it only controls prompt 
message? Is it not the behavior we expect?

Libo

>> Libo
>>> 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