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

Aneesh Kumar K V
2022-06-13T05:49:46Z

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