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

[Ying Huang]

On Mon, 2022-06-13 at 11:18 +0530, Aneesh Kumar K V wrote:
> 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@xxxxxxxxxxxxx>
> > > > > ---
> > > > >    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.

Per my understanding, we can enclose most code about
demoting/promoting inside CONFIG_MIGRATION, including
numa/demotion_enabled sysfs interface.  In this way, the code size can
be reduced.

Best Regards,
Huang, Ying