Re: [PATCH v6 03/13] mm/demotion: Return error on write to numa_demotion sysfs

[Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>]

On 6/13/22 11:03 AM, Ying Huang wrote:
> On Mon, 2022-06-13 at 09:05 +0530, Aneesh Kumar K V wrote:
>> On 6/13/22 8:56 AM, Ying Huang wrote:
>>> On Fri, 2022-06-10 at 19:22 +0530, Aneesh Kumar K.V wrote:
>>>> With CONFIG_MIGRATION disabled return EINVAL on write.
>>>>
>>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>>>> ---
>>>>    mm/memory-tiers.c | 3 +++
>>>>    1 file changed, 3 insertions(+)
>>>>
>>>> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
>>>> index 9c6b40d7e0bf..c3123a457d90 100644
>>>> --- a/mm/memory-tiers.c
>>>> +++ b/mm/memory-tiers.c
>>>> @@ -105,6 +105,9 @@ static ssize_t numa_demotion_enabled_store(struct kobject *kobj,
>>>>    {
>>>>    	ssize_t ret;
>>>>    
>>>>
>>>>
>>>> +	if (!IS_ENABLED(CONFIG_MIGRATION))
>>>> +		return -EINVAL;
>>>> +
>>>
>>> How about enclose numa_demotion_enabled_xxx related code with CONFIG_MIGRATION?
>>>
>>
>> IIUC there is a desire to use IS_ENABLED() in the kernel instead of
>> #ifdef since that helps in more compile time checks. Because there are
>> no dead codes during compile now with IS_ENABLED().
> 
> IS_ENABLED() is used to reduce usage of "#ifdef" in ".c" file,
> especially inside a function.  We have good build test coverage with
> 0Day now.
> 
> To avoid code size inflate, it's better to use #ifdef CONFIG_MIGRATION.
> 

For a diff like below I am finding IS_ENABLED better.

size memory-tiers.o.isenabled memory-tiers.o
    text    data     bss     dec     hex filename
    4776     989       5    5770    168a memory-tiers.o.isenabled
    5257     990       5    6252    186c memory-tiers.o


modified   mm/memory-tiers.c
@@ -710,12 +710,11 @@ static int __meminit 
migrate_on_reclaim_callback(struct notifier_block *self,

  static void __init migrate_on_reclaim_init(void)
  {
-
-	if (IS_ENABLED(CONFIG_MIGRATION)) {
+#ifdef CONFIG_MIGRATION
  		node_demotion = kcalloc(MAX_NUMNODES, sizeof(struct demotion_nodes),
  					GFP_KERNEL);
  		WARN_ON(!node_demotion);
-	}
+#endif
  	hotplug_memory_notifier(migrate_on_reclaim_callback, 100);
  }

@@ -844,14 +843,19 @@ static ssize_t numa_demotion_enabled_show(struct 
kobject *kobj,
  			  numa_demotion_enabled ? "true" : "false");
  }

+#ifdef CONFIG_MIGRATION
  static ssize_t numa_demotion_enabled_store(struct kobject *kobj,
  					   struct kobj_attribute *attr,
  					   const char *buf, size_t count)
  {
-	ssize_t ret;
-
-	if (!IS_ENABLED(CONFIG_MIGRATION))
-		return -EINVAL;
+	return -EINVAL;
+}
+#else
+static ssize_t numa_demotion_enabled_store(struct kobject *kobj,
+					   struct kobj_attribute *attr,
+					   const char *buf, size_t count)
+{
+		ssize_t ret;

  	ret = kstrtobool(buf, &numa_demotion_enabled);
  	if (ret)
@@ -859,6 +863,7 @@ static ssize_t numa_demotion_enabled_store(struct 
kobject *kobj,

  	return count;
  }
+#endif

  static struct kobj_attribute numa_demotion_enabled_attr =
  	__ATTR(demotion_enabled, 0644, numa_demotion_enabled_show,

I also find that #ifdef config not easier to the eyes. If there is a 
large code that we can end up #ifdef out, then it might be worth it. 
IIUC, we might want to keep the establish_migration target to find 
top_tier rank and lower_tier mask. Once we do that only thing that we 
could comment out is the node_demotion sysfs creation and I was 
considering to keep that even if migration is disabled with a write to 
the file returning EINVAL. I could switch that if you strongly feel that 
we should hide node_demotion sysfs file.

-aneesh