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 22:54:30 -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 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 think that you are arguing for a patch like this:

--- 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
 	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.


> 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
> 

-- 
~Randy