Re: [PATCH RESEND 1/1] lib/Kconfig: remove DEBUG_PER_CPU_MAPS dependency for CPUMASK_OFFSTACK

From: Libo Chen
Date: Wed Apr 13 2022 - 15:29:24 EST



On 4/13/22 08:41, Randy Dunlap wrote:

On 4/12/22 23:56, Libo Chen wrote:
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@xxxxxxxxxx>
---
    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.
This patch builds cleanly for me:

---
arch/arm64/Kconfig | 1 +
lib/Kconfig | 2 +-
2 files changed, 2 insertions(+), 1 deletion(-)

--- 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
+ 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
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -316,6 +316,7 @@ config ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE
config SMP
def_bool y
+ select CPUMASK_OFFSTACK
config KERNEL_MODE_NEON
def_bool y

along with:
# CONFIG_DEBUG_PER_CPU_MAPS is not set


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.
I agree.

--- 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?
Yes, it is. I don't know that the problem is...
Masahiro explained that CPUMASK_OFFSTACK can only be configured by options not users if DEBUG_PER_CPU_MASK is not enabled. This doesn't seem to be what we want.

Libo